Re:RE: Charge moving from decoupling capacitors
Forwarded for George.. Reply Separator Subject:RE: Charge moving from decoupling capacitors Author: george_t...@dell.com List-Post: emc-pstc@listserv.ieee.org Date: 5/23/00 12:35 PM Let's say that you have a cap placed at 1/4 wavelength away from an IC pwr pin. When the IC draws current from the Pwr/gnd planes, it causes a voltage dip/pwr bounce on the parallel planes. This voltage dip propagates to the cap at 1/4 wavelength away and draws current out of the cap. The current from the cap propagates back to the IC pwr pin at 1/4 wavelength away. The total travel for the incident and reflected wave is 1/4 + 1/4 = 1/2 wavelength. The supply current is 180 degrees out of phase from the IC switching current. Some people say that the cap needs to be closer than 1/2 of rising edge. But you can also calculate the wavelength of the 3rd and 5th harmonic of your pulse to determine your cap placement. Regards, George Tang george_t...@dell.com -Original Message- From: Roncone Paolo [mailto:paolo.ronc...@compuprint.it] Sent: Tuesday, May 23, 2000 7:15 AM To: 'george_t...@dell.com '; 'barry...@altavista.com ' Cc: 'emc-p...@ieee.org ' Subject: RE: Charge moving from decoupling capacitors George, can you pls explain your correction ? I supposed your first statement (At 1/4 wavelength, the charges are 90 degrees out of phase) was the correct one ! Paolo Roncone Compuprint s.p.a. Reply Separator Subject:RE: Charge moving from decoupling capacitors Author: george_t...@dell.com List-Post: emc-pstc@listserv.ieee.org Date: 5/22/00 9:14 PM Barry, I need to make a correction. I was rushing to lunch on Thursday, so I did not read over what I wrote. Here is the correction for the 2nd comment below: At 1/4 wavelength, the charges are 180 degrees out of phase, so they are working against the IC current draw. 1/8 wavelength (90 degrees out of phase) is what I consider to be acceptable. Regards, George Tang george_t...@dell.com -Original Message- From: Tang, George Sent: Thursday, May 18, 2000 12:31 PM To: 'Barry Ma'; Tang, George Cc: si-l...@silab.eng.sun.com; emc-p...@ieee.org Subject: RE: Charge moving from decoupling capacitors Barry, Thanks for the comments. Here are my comments: Ok, you put caps at a certain distance away from the IC because you only want them to work at 100 MHz. But that distance turns out to be the 1/4 wave distance at 400 MHz, and you placed enough caps at the 1/4 wave distance to cause board resonance. Now what? Do you tell the caps not to work at 400 MHz because it's not their frequency? For your 2nd comment: I used the words loosely define for that reason. If you are interested in high frequency decoupling and instantaneous current, you really want to have all your charges moving in phase. At 1/4 wavelength, the charges are 90 degrees out of phase, so they will not do much for your instantaneous current. 1/8 wavelength is what I consider to be acceptable. You can certainly pick a different number. Regards, George Tang george_t...@dell.com --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org
[SI-LIST] : RE: Charge moving from decoupling capacitors
Barry, The current can flow from the cap, but it will get to the IC at the wrong time. Regards, George george_t...@dell.com -Original Message- From: Barry Ma [mailto:barry...@altavista.com] Sent: Tuesday, May 23, 2000 10:22 AM To: george_t...@exchange.dell.com Cc: si-l...@silab.eng.sun.com; emc-p...@ieee.org Subject: RE: Charge moving from decoupling capacitors George, I am impressed by your attitude to pursue the correctness, and glad to discuss with you further on “How does a decoupling capacitor support an IC?” Here is my two cents worth. The decap supplies necessary charge to the IC during Tr through a transmission line. As you mentioned before: “The current is an impulse function, although the voltage waveform is a step function.” This impulse function, actually a bell-like function on Tr, happens every time period T when the IC gate switches from low to high. The corresponding frequency spectrum contains lots of frequencies. There must be some frequencies making the transmission line a 1/4, 3/4, ... wavelength. It is hard for me to be convinced that currents of those frequencies cannot flow from the decap to the IC. ... Pleas correct me if misunderstood. Thanks. Regards, Barry Ma b...@anritsu.com On Mon, 22 May 2000, george_t...@dell.com wrote: Barry, I need to make a correction. I was rushing to lunch on Thursday, so I did not read over what I wrote. Here is the correction for the 2nd comment below: At 1/4 wavelength, the charges are 180 degrees out of phase, so they are working against the IC current draw. 1/8 wavelength (90 degrees out of phase) is what I consider to be acceptable. Regards, George Tang george_t...@dell.com ___ Why pay when you don't have to? Get AltaVista Free Internet Access now! http://jump.altavista.com/freeaccess4.go ___ --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org To unsubscribe from si-list or si-list-digest: send e-mail to majord...@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu
RE: Charge moving from decoupling capacitors
George, I am impressed by your attitude to pursue the correctness, and glad to discuss with you further on How does a decoupling capacitor support an IC? Here is my two cents worth. The decap supplies necessary charge to the IC during Tr through a transmission line. As you mentioned before: The current is an impulse function, although the voltage waveform is a step function. This impulse function, actually a bell-like function on Tr, happens every time period T when the IC gate switches from low to high. The corresponding frequency spectrum contains lots of frequencies. There must be some frequencies making the transmission line a 1/4, 3/4, ... wavelength. It is hard for me to be convinced that currents of those frequencies cannot flow from the decap to the IC. ... Pleas correct me if misunderstood. Thanks. Regards, Barry Ma b...@anritsu.com On Mon, 22 May 2000, george_t...@dell.com wrote: Barry, I need to make a correction. I was rushing to lunch on Thursday, so I did not read over what I wrote. Here is the correction for the 2nd comment below: At 1/4 wavelength, the charges are 180 degrees out of phase, so they are working against the IC current draw. 1/8 wavelength (90 degrees out of phase) is what I consider to be acceptable. Regards, George Tang george_t...@dell.com ___ Why pay when you don't have to? Get AltaVista Free Internet Access now! http://jump.altavista.com/freeaccess4.go ___ --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org
RE: Charge moving from decoupling capacitors
George, can you pls explain your correction ? I supposed your first statement (At 1/4 wavelength, the charges are 90 degrees out of phase) was the correct one ! Paolo Roncone Compuprint s.p.a. Reply Separator Subject:RE: Charge moving from decoupling capacitors Author: george_t...@dell.com List-Post: emc-pstc@listserv.ieee.org Date: 5/22/00 9:14 PM Barry, I need to make a correction. I was rushing to lunch on Thursday, so I did not read over what I wrote. Here is the correction for the 2nd comment below: At 1/4 wavelength, the charges are 180 degrees out of phase, so they are working against the IC current draw. 1/8 wavelength (90 degrees out of phase) is what I consider to be acceptable. Regards, George Tang george_t...@dell.com -Original Message- From: Tang, George Sent: Thursday, May 18, 2000 12:31 PM To: 'Barry Ma'; Tang, George Cc: si-l...@silab.eng.sun.com; emc-p...@ieee.org Subject: RE: Charge moving from decoupling capacitors Barry, Thanks for the comments. Here are my comments: Ok, you put caps at a certain distance away from the IC because you only want them to work at 100 MHz. But that distance turns out to be the 1/4 wave distance at 400 MHz, and you placed enough caps at the 1/4 wave distance to cause board resonance. Now what? Do you tell the caps not to work at 400 MHz because it's not their frequency? For your 2nd comment: I used the words loosely define for that reason. If you are interested in high frequency decoupling and instantaneous current, you really want to have all your charges moving in phase. At 1/4 wavelength, the charges are 90 degrees out of phase, so they will not do much for your instantaneous current. 1/8 wavelength is what I consider to be acceptable. You can certainly pick a different number. Regards, George Tang george_t...@dell.com --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org
Fwd:RE: Charge moving from decoupling capacitors
for George Reply Separator Subject:RE: Charge moving from decoupling capacitors Author: george_t...@dell.com List-Post: emc-pstc@listserv.ieee.org Date: 5/22/00 9:14 PM Barry, I need to make a correction. I was rushing to lunch on Thursday, so I did not read over what I wrote. Here is the correction for the 2nd comment below: At 1/4 wavelength, the charges are 180 degrees out of phase, so they are working against the IC current draw. 1/8 wavelength (90 degrees out of phase) is what I consider to be acceptable. Regards, George Tang george_t...@dell.com -Original Message- From: Tang, George Sent: Thursday, May 18, 2000 12:31 PM To: 'Barry Ma'; Tang, George Cc: si-l...@silab.eng.sun.com; emc-p...@ieee.org Subject: RE: Charge moving from decoupling capacitors Barry, Thanks for the comments. Here are my comments: Ok, you put caps at a certain distance away from the IC because you only want them to work at 100 MHz. But that distance turns out to be the 1/4 wave distance at 400 MHz, and you placed enough caps at the 1/4 wave distance to cause board resonance. Now what? Do you tell the caps not to work at 400 MHz because it's not their frequency? For your 2nd comment: I used the words loosely define for that reason. If you are interested in high frequency decoupling and instantaneous current, you really want to have all your charges moving in phase. At 1/4 wavelength, the charges are 90 degrees out of phase, so they will not do much for your instantaneous current. 1/8 wavelength is what I consider to be acceptable. You can certainly pick a different number. Regards, George Tang george_t...@dell.com -Original Message- From: Barry Ma [mailto:barry...@altavista.com] Sent: Thursday, May 18, 2000 10:50 AM To: george_t...@exchange.dell.com Cc: si-l...@silab.eng.sun.com; emc-p...@ieee.org Subject: RE: Charge moving from decoupling capacitors George, Thanks for your long input. I'd like to make some comments below. - On Wed, 17 May 2000, george_t...@dell.com wrote: Large parallel plates behave as transmission lines. A quarter wavelength transmission line with a short at the end has infinite impedance, so capacitors placed 1/4 wavelength away are bad. That's why decaps work on low frequency portion. Let's set 100 MHz and below for decaps to cover. The wavelength at 100 MHz is 3 meters. A quarter of it is 75 cm. It's long enough to ordinary PCB size. (The cap is directly connected to pwr/gnd planes.) This means that we can loosely define the largest usable board area capacitance as 1/8 wavelength radius of copper surrounding the IC power pin. Charges stored on the planes further than 1/8 wavelength away are not very usable due to the time delay. At 500MHz in FR4, 1/8 wavelength is 1.5 inches. Is such a board capacitor good enough for your IC? George, I beg for differentials. How did you jump from capacitors placed 1/4 wavelength away are bad to the largest usable board area capacitance as 1/8 wavelength radius? Can I use the same token to infer from caps placed one wavelength away are good to the largest usable board area capacitance is within 1/2 wavelength radius? And so, and so on. Regards, Barry Ma b...@anritsu.com ___ Why pay when you don't have to? Get AltaVista Free Internet Access now! http://jump.altavista.com/freeaccess4.go ___ --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org
[SI-LIST] : RE: Charge moving from decoupling capacitors
Barry, I need to make a correction. I was rushing to lunch on Thursday, so I did not read over what I wrote. Here is the correction for the 2nd comment below: At 1/4 wavelength, the charges are 180 degrees out of phase, so they are working against the IC current draw. 1/8 wavelength (90 degrees out of phase) is what I consider to be acceptable. Regards, George Tang george_t...@dell.com -Original Message- From: Tang, George Sent: Thursday, May 18, 2000 12:31 PM To: 'Barry Ma'; Tang, George Cc: si-l...@silab.eng.sun.com; emc-p...@ieee.org Subject: RE: Charge moving from decoupling capacitors Barry, Thanks for the comments. Here are my comments: Ok, you put caps at a certain distance away from the IC because you only want them to work at 100 MHz. But that distance turns out to be the 1/4 wave distance at 400 MHz, and you placed enough caps at the 1/4 wave distance to cause board resonance. Now what? Do you tell the caps not to work at 400 MHz because it's not their frequency? For your 2nd comment: I used the words loosely define for that reason. If you are interested in high frequency decoupling and instantaneous current, you really want to have all your charges moving in phase. At 1/4 wavelength, the charges are 90 degrees out of phase, so they will not do much for your instantaneous current. 1/8 wavelength is what I consider to be acceptable. You can certainly pick a different number. Regards, George Tang george_t...@dell.com -Original Message- From: Barry Ma [mailto:barry...@altavista.com] Sent: Thursday, May 18, 2000 10:50 AM To: george_t...@exchange.dell.com Cc: si-l...@silab.eng.sun.com; emc-p...@ieee.org Subject: RE: Charge moving from decoupling capacitors George, Thanks for your long input. I'd like to make some comments below. - On Wed, 17 May 2000, george_t...@dell.com wrote: Large parallel plates behave as transmission lines. A quarter wavelength transmission line with a short at the end has infinite impedance, so capacitors placed 1/4 wavelength away are bad. That’s why decaps work on low frequency portion. Let’s set 100 MHz and below for decaps to cover. The wavelength at 100 MHz is 3 meters. A quarter of it is 75 cm. It’s long enough to ordinary PCB size. (The cap is directly connected to pwr/gnd planes.) This means that we can loosely define the largest usable board area capacitance as 1/8 wavelength radius of copper surrounding the IC power pin. Charges stored on the planes further than 1/8 wavelength away are not very usable due to the time delay. At 500MHz in FR4, 1/8 wavelength is 1.5 inches. Is such a board capacitor good enough for your IC? George, I beg for differentials. How did you jump from capacitors placed 1/4 wavelength away are bad to the largest usable board area capacitance as 1/8 wavelength radius? Can I use the same token to infer from caps placed one wavelength away are good to the largest usable board area capacitance is within 1/2 wavelength radius? And so, and so on. Regards, Barry Ma b...@anritsu.com ___ Why pay when you don't have to? Get AltaVista Free Internet Access now! http://jump.altavista.com/freeaccess4.go ___ To unsubscribe from si-list or si-list-digest: send e-mail to majord...@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu
Re: Charge moving from decoupling capacitors
Barry Thanks for correcting me. Practical solutions have changed from the time I made many measurements on PCBs with discrete components. I'll be interested to read the article you mentioned since I wasn't aware a single SMA cap could do a better job. Regards, Ralph Cameron - Original Message - From: Barry Ma barry...@altavista.com To: ral...@igs.net Cc: emc-p...@ieee.org Sent: Friday, May 19, 2000 11:33 AM Subject: Re: Charge moving from decoupling capacitors Ralph, Please be cautious about what you said below: the reason why most power buses on PCBs use several values of decoupling is to ensure that a wide range of frequencies are covered. Several V curves shown in the figure of impedance vs. frequency, which we are all familiar with, would easily convince us the above statement. But those V curves only show the absolute value of impedance around self-resonance frequency of various capacitors. If considering related phase relations, the total impedance of several values of decaps would become not as simple as we expected - having low impedance over wider frequecy range. You may refer to an article by Paul, C. R.: Effectiveness of multiple decoupling capacitors, IEEE EMC Vol. 34, p. 130, May 1992. In my practice of using SMA caps, only the largest value of capacitance available for given SMA size is selected, if I have a PCB with 10 mil or less plane spacing. Regards, Barry Ma b...@anritsu.com On Thu, 18 May 2000, Ralph Cameron wrote: As I read in an article related to bypassing for good decoupling ( in 1971) one can select from a number of EIA values and by cutting the lead lengths correctly ( e.g. from 1/2- less than 1/4 ) the series reonant frequency will drop by a considerable amount so - yes, the reason why most power buses on PCBs use several values of decoupling is to ensure that a wide range of requencies are covered. Perhaps, with surface mount caps, that is easier to predict because they are essentially leadless. I once cured a very severe case of an FM receiver responding to the 7th harmonic of a 14Mhz transmitter because an untuned mixer was used. Placing a 100pf cap with 1/4 leads right across the mixer IC completely cured the problem without degrading mixer sensitivity. Ralph Cameron EMC Consultant for Suppression of Consumer Electronics (After Sale) ___ Why pay when you don't have to? Get AltaVista Free Internet Access now! http://jump.altavista.com/freeaccess4.go ___ --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org
Re: Charge moving from decoupling capacitors
Ralph, Please be cautious about what you said below: the reason why most power buses on PCBs use several values of decoupling is to ensure that a wide range of frequencies are covered. Several V curves shown in the figure of impedance vs. frequency, which we are all familiar with, would easily convince us the above statement. But those V curves only show the absolute value of impedance around self-resonance frequency of various capacitors. If considering related phase relations, the total impedance of several values of decaps would become not as simple as we expected - having low impedance over wider frequecy range. You may refer to an article by Paul, C. R.: Effectiveness of multiple decoupling capacitors, IEEE EMC Vol. 34, p. 130, May 1992. In my practice of using SMA caps, only the largest value of capacitance available for given SMA size is selected, if I have a PCB with 10 mil or less plane spacing. Regards, Barry Ma b...@anritsu.com On Thu, 18 May 2000, Ralph Cameron wrote: As I read in an article related to bypassing for good decoupling ( in 1971) one can select from a number of EIA values and by cutting the lead lengths correctly ( e.g. from 1/2- less than 1/4 ) the series reonant frequency will drop by a considerable amount so - yes, the reason why most power buses on PCBs use several values of decoupling is to ensure that a wide range of requencies are covered. Perhaps, with surface mount caps, that is easier to predict because they are essentially leadless. I once cured a very severe case of an FM receiver responding to the 7th harmonic of a 14Mhz transmitter because an untuned mixer was used. Placing a 100pf cap with 1/4 leads right across the mixer IC completely cured the problem without degrading mixer sensitivity. Ralph Cameron EMC Consultant for Suppression of Consumer Electronics (After Sale) ___ Why pay when you don't have to? Get AltaVista Free Internet Access now! http://jump.altavista.com/freeaccess4.go ___ --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org
Re: Re:RE: Charge moving from decoupling capacitors
As I read in an article related to bypassing for good decoupling ( in 1971) one can select from a number of EIA values and by cutting the lead lengths correctly ( e.g. from 1/2- less than 1/4 ) the series reonant frequency will drop by a considerable amount so - yes, the reason why most power buses on PCBs use several values of decoupling is to ensure that a wide range of requencies are covered. Perhaps, with surface mount caps, that is easier to predict because they are essentially leadless. I once cured a very severe case of an FM receiver responding to the 7th harmonic of a 14Mhz transmitter because an untuned mixer was used. Placing a 100pf cap with 1/4 leads right across the mixer IC completely cured the problem without degrading mixer sensitivity. Ralph Cameron EMC Consultant for Suppression of Consumer Electronics (After Sale) - Original Message - From: Jim Bacher jim_bac...@mail.monarch.com To: george_t...@dell.com Cc: emc-p...@ieee.org Sent: Thursday, May 18, 2000 4:24 PM Subject: Re:RE: Charge moving from decoupling capacitors Forwarded for George. Reply Separator Subject:RE: Charge moving from decoupling capacitors Author: george_t...@dell.com Date: 5/18/00 2:30 PM Barry, Thanks for the comments. Here are my comments: Ok, you put caps at a certain distance away from the IC because you only want them to work at 100 MHz. But that distance turns out to be the 1/4 wave distance at 400 MHz, and you placed enough caps at the 1/4 wave distance to cause board resonance. Now what? Do you tell the caps not to work at 400 MHz because it's not their frequency? For your 2nd comment: I used the words loosely define for that reason. If you are interested in high frequency decoupling and instantaneous current, you really want to have all your charges moving in phase. At 1/4 wavelength, the charges are 90 degrees out of phase, so they will not do much for your instantaneous current. 1/8 wavelength is what I consider to be acceptable. You can certainly pick a different number. Regards, George Tang george_t...@dell.com -Original Message- From: Barry Ma [mailto:barry...@altavista.com] Sent: Thursday, May 18, 2000 10:50 AM To: george_t...@exchange.dell.com Cc: si-l...@silab.eng.sun.com; emc-p...@ieee.org Subject: RE: Charge moving from decoupling capacitors George, Thanks for your long input. I'd like to make some comments below. - On Wed, 17 May 2000, george_t...@dell.com wrote: Large parallel plates behave as transmission lines. A quarter wavelength transmission line with a short at the end has infinite impedance, so capacitors placed 1/4 wavelength away are bad. That's why decaps work on low frequency portion. Let's set 100 MHz and below for decaps to cover. The wavelength at 100 MHz is 3 meters. A quarter of it is 75 cm. It's long enough to ordinary PCB size. (The cap is directly connected to pwr/gnd planes.) This means that we can loosely define the largest usable board area capacitance as 1/8 wavelength radius of copper surrounding the IC power pin. Charges stored on the planes further than 1/8 wavelength away are not very usable due to the time delay. At 500MHz in FR4, 1/8 wavelength is 1.5 inches. Is such a board capacitor good enough for your IC? George, I beg for differentials. How did you jump from capacitors placed 1/4 wavelength away are bad to the largest usable board area capacitance as 1/8 wavelength radius? Can I use the same token to infer from caps placed one wavelength away are good to the largest usable board area capacitance is within 1/2 wavelength radius? And so, and so on. Regards, Barry Ma b...@anritsu.com --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org
RE: Charge moving from decoupling capacitors
George, Sorry, I forgot to repeat what I wrote to you 5/15/00: It is generally acknowledged that decaps and plane cap are complementary (supposing a 10 mil or less spacing between pwr and gnd planes). Decaps cover low end of frequency range, while the plane cap takes care of high frequencies. I wrote similar words to Steve wei...@atdial.net 5/17/00: If there's a 10 mil or less spacing between pwr and gnd planes, the plane cap is available. The plane cap and the decaps are complementary in whole frequency range. Plane cap takes care of high end, and decaps cover low portion. Then locations of decaps are not critical. And then decaps can be shared by other chips, according to the excellent research conducted by EMC lab at UMR. That's my fault omitting this prerequisite today. In a PCB with 10 mil or less plane spacing, the plane capacitance would work better at 400 MHz than any decaps. Please allow me to put some words about plane cap. Why do we - actually the EMC lab at UMR, insist on 10 mil or less? Three reasons: (1) Utilizable plane capacitor. (2) Low inductance when pwr/gnd serve as a transmission line. (3) Low mutual inductance between vias. I apologize for not phrasing properly. Thank you Best Regards, Barry --- On Thu, 18 May 2000, george_t...@dell.com wrote: Barry, Thanks for the comments. Here are my comments: Ok, you put caps at a certain distance away from the IC because you only want them to work at 100 MHz. But that distance turns out to be the 1/4 wave distance at 400 MHz, and you placed enough caps at the 1/4 wave distance to cause board resonance. Now what? Do you tell the caps not to work at 400 MHz because it's not their frequency? For your 2nd comment: I used the words loosely define for that reason. If you are interested in high frequency decoupling and instantaneous current, you really want to have all your charges moving in phase. At 1/4 wavelength, the charges are 90 degrees out of phase, so they will not do much for your instantaneous current. 1/8 wavelength is what I consider to be acceptable. You can certainly pick a different number. Regards, George Tang george_t...@dell.com ___ Why pay when you don't have to? Get AltaVista Free Internet Access now! http://jump.altavista.com/freeaccess4.go ___ --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org
Re:RE: Charge moving from decoupling capacitors
Forwarded for George. Reply Separator Subject:RE: Charge moving from decoupling capacitors Author: george_t...@dell.com List-Post: emc-pstc@listserv.ieee.org Date: 5/18/00 2:30 PM Barry, Thanks for the comments. Here are my comments: Ok, you put caps at a certain distance away from the IC because you only want them to work at 100 MHz. But that distance turns out to be the 1/4 wave distance at 400 MHz, and you placed enough caps at the 1/4 wave distance to cause board resonance. Now what? Do you tell the caps not to work at 400 MHz because it's not their frequency? For your 2nd comment: I used the words loosely define for that reason. If you are interested in high frequency decoupling and instantaneous current, you really want to have all your charges moving in phase. At 1/4 wavelength, the charges are 90 degrees out of phase, so they will not do much for your instantaneous current. 1/8 wavelength is what I consider to be acceptable. You can certainly pick a different number. Regards, George Tang george_t...@dell.com -Original Message- From: Barry Ma [mailto:barry...@altavista.com] Sent: Thursday, May 18, 2000 10:50 AM To: george_t...@exchange.dell.com Cc: si-l...@silab.eng.sun.com; emc-p...@ieee.org Subject: RE: Charge moving from decoupling capacitors George, Thanks for your long input. I'd like to make some comments below. - On Wed, 17 May 2000, george_t...@dell.com wrote: Large parallel plates behave as transmission lines. A quarter wavelength transmission line with a short at the end has infinite impedance, so capacitors placed 1/4 wavelength away are bad. That's why decaps work on low frequency portion. Let's set 100 MHz and below for decaps to cover. The wavelength at 100 MHz is 3 meters. A quarter of it is 75 cm. It's long enough to ordinary PCB size. (The cap is directly connected to pwr/gnd planes.) This means that we can loosely define the largest usable board area capacitance as 1/8 wavelength radius of copper surrounding the IC power pin. Charges stored on the planes further than 1/8 wavelength away are not very usable due to the time delay. At 500MHz in FR4, 1/8 wavelength is 1.5 inches. Is such a board capacitor good enough for your IC? George, I beg for differentials. How did you jump from capacitors placed 1/4 wavelength away are bad to the largest usable board area capacitance as 1/8 wavelength radius? Can I use the same token to infer from caps placed one wavelength away are good to the largest usable board area capacitance is within 1/2 wavelength radius? And so, and so on. Regards, Barry Ma b...@anritsu.com --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org
[SI-LIST] : RE: Charge moving from decoupling capacitors
Barry, Thanks for the comments. Here are my comments: Ok, you put caps at a certain distance away from the IC because you only want them to work at 100 MHz. But that distance turns out to be the 1/4 wave distance at 400 MHz, and you placed enough caps at the 1/4 wave distance to cause board resonance. Now what? Do you tell the caps not to work at 400 MHz because it's not their frequency? For your 2nd comment: I used the words loosely define for that reason. If you are interested in high frequency decoupling and instantaneous current, you really want to have all your charges moving in phase. At 1/4 wavelength, the charges are 90 degrees out of phase, so they will not do much for your instantaneous current. 1/8 wavelength is what I consider to be acceptable. You can certainly pick a different number. Regards, George Tang george_t...@dell.com -Original Message- From: Barry Ma [mailto:barry...@altavista.com] Sent: Thursday, May 18, 2000 10:50 AM To: george_t...@exchange.dell.com Cc: si-l...@silab.eng.sun.com; emc-p...@ieee.org Subject: RE: Charge moving from decoupling capacitors George, Thanks for your long input. I'd like to make some comments below. - On Wed, 17 May 2000, george_t...@dell.com wrote: Large parallel plates behave as transmission lines. A quarter wavelength transmission line with a short at the end has infinite impedance, so capacitors placed 1/4 wavelength away are bad. That’s why decaps work on low frequency portion. Let’s set 100 MHz and below for decaps to cover. The wavelength at 100 MHz is 3 meters. A quarter of it is 75 cm. It’s long enough to ordinary PCB size. (The cap is directly connected to pwr/gnd planes.) This means that we can loosely define the largest usable board area capacitance as 1/8 wavelength radius of copper surrounding the IC power pin. Charges stored on the planes further than 1/8 wavelength away are not very usable due to the time delay. At 500MHz in FR4, 1/8 wavelength is 1.5 inches. Is such a board capacitor good enough for your IC? George, I beg for differentials. How did you jump from capacitors placed 1/4 wavelength away are bad to the largest usable board area capacitance as 1/8 wavelength radius? Can I use the same token to infer from caps placed one wavelength away are good to the largest usable board area capacitance is within 1/2 wavelength radius? And so, and so on. Regards, Barry Ma b...@anritsu.com ___ Why pay when you don't have to? Get AltaVista Free Internet Access now! http://jump.altavista.com/freeaccess4.go ___ To unsubscribe from si-list or si-list-digest: send e-mail to majord...@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu
RE: Charge moving from decoupling capacitors
George, Thanks for your long input. I'd like to make some comments below. - On Wed, 17 May 2000, george_t...@dell.com wrote: Large parallel plates behave as transmission lines. A quarter wavelength transmission line with a short at the end has infinite impedance, so capacitors placed 1/4 wavelength away are bad. Thats why decaps work on low frequency portion. Lets set 100 MHz and below for decaps to cover. The wavelength at 100 MHz is 3 meters. A quarter of it is 75 cm. Its long enough to ordinary PCB size. (The cap is directly connected to pwr/gnd planes.) This means that we can loosely define the largest usable board area capacitance as 1/8 wavelength radius of copper surrounding the IC power pin. Charges stored on the planes further than 1/8 wavelength away are not very usable due to the time delay. At 500MHz in FR4, 1/8 wavelength is 1.5 inches. Is such a board capacitor good enough for your IC? George, I beg for differentials. How did you jump from capacitors placed 1/4 wavelength away are bad to the largest usable board area capacitance as 1/8 wavelength radius? Can I use the same token to infer from caps placed one wavelength away are good to the largest usable board area capacitance is within 1/2 wavelength radius? And so, and so on. Regards, Barry Ma b...@anritsu.com ___ Why pay when you don't have to? Get AltaVista Free Internet Access now! http://jump.altavista.com/freeaccess4.go ___ --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org
Re:RE: Charge moving from decoupling capacitors
forwarding for George Reply Separator Subject:RE: Charge moving from decoupling capacitors Author: george_t...@dell.com List-Post: emc-pstc@listserv.ieee.org Date: 5/17/00 11:16 PM Barry, This is true on most boards. But that is because the caps are tied to the planes through vias and lead traces, which are inductive. Also, the caps are further away from the IC power pins than the planes. This does not mean that the cap by itself cannot work at 1 GHz. If you have a 50 mil trace above a gnd plane terminated to a cap without using vias or other lead traces and measure VSWR and reflections from the cap, you will see that the cap works well at 1 GHz and beyond. You must solder the cap to the gnd plane and to the 50 mil trace directly. Regards, George Tang -Original Message- From: Barry Ma [mailto:barry...@altavista.com] Sent: Wednesday, May 17, 2000 9:16 AM To: si-l...@silab.eng.sun.com; EMC-PSTC Cc: wei...@atdial.net Subject: Re: Charge moving from decoupling capacitors Steve, Thanks a lot for the very nice hierarchy description below. If there's a 10 mil or less spacing between pwr and gnd planes, the plane cap is available. The plane cap and the decaps are complementary in whole frequency range. Plane cap takes care of high end, and decaps cover low portion. Then the location of decaps are not critical. And then decaps can be shared by other chips, according to the excellent research conducted by EMC lab at UMR. Please allow me to modify a bit your description as follows. The capacitance inside the device supports the chip first, but usually not enough. Charge from the planes also supports the chip and replenishes the device capacitance, Decaps replenish the plane on low frequency portion, while plane cap responds itself on HF end, Bulk capacitors replenish decaps and plane cap, The voltage regulator replenishes the bulk capacitors. Please correct me. Thanks. Regards, Barry b...@anritsu.com - From: sweir wei...@atdial.net, on 5/11/00 9:28 PM: The capacitance inside the device supports the chip, Charge from the planes replenishes the device capacitance, HF capacitors on the board replenish the planes, Bulk capacitors replenish the HF capacitors, The voltage regulator replenishes the bulk capacitors. [edited by bm] --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org
Fwd:RE: Charge moving from decoupling capacitors
forwarding for George.. Reply Separator Subject:RE: Charge moving from decoupling capacitors Author: george_t...@dell.com List-Post: emc-pstc@listserv.ieee.org Date: 5/17/00 10:50 PM You ask an open question, so the open answer to that is it depends. It depends on how much current the IC is drawing, what frequency and rise time it has, what type of load the IC output drivers are driving, how much noise on the power plane the other ICs on the same board can tolerate . . . . You are looking for a solution that will work in every case, but you will find that there are problems or exceptions to every solution. A capacitor stores charges and supplies current to ICs when needed. By that definition, the charges on the capacitor must move in phase from each other. For parallel plates to behave as a good capacitor, their dimensions should be much smaller than a wavelength so the charges on the plates will move in phase. Large parallel plates behave as transmission lines. A quarter wavelength transmission line with a short at the end has infinite impedance, so capacitors placed 1/4 wavelength away are bad. This means that we can loosely define the largest usable board area capacitance as 1/8 wavelength radius of copper surrounding the IC power pin. Charges stored on the planes further than 1/8 wavelength away are not very usable due to the time delay. At 500MHz in FR4, 1/8 wavelength is 1.5 inches. Is such a board capacitor good enough for your IC? It might be if you have a CMOS IC driving another CMOS IC less than 2 inches away, so the load on the output of the 1st IC is mainly the CMOS gate capacitance at the input of the 2nd IC at the end of the 2 inch transmission line. During switching, the 2nd IC draws current from the output of the 1st IC for the 1st 200 or 300 ps to charge up the input gate capacitance on the 2nd IC. The current is an impulse function, although the voltage waveform is a step function. If these ICs are small and uses little power, the board capacitance might be enough to supply the impulse current for the 1st IC. If the load on the transmission line is a termination resistor, the current draw will be a step function, and the board capacitance alone may not be good enough. But here is an exception. You have a board that uses only CMOS devices, and the largest IC is a 500 MHz processor that consumes 50W of power at 2.5v, so it switches 20A of current at 500 MHz. It is a CMOS device, so its current draws are mostly impulse functions. Would the board capacitance be good enough for this 20A switching current? Probably not. Making the pwr plane larger will not help, but using more layers in parallel will help. You might have to use 4, 8, or 16 pwr/gnd layer pairs in parallel for this board, the more layers the better. But wait!! Isn't that what a multilayer ceramic capacitor is? It has many pwr/gnd layers in parallel . . . . Hmm, if we could only take advantage of that . . . I'm thinking that if you have to use a 50W, 500MHz processor, and your boss tells you that you cannot have 8 pwr/gnd layers on your board, you or someone will probably find a way to make the ceramic capacitor work effectively beyond 1 GHz!! Another question you might ask is that do I really want to dump the 20A switching noise directly into the pwr/gnd planes and create pwr/gnd bounce and board resonance to interfere with all the ICs on the board, not to mention EMI problems? Regards, George Tang -Original Message- From: Barry Ma [mailto:barry...@altavista.com] Sent: Monday, May 15, 2000 1:33 PM To: george_t...@exchange.dell.com; si-l...@silab.eng.sun.com Cc: emc-p...@ieee.org Subject: RE: Charge moving from decoupling capacitors Thanks a lot for your inputs. All responses to my second question are only concerned with the inductance due to long distance between chip and decap. Nobody seems to agree imposing another constrain to the distance. My question was Do we really have to limit the distance letting the charge have enough time to move from the cap to the chip during the rise time interval? I doubt it. But I really read an article implying this extra concern. George, you wrote: This is true if you have only DC current. For AC, you may have water in the pipe but no water out of the faucet if the faucet is switching out of phase from the water in the pipe. Thank you for reminding me of Frequency Domain analysis. Yes, I should have described and analyzed a transient problem (charge travel during Tr) in both TD and FD, and then correlate the results. Let me have a try this time: It is generally acknowledged that decaps and plane cap are complementary (supposing a 10 mil or less spacing between pwr and gnd planes). Decaps cover low end of frequency range, while the plane cap takes care of high frequencies. Thus the interplane cap would play more and more important role in high-speed PCB design, as the speed
[SI-LIST] : RE: Charge moving from decoupling capacitors
Barry, This is true on most boards. But that is because the caps are tied to the planes through vias and lead traces, which are inductive. Also, the caps are further away from the IC power pins than the planes. This does not mean that the cap by itself cannot work at 1 GHz. If you have a 50 mil trace above a gnd plane terminated to a cap without using vias or other lead traces and measure VSWR and reflections from the cap, you will see that the cap works well at 1 GHz and beyond. You must solder the cap to the gnd plane and to the 50 mil trace directly. Regards, George Tang -Original Message- From: Barry Ma [mailto:barry...@altavista.com] Sent: Wednesday, May 17, 2000 9:16 AM To: si-l...@silab.eng.sun.com; EMC-PSTC Cc: wei...@atdial.net Subject: Re: Charge moving from decoupling capacitors Steve, Thanks a lot for the very nice hierarchy description below. If there's a 10 mil or less spacing between pwr and gnd planes, the plane cap is available. The plane cap and the decaps are complementary in whole frequency range. Plane cap takes care of high end, and decaps cover low portion. Then the location of decaps are not critical. And then decaps can be shared by other chips, according to the excellent research conducted by EMC lab at UMR. Please allow me to modify a bit your description as follows. The capacitance inside the device supports the chip first, but usually not enough. Charge from the planes also supports the chip and replenishes the device capacitance, Decaps replenish the plane on low frequency portion, while plane cap responds itself on HF end, Bulk capacitors replenish decaps and plane cap, The voltage regulator replenishes the bulk capacitors. Please correct me. Thanks. Regards, Barry b...@anritsu.com - From: sweir wei...@atdial.net, on 5/11/00 9:28 PM: The capacitance inside the device supports the chip, Charge from the planes replenishes the device capacitance, HF capacitors on the board replenish the planes, Bulk capacitors replenish the HF capacitors, The voltage regulator replenishes the bulk capacitors. [edited by bm] ___ Why pay when you don't have to? Get AltaVista Free Internet Access now! http://jump.altavista.com/freeaccess4.go ___ --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org To unsubscribe from si-list or si-list-digest: send e-mail to majord...@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu
[SI-LIST] : RE: Charge moving from decoupling capacitors
You ask an open question, so the open answer to that is it depends. It depends on how much current the IC is drawing, what frequency and rise time it has, what type of load the IC output drivers are driving, how much noise on the power plane the other ICs on the same board can tolerate . . . . You are looking for a solution that will work in every case, but you will find that there are problems or exceptions to every solution. A capacitor stores charges and supplies current to ICs when needed. By that definition, the charges on the capacitor must move in phase from each other. For parallel plates to behave as a good capacitor, their dimensions should be much smaller than a wavelength so the charges on the plates will move in phase. Large parallel plates behave as transmission lines. A quarter wavelength transmission line with a short at the end has infinite impedance, so capacitors placed 1/4 wavelength away are bad. This means that we can loosely define the largest usable board area capacitance as 1/8 wavelength radius of copper surrounding the IC power pin. Charges stored on the planes further than 1/8 wavelength away are not very usable due to the time delay. At 500MHz in FR4, 1/8 wavelength is 1.5 inches. Is such a board capacitor good enough for your IC? It might be if you have a CMOS IC driving another CMOS IC less than 2 inches away, so the load on the output of the 1st IC is mainly the CMOS gate capacitance at the input of the 2nd IC at the end of the 2 inch transmission line. During switching, the 2nd IC draws current from the output of the 1st IC for the 1st 200 or 300 ps to charge up the input gate capacitance on the 2nd IC. The current is an impulse function, although the voltage waveform is a step function. If these ICs are small and uses little power, the board capacitance might be enough to supply the impulse current for the 1st IC. If the load on the transmission line is a termination resistor, the current draw will be a step function, and the board capacitance alone may not be good enough. But here is an exception. You have a board that uses only CMOS devices, and the largest IC is a 500 MHz processor that consumes 50W of power at 2.5v, so it switches 20A of current at 500 MHz. It is a CMOS device, so its current draws are mostly impulse functions. Would the board capacitance be good enough for this 20A switching current? Probably not. Making the pwr plane larger will not help, but using more layers in parallel will help. You might have to use 4, 8, or 16 pwr/gnd layer pairs in parallel for this board, the more layers the better. But wait!! Isn't that what a multilayer ceramic capacitor is? It has many pwr/gnd layers in parallel . . . . Hmm, if we could only take advantage of that . . . I'm thinking that if you have to use a 50W, 500MHz processor, and your boss tells you that you cannot have 8 pwr/gnd layers on your board, you or someone will probably find a way to make the ceramic capacitor work effectively beyond 1 GHz!! Another question you might ask is that do I really want to dump the 20A switching noise directly into the pwr/gnd planes and create pwr/gnd bounce and board resonance to interfere with all the ICs on the board, not to mention EMI problems? Regards, George Tang -Original Message- From: Barry Ma [mailto:barry...@altavista.com] Sent: Monday, May 15, 2000 1:33 PM To: george_t...@exchange.dell.com; si-l...@silab.eng.sun.com Cc: emc-p...@ieee.org Subject: RE: Charge moving from decoupling capacitors Thanks a lot for your inputs. All responses to my second question are only concerned with the inductance due to “long” distance between chip and decap. Nobody seems to agree imposing another constrain to the distance. My question was “Do we really have to limit the distance letting the charge have enough time to move from the cap to the chip during the rise time interval? I doubt it.” But I really read an article implying this extra concern. George, you wrote: This is true if you have only DC current. For AC, you may have water in the pipe but no water out of the faucet if the faucet is switching out of phase from the water in the pipe. Thank you for reminding me of Frequency Domain analysis. Yes, I should have described and analyzed a transient problem (charge travel during Tr) in both TD and FD, and then correlate the results. Let me have a try this time: It is generally acknowledged that decaps and plane cap are complementary (supposing a 10 mil or less spacing between pwr and gnd planes). Decaps cover low end of frequency range, while the plane cap takes care of high frequencies. Thus the interplane cap would play more and more important role in high-speed PCB design, as the speed gets faster and faster. On the other hand, nobody objects closer distances from decaps to the chip, if possible. . When a chip drains necessary charges from pwr/gnd planes during Tr, decaps would supply charges to pwr and gnd
Re: Charge moving from decoupling capacitors
Hi Yu, Lets begin with the excellent description written by Andrew Ingraham: The voltage sag propagates outward from the chip, consuming charge stored in the intrinsic capacitance of the planes bit by bit (not all of it at once!), and eventually reaching external capacitors which help hold up the voltage. When a chip gate opens, the V (and Q) sag on the metal plane propagates outward from the chip at circle front, consuming charges inside the circle. If the outgoing circle front meets a decap, charge would be moving from the decap to the chip. ... What if the circle does not reach any decap before the end of Tr? The gate has closed, no current loop can be formed. That is the scenario I was interested in. Now an electrical potential imbalance happens in a metallic plane. Charge on the plane would flow toward the circle for regaining eqi-potential. ... Regards, Barry Ma ANRITSU company Morgan Hill, CA 95037 - From: Yu Wang wangy...@yahoo.com, on 5/17/00 7:45 AM: Hi, Barry, I think we all do agree the 1st point you said. But on the 2nd point, I have another opinion. Say, you are right,When an electrical potential imbalance happens in a metallic plane, a current would flow on the plane for regaining the equi-potential. But we regard the metallic plane as a reference plane, then, theoretically, there is no an electrical potential imbalance happens in a metallic plane. In fact, we can never get a ideal reference plane(even the earth is not IDEAL). Based on this, I would say yes on your question Does it need EM field support from the dielectric?. Still but, the need of the support is usually slight if the electical size of the metallic plane is big enough. because we know the metallic plane is a equi-potential plane. regards, Yu Wang, Ph.D U.T. MD Anderson Cancer Center 1100 Holcombe Blvd., Box 217 Houston, TX, 77030 Tel:713-745-1671 ___ Why pay when you don't have to? Get AltaVista Free Internet Access now! http://jump.altavista.com/freeaccess4.go ___ --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org
Re: Charge moving from decoupling capacitors
Steve, Thanks a lot for the very nice hierarchy description below. If there's a 10 mil or less spacing between pwr and gnd planes, the plane cap is available. The plane cap and the decaps are complementary in whole frequency range. Plane cap takes care of high end, and decaps cover low portion. Then the location of decaps are not critical. And then decaps can be shared by other chips, according to the excellent research conducted by EMC lab at UMR. Please allow me to modify a bit your description as follows. The capacitance inside the device supports the chip first, but usually not enough. Charge from the planes also supports the chip and replenishes the device capacitance, Decaps replenish the plane on low frequency portion, while plane cap responds itself on HF end, Bulk capacitors replenish decaps and plane cap, The voltage regulator replenishes the bulk capacitors. Please correct me. Thanks. Regards, Barry b...@anritsu.com - From: sweir wei...@atdial.net, on 5/11/00 9:28 PM: The capacitance inside the device supports the chip, Charge from the planes replenishes the device capacitance, HF capacitors on the board replenish the planes, Bulk capacitors replenish the HF capacitors, The voltage regulator replenishes the bulk capacitors. [edited by bm] ___ Why pay when you don't have to? Get AltaVista Free Internet Access now! http://jump.altavista.com/freeaccess4.go ___ --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org
RE: Charge moving from decoupling capacitors
Barry, You're hitting upon one of the basic fundamentals of how radiated emissions occur. The situation that Andrew is describing occurs if we design our boards properly, with a low inductance return path for each current and with proper impedance matching; then our traces will act more like waveguides and transmissions lines where the charge flows in the conductors and the corresponding field stays mostly contained in the dielectric. The other situation that you're describing is the situation where there is no loop for return current. In this situation, the trace is acting more like an antenna which is more or less a conductor with a time varying potential at one end which causes charge to accelerate back and forth within the conductor. Since there is no readily available return path, the current is more of a common mode current which will find it's own return path by radiating a field (displacement current?) to another conductor. The problem frequencies will be determined by the frequency at which the charge accelerates (changes speed or direction) and by the distance that the charge accelerates (length of antenna). This is the main mechanism for the radiated emissions that we get paid to minimize. As for the velocity. The equations for a stripline may be used (too complex for me to write using an email editor). One good reference is pages 407-411 of Fields and Waves in Communication Electronics by Ramo, Whinnery and Van Duzer, published by Wiley. In short, you can only use the velocity of propogation from the dielectric if you have a perfect stripline (strip conductor sandwiched in dielectric between two very wide conductive planes). Correction factors need to be applied in other situations. Thanks for bringing up such a challenging topic. It helps keep the juices flowing. Chris Maxwell, Design Engineer GN Nettest Optical Division 109 N. Genesee St. Utica, NY 13502 PH: 315-797-4449 FAX: 315-797-8024 EMAIL: chr...@gnlp.com -Original Message- From: Barry Ma [SMTP:barry...@altavista.com] Sent: Tuesday, May 16, 2000 4:29 PM To: EMC-PSTC; si-l...@silab.eng.sun.com Subject: Re: Charge moving from decoupling capacitors Hi Andrew, You said: It is just like an ordinary transmission line such as stripline. Please allow me to say something different. (1) When a signal propagates along a transmission line, we could observe a current loop from source to load through the transmission line. The signal velocity is the same as the speed of light in the dielectric. You are right. (2) When an electrical potential imbalance happens in a metallic plane, a current would flow on the plane for regaining the equi-potential. This current looks different from the signal current. There's no current loop here. Does it need EM field support from the dielectric? If not, should it have a different velocity? That is my point. I have no answer, and appreciate any input. Thanks. Bets Regards, Barry Ma b...@anritsu.com Barry Ma wrote: As the speed of digital signals gets faster and faster, people begin being concerned with the distance for electric charge to move on power and ground planes of multilayer PCB during the signal rise time from a decoupling capacitor (cap) to a chip it serves. I would like to raise two questions. (1) The charge is moving in a metalic plane, not inside the dielectric between pwr and gnd planes. Please let me know why you have to use the propagation velocity in the dielectric, instead of that in the metal. -- Ingraham, Andrew wrote: The charge may be moving in the metal, but the energy (which makes the charge keep moving) is primarily in the electro-magnetic field between the planes, in the dielectric. The charge won't move unless there is an E-M field to push it. It is just like an ordinary transmission line such as stripline. The propagation velocity of a trace is that of the dielectric, even though the charge moves only in the metal trace and planes. (Edited by BM) ___ Why pay when you don't have to? Get AltaVista Free Internet Access now! http://jump.altavista.com/freeaccess4.go ___ --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org --- This message is from the IEEE EMC Society Product Safety Technical Committee
Re: Charge moving from decoupling capacitors
Hi Andrew, You said: It is just like an ordinary transmission line such as stripline. Please allow me to say something different. (1) When a signal propagates along a transmission line, we could observe a current loop from source to load through the transmission line. The signal velocity is the same as the speed of light in the dielectric. You are right. (2) When an electrical potential imbalance happens in a metallic plane, a current would flow on the plane for regaining the equi-potential. This current looks different from the signal current. There's no current loop here. Does it need EM field support from the dielectric? If not, should it have a different velocity? That is my point. I have no answer, and appreciate any input. Thanks. Bets Regards, Barry Ma b...@anritsu.com Barry Ma wrote: As the speed of digital signals gets faster and faster, people begin being concerned with the distance for electric charge to move on power and ground planes of multilayer PCB during the signal rise time from a decoupling capacitor (cap) to a chip it serves. I would like to raise two questions. (1) The charge is moving in a metalic plane, not inside the dielectric between pwr and gnd planes. Please let me know why you have to use the propagation velocity in the dielectric, instead of that in the metal. -- Ingraham, Andrew wrote: The charge may be moving in the metal, but the energy (which makes the charge keep moving) is primarily in the electro-magnetic field between the planes, in the dielectric. The charge won't move unless there is an E-M field to push it. It is just like an ordinary transmission line such as stripline. The propagation velocity of a trace is that of the dielectric, even though the charge moves only in the metal trace and planes. (Edited by BM) ___ Why pay when you don't have to? Get AltaVista Free Internet Access now! http://jump.altavista.com/freeaccess4.go ___ --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org
RE: Charge moving from decoupling capacitors
Thanks a lot for your inputs. All responses to my second question are only concerned with the inductance due to long distance between chip and decap. Nobody seems to agree imposing another constrain to the distance. My question was Do we really have to limit the distance letting the charge have enough time to move from the cap to the chip during the rise time interval? I doubt it. But I really read an article implying this extra concern. George, you wrote: This is true if you have only DC current. For AC, you may have water in the pipe but no water out of the faucet if the faucet is switching out of phase from the water in the pipe. Thank you for reminding me of Frequency Domain analysis. Yes, I should have described and analyzed a transient problem (charge travel during Tr) in both TD and FD, and then correlate the results. Let me have a try this time: It is generally acknowledged that decaps and plane cap are complementary (supposing a 10 mil or less spacing between pwr and gnd planes). Decaps cover low end of frequency range, while the plane cap takes care of high frequencies. Thus the interplane cap would play more and more important role in high-speed PCB design, as the speed gets faster and faster. On the other hand, nobody objects closer distances from decaps to the chip, if possible. . When a chip drains necessary charges from pwr/gnd planes during Tr, decaps would supply charges to pwr and gnd planes on lower frequencies, while interplane cap can respond itself on higher frequencies. Best Regards, Barry Ma b...@anritsu.com ___ Why pay when you don't have to? Get AltaVista Free Internet Access now! http://jump.altavista.com/freeaccess4.go ___ --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org
Re:RE: [SI-LIST] : Charge moving from decoupling capacitors
forwarding for Francis Reply Separator Subject:RE: [SI-LIST] : Charge moving from decoupling capacitors Author: Kai; Francis francis@intel.com List-Post: emc-pstc@listserv.ieee.org Date: 5/12/00 9:02 AM Dear Electrical Engineers, Based on Mr. Ma's questions, I would like to know if you guys are really interested in the electron movements in metal. The transport of electrons in a solid is dominated by Boltzmann's Equation. How do you solve this equation for an electron to move from the decoupling capacitor to a chip (package and...) it serves in a metal plane? This might be something that the tools vendors are interested in for the next generation high-speed tools. Kind Regards, Francis Kai Intel Corporation -Original Message- From: Barry Ma [mailto:barry...@altavista.com] Sent: Thursday, May 11, 2000 3:50 PM To: EMC-PSTC; si-l...@silab.eng.sun.com Subject: [SI-LIST] : Charge moving from decoupling capacitors Hi, As the speed of digital signals gets faster and faster, people begin being concerned with the distance for electric charge to move on power and ground planes of multilayer PCB during the signal rise time from a decoupling capacitor (cap) to a chip it serves. I would like to raise two questions. (1) The charge is moving in a metalic plane, not inside the dielectric between pwr and gnd planes. Please let me know why you have to use the propagation velocity in the dielectric, instead of that in the metal. (2) The second question is regarding distance between the cap and the chip. Do we really have to limit the distance letting the charge have enough time to move from the cap to the chip during the rise time interval? I doubt it. Take the running water system for example. When we open, then close the water faucet within one second, does the water we've got in basin come from water tower (or water station, or reservoir)? No, it is the water that resides in the pipe. As a matter of fact, we have a very large pipe - pwr/gnd planes. Well, of cause you know, I did not mean we don't need water tower - the cap. .. Regards, Barry Ma b...@anritsu.com Morgan Hill, CA 95037 Tel. 408-778-2000 ___ Why pay when you don't have to? Get AltaVista Free Internet Access now! http://jump.altavista.com/freeaccess4.go ___ To unsubscribe from si-list or si-list-digest: send e-mail to majord...@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org
Re: Re[2]: [SI-LIST] : Charge moving from decoupling capacitors
Equally important is that respect has to be paid to impedance matching ; other wise, energy is reflected from the load and this will degrade the risetime as well as cause some energy to be radiated. The dielectric material must be low loss so that you ensure the signal is conducted with minimum attenuation.and keeping the path length to the chip to a minimum keeps the undesired switching products very low. Ralph Cameron EMC Consultant for Suppression of Consumer Electronics (after Sale). - Original Message - From: Jim Bacher jim_bac...@mail.monarch.com To: Larry Miller ldmil...@nortelnetworks.com; EMC-PSTC emc-p...@ieee.org Sent: Friday, May 12, 2000 11:24 AM Subject: Re[2]: [SI-LIST] : Charge moving from decoupling capacitors Forwarded for Larry. Reply Separator Subject:Re: [SI-LIST] : Charge moving from decoupling capacitors Author: Larry Miller ldmil...@nortelnetworks.com Date: 5/12/00 7:37 AM Hi, Barry, Responses below: At 03:49 PM 5/11/00 -0700, Barry Ma wrote: Hi, As the speed of digital signals gets faster and faster, people begin being concerned with the distance for electric charge to move on power and ground planes of multilayer PCB during the signal rise time from a decoupling capacitor (cap) to a chip it serves. I would like to raise two questions. (1) The charge is moving in a metalic plane, not inside the dielectric between pwr and gnd planes. Please let me know why you have to use the propagation velocity in the dielectric, instead of that in the metal. Due to skin effect at anything above a few tens of kHz, the current flows mainly on the surface of the trace, so it has to interact with the dielectric in accordance with Maxwell's equations for electromagnetic waves. (2) The second question is regarding distance between the cap and the chip. Do we really have to limit the distance letting the charge have enough time to move from the cap to the chip during the rise time interval? I doubt it. No, you have to let the wave propagate (see below). Take the running water system for example. When we open, then close the water faucet within one second, does the water we've got in basin come from water tower (or water station, or reservoir)? No, it is the water that resides in the pipe. As a matter of fact, we have a very large pipe - pwr/gnd planes. Well, of cause you know, I did not mean we don't need water tower - the cap. .. The running water analogy breaks down here (at AC). Another analogy would be that the current is comprised of many successive collisions between billiard balls, not the motion of one single ball. Yet another analogy would be to look at the current as a game of Chinese checkers, where an individual electron can only move into a hole in the board vacated by another electron (if you overcome this you get superconduction!). As I recollect, the actual speed of an identifiable electron, assuming you could actually identify it, is on the order of a few meters/sec, though the electromagnetic wave caused by the transfer of energy between electrons travels at the speed of light in the medium. Larry Miller --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org
RE: [SI-LIST] : Charge moving from decoupling capacitors
As the speed of digital signals gets faster and faster, people begin being concerned with the distance for electric charge to move on power and ground planes of multilayer PCB during the signal rise time from a decoupling capacitor (cap) to a chip it serves. I would like to raise two questions. (1) The charge is moving in a metalic plane, not inside the dielectric between pwr and gnd planes. Please let me know why you have to use the propagation velocity in the dielectric, instead of that in the metal. The charge may be moving in the metal, but the energy (which makes the charge keep moving) is primarily in the electro-magnetic field between the planes, in the dielectric. The charge won't move unless there is an E-M field to push it. It is just like an ordinary transmission line such as stripline. The propagation velocity of a trace is that of the dielectric, even though the charge moves only in the metal trace and planes. (2) The second question is regarding distance between the cap and the chip. Do we really have to limit the distance letting the charge have enough time to move from the cap to the chip during the rise time interval? I doubt it. That depends whether you need the capacitor to help during the rise time interval itself. If you had a single 1.0 Farad cap and attached it with 20 foot long jumper cables to your chip, it would do nothing to help the chip during the rise time interval. The jumper cable is a transmission line. If the voltage sags at your chip, it takes many nanoseconds for the sag to reach the capacitor. Until the sag reaches it, charge doesn't even start moving out of it, i.e., the cap might as well not be there. Now flatten the jumper cable into two planes. The planes are a fat transmission line (really!). The voltage sag propagates outward from the chip, consuming charge stored in the intrinsic capacitance of the planes bit by bit (not all of it at once!), and eventually reaching external capacitors which help hold up the voltage. Take the running water system for example. When we open, then close the water faucet within one second, does the water we've got in basin come from water tower (or water station, or reservoir)? No, it is the water that resides in the pipe. As a matter of fact, we have a very large pipe - pwr/gnd planes. Well, of cause you know, I did not mean we don't need water tower - the cap. .. Pwr/gnd planes are similar to a long pipe. At first they help hold up the pressure, but without a tank, the pressure would disappear. Even with a tank, the pressure does drop a little when you open the faucet. Open a very large faucet, and the pressure immediately drops a lot ... especially if you are the house at the end of the water main. Open a very large faucet right at the tank or pumping station, and the drop in pressure is much less. (This analogy is not very good, however, because water pipes also have resistance.) Regards, Andy To unsubscribe from si-list or si-list-digest: send e-mail to majord...@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu
RE: [SI-LIST] : Charge moving from decoupling capacitors
Dear Electrical Engineers, Based on Mr. Ma's questions, I would like to know if you guys are really interested in the electron movements in metal. The transport of electrons in a solid is dominated by Boltzmann's Equation. How do you solve this equation for an electron to move from the decoupling capacitor to a chip (package and...) it serves in a metal plane? This might be something that the tools vendors are interested in for the next generation high-speed tools. Kind Regards, Francis Kai Intel Corporation -Original Message- From: Barry Ma [mailto:barry...@altavista.com] Sent: Thursday, May 11, 2000 3:50 PM To: EMC-PSTC; si-l...@silab.eng.sun.com Subject: [SI-LIST] : Charge moving from decoupling capacitors Hi, As the speed of digital signals gets faster and faster, people begin being concerned with the distance for electric charge to move on power and ground planes of multilayer PCB during the signal rise time from a decoupling capacitor (cap) to a chip it serves. I would like to raise two questions. (1) The charge is moving in a metalic plane, not inside the dielectric between pwr and gnd planes. Please let me know why you have to use the propagation velocity in the dielectric, instead of that in the metal. (2) The second question is regarding distance between the cap and the chip. Do we really have to limit the distance letting the charge have enough time to move from the cap to the chip during the rise time interval? I doubt it. Take the running water system for example. When we open, then close the water faucet within one second, does the water we've got in basin come from water tower (or water station, or reservoir)? No, it is the water that resides in the pipe. As a matter of fact, we have a very large pipe - pwr/gnd planes. Well, of cause you know, I did not mean we don't need water tower - the cap. .. Regards, Barry Ma b...@anritsu.com Morgan Hill, CA 95037 Tel. 408-778-2000 ___ Why pay when you don't have to? Get AltaVista Free Internet Access now! http://jump.altavista.com/freeaccess4.go ___ To unsubscribe from si-list or si-list-digest: send e-mail to majord...@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu To unsubscribe from si-list or si-list-digest: send e-mail to majord...@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu
Re[2]: [SI-LIST] : Charge moving from decoupling capacitors
Forwarded for Larry. Reply Separator Subject:Re: [SI-LIST] : Charge moving from decoupling capacitors Author: Larry Miller ldmil...@nortelnetworks.com List-Post: emc-pstc@listserv.ieee.org Date: 5/12/00 7:37 AM Hi, Barry, Responses below: At 03:49 PM 5/11/00 -0700, Barry Ma wrote: Hi, As the speed of digital signals gets faster and faster, people begin being concerned with the distance for electric charge to move on power and ground planes of multilayer PCB during the signal rise time from a decoupling capacitor (cap) to a chip it serves. I would like to raise two questions. (1) The charge is moving in a metalic plane, not inside the dielectric between pwr and gnd planes. Please let me know why you have to use the propagation velocity in the dielectric, instead of that in the metal. Due to skin effect at anything above a few tens of kHz, the current flows mainly on the surface of the trace, so it has to interact with the dielectric in accordance with Maxwell's equations for electromagnetic waves. (2) The second question is regarding distance between the cap and the chip. Do we really have to limit the distance letting the charge have enough time to move from the cap to the chip during the rise time interval? I doubt it. No, you have to let the wave propagate (see below). Take the running water system for example. When we open, then close the water faucet within one second, does the water we've got in basin come from water tower (or water station, or reservoir)? No, it is the water that resides in the pipe. As a matter of fact, we have a very large pipe - pwr/gnd planes. Well, of cause you know, I did not mean we don't need water tower - the cap. .. The running water analogy breaks down here (at AC). Another analogy would be that the current is comprised of many successive collisions between billiard balls, not the motion of one single ball. Yet another analogy would be to look at the current as a game of Chinese checkers, where an individual electron can only move into a hole in the board vacated by another electron (if you overcome this you get superconduction!). As I recollect, the actual speed of an identifiable electron, assuming you could actually identify it, is on the order of a few meters/sec, though the electromagnetic wave caused by the transfer of energy between electrons travels at the speed of light in the medium. Larry Miller --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org
Re: Charge moving from decoupling capacitors
Barry Ma wrote: Hi, As the speed of digital signals gets faster and faster, people begin being concerned with the distance for electric charge to move on power and ground planes of multilayer PCB during the signal rise time from a decoupling capacitor (cap) to a chip it serves. I would like to raise two questions. (1) The charge is moving in a metalic plane, not inside the dielectric between pwr and gnd planes. Please let me know why you have to use the propagation velocity in the dielectric, instead of that in the metal. Simple. The power/ground plane construction is a 2-dimensional transmission line. Just as you determine the velocity of propagation in a 1-dimensional transmission line, the dielectric is part of the equation for the capacitive component. And so it follows with the planar construction. (2) The second question is regarding distance between the cap and the chip. Do we really have to limit the distance letting the charge have enough time to move from the cap to the chip during the rise time interval? I doubt it. Depends. Distance can mean inductance. Since, v(t) = L di/dt and with L in nano-henries (10^-9) and suppose dt in pico-seconds (10^-12), we're already up in the 10^3 range ... - Doug McKean --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org
Charge moving from decoupling capacitors
Hi, As the speed of digital signals gets faster and faster, people begin being concerned with the distance for electric charge to move on power and ground planes of multilayer PCB during the signal rise time from a decoupling capacitor (cap) to a chip it serves. I would like to raise two questions. (1) The charge is moving in a metalic plane, not inside the dielectric between pwr and gnd planes. Please let me know why you have to use the propagation velocity in the dielectric, instead of that in the metal. (2) The second question is regarding distance between the cap and the chip. Do we really have to limit the distance letting the charge have enough time to move from the cap to the chip during the rise time interval? I doubt it. Take the running water system for example. When we open, then close the water faucet within one second, does the water we've got in basin come from water tower (or water station, or reservoir)? No, it is the water that resides in the pipe. As a matter of fact, we have a very large pipe - pwr/gnd planes. Well, of cause you know, I did not mean we don't need water tower - the cap. .. Regards, Barry Ma b...@anritsu.com Morgan Hill, CA 95037 Tel. 408-778-2000 ___ Why pay when you don't have to? Get AltaVista Free Internet Access now! http://jump.altavista.com/freeaccess4.go ___ --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org
