RE: Q on Correlation of Votage ripple with a Spectrum Analyser
Charles, are you using the same probe for both measurements? I have found that probing a Vcc plane accurately requires a probe with decent common mode rejection. About the best is to solder a coaxial cable in place. Dave Cuthbert Micron Technology From: Charles Grasso [mailto:cgrassospri...@earthlink.net] Sent: Saturday, July 26, 2003 7:11 PM To: Ken Javor; Emc-Pstc Subject: RE: Q on Correlation of Votage ripple with a Spectrum Analyser Hi all, Actually I was using a good ole Spectrim Analyser so I sidestepped the windowing issue/software issues altogether. What I was(am)trying to do was match the max voltage as measured on a scope with the value as measured on a SA. I first calibrated myslef using a known source - a sine wave. The amplitudes fell in just as theory predicted. Encouraged, I then probed the Vcc plane on a product I was working on and was not so happy!! Any ideas? From: Ken Javor [mailto:ken.ja...@emccompliance.com] Sent: Saturday, July 26, 2003 4:19 PM To: Charles Grasso; Emc-Pstc Subject: Re: Q on Correlation of Votage ripple with a Spectrum Analyser I presented a paper on that very subject about a decade ago at one of the EMC TD magazine EMC symposia. I used a Fluke Scopemeter and some FFT software that came with it. The Fluke interfaced to the PC through an optically isolated RS-232 protocol. It worked quite well from a pre-compliance or troubleshooting point-of-view. You could use time windowing to separate the signals deriving from leading and falling edges from the signals deriving from the pulse itself. I used LISNMATE and LISNMARK mode separation devices to show that the rising/falling edge signals were common mode, while the pulse itself generated differential mode signals. From: Charles Grasso cgrassospri...@earthlink.net Reply-To: Charles Grasso cgrassospri...@earthlink.net Date: Sat, 26 Jul 2003 16:18:36 -0700 To: Emc-Pstc emc-p...@ieee.org Subject: Q on Correlation of Votage ripple with a Spectrum Analyser Hi All, Has anyone tried correlating the voltage ripple as seen on a scope with the amplitudes measured on a Spectrum Analyser? I tried doing that the other day with ..umm. minimal success. I think that due to the comples convoltions that would have to occur when FFT'ing an irregular voltage shape. Charles Grasso Echostar Communications. --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ 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: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ 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: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ 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: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
Re: Q on Correlation of Votage ripple with a Spectrum Analyser
Mr. Novak makes some excellent points. I was under the assumption that the phrase voltage ripple implied conducted emission measurements at a LISN port. Hence my comments on mode separation. Across a spectrum of even 30 MHz, any normal scope probe I know of (1 or 10 M Ohm in parallel with 5/10/20 pF) will present a varying load to the measured waveform. I know there are some broadband active differential probes with low shunt capacity, but I have no experience using them. The scope should be made to look like a flat 50 Ohm load to help correlate to a spectrum analyzer. Instead of using long leads to make measurements from one point of the Vcc plane to another, I would make coaxial measurements from one point on the Vcc plane to chassis ground or the reference/image plane if one exists, and then make the same measurement at another point on the Vcc plane, and compare the waveforms, perhaps using a built-in math function if your scope has that. Of course this technique requires the reference/image plane/chassis ground to be an equipotential plane... By coaxial measurement I mean using coax rather than a scope probe, and terminating the shield at the reference point, while extending the center conductor just far enough to connect to the point of interest on the Vcc plane. Theoretically, an even better technique would be to have a place on the board where the power plane reference was available circumferentially around a Vcc via, and connect the coax shield to the reference plane and the center conductor to the Vcc contact. Given a 50 Ohm load at the other end (with a blocking cap to protect it), and an FFT capability with enough memory, I believe you could achieve correlation with the individual spectral components measured by a spectrum analyzer (which of course would also need a blocking cap with this config). From: istvan novak istvan.no...@worldnet.att.net Date: Sun, 27 Jul 2003 11:02:02 -0400 To: Charles Grasso cgrassospri...@earthlink.net, Ken Javor ken.ja...@emccompliance.com, Emc-Pstc emc-p...@ieee.org Subject: Re: Q on Correlation of Votage ripple with a Spectrum Analyser Charles, Doing this kind of correlation is very difficult for the following reasons: - unless you measure a very simple and dummy system, hardware today is so complex that you cant predict for sure its activity; it is a strong function of time. -tThe spectrum analyzer and scope will look at the same signal in different ways: analog spectrum analyzers have a seep time and settling time determining the frequency and aperture of visit each frequency. If you have a spectrumn analyzer used for compliance tests, probably the CISPR filter is on. Scopes on the other hand (digital scopes) undersample the signal, whether it is called real-time or not. Memory and displey refresh rate does not allow scopes to display and process all data points of high-frequency signals. Real-time scopes do it for a given time window, but it is usually way less than the time constant of a CISPR filter on the spectrum analyzer. - connection to the source makes a big difference. I assume when you calibrated the reading with a sine wave, a coaxial cable with coax connectors at both ends was used. Presumably the product does not have a coaxial connector on the Vcc plane, so you have to make your own connection or use a hand-held probe. This is very extra noise usually gets in the path, and the scope reading becomes unrealistically high. I have found no active scope probes so far, which would give a correct reading in a noisy environment. We hopefully should not see noise on the Vcc planes more than a few hundred mV. In contrast, many scope probes can pick up spikes as big as volts from the environment. If you want to measure noise levels below 100mVpp, double-shielded coax is necessary in noisy environments. Here the 'noisy environment' refers to the close vicinity of the point you test. The simplest test is: take your present probe, and hook up a good double-shielded coax to the same points. Check both readings on the same scope at the same time, and compare. I hope this helps. Best regards, Istvan Novak SUN Microsystems - Original Message - From: Charles Grasso cgrassospri...@earthlink.net To: Ken Javor ken.ja...@emccompliance.com; Emc-Pstc emc-p...@ieee.org Sent: Saturday, July 26, 2003 9:11 PM Subject: RE: Q on Correlation of Votage ripple with a Spectrum Analyser Hi all, Actually I was using a good ole Spectrim Analyser so I sidestepped the windowing issue/software issues altogether. What I was(am)trying to do was match the max voltage as measured on a scope with the value as measured on a SA. I first calibrated myslef using a known source - a sine wave. The amplitudes fell in just as theory predicted. Encouraged, I then probed the Vcc plane on a product I was working on and was not so happy!! Any ideas? -Original Message- From:
Re: Q on Correlation of Votage ripple with a Spectrum Analyser
Zero span can show rep rate or modulation but it cannot correlate the MHz bandwidth waveform amplitude the scope sees with the amplitude of any particular spectral component to which the spectrum analyzer is tuned. Further, the spectrum analyzer is a much more sensitive device than the scope, and it typically has about 80 dB of on screen dynamic range, whereas an 8 bit oscilloscope has 48 dB of dynamic range. From: Cortland Richmond 72146@compuserve.com Reply-To: Cortland Richmond 72146@compuserve.com Date: Sat, 26 Jul 2003 20:45:18 -0400 To: Charles Grasso cgrassospri...@earthlink.net, ieee pstc list emc-p...@ieee.org Subject: RE: Q on Correlation of Votage ripple with a Spectrum Analyser Charles Grasso wrote: What I was(am)trying to do was match the max voltage as measured on a scope with the value as measured on a SA. Try zero span on the SA. Compare THAT with the scope. Cortland --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ 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: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ 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: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
Re: Q on Correlation of Votage ripple with a Spectrum Analyser
As I mentioned earlier, the waveform from a switching power supply has two distinct components, due to the fast rise-time driving current into ground (common mode) and the pulse itself which is differential mode. Separating modes allows you to time window properly to really resolve the waveform to be FFT'd. In my experience, that makes a world of difference. To specify further, I had maybe 32 k of memory. If you have infinite memory, you can FFT a 10 us pulse with a sub-microsecond rise-time, but with limited memory, you get better results doing a separate FFT on the rise-time and then the pulse. From: John Woodgate j...@jmwa.demon.co.uk Reply-To: John Woodgate j...@jmwa.demon.co.uk Date: Sun, 27 Jul 2003 07:46:29 +0100 To: emc-p...@majordomo.ieee.org Subject: Re: Q on Correlation of Votage ripple with a Spectrum Analyser I read in !emc-pstc that Charles Grasso cgrassospri...@earthlink.net wrote (in ekeeipjkkmpklafoobmcoelhcfaa.cgrassospri...@earthlink.net) about 'Q on Correlation of Votage ripple with a Spectrum Analyser' on Sat, 26 Jul 2003: Has anyone tried correlating the voltage ripple as seen on a scope with the amplitudes measured on a Spectrum Analyser? More information, please. What voltage is it that has ripple on it? Do you mean the ripple voltage across a rectifier filter capacitor? I tried doing that the other day with ..umm. minimal success. I think that due to the comples convoltions that would have to occur when FFT'ing an irregular voltage shape. It's really no more difficult to FFT one waveform than another. The FFT doesn't 'know' whether the waveform looks regular or irregular to a human. If it is power supply ripple that you are considering, the waveform is approximately a sawtooth, but the short rising branch is a small section of the crest of a sine wave. If the ripple voltage is very small compared with the d.c. voltage and the ESR of the filter capacitor is very low, the spectrum of the ripple is often quite close to that of a sawtooth. -- Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk Interested in professional sound reinforcement and distribution? Then go to http://www.isce.org.uk PLEASE do NOT copy news posts to me by E-MAIL! --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ 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: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ 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: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
Re: Q on Correlation of Votage ripple with a Spectrum Analyser
Charles, Doing this kind of correlation is very difficult for the following reasons: - unless you measure a very simple and dummy system, hardware today is so complex that you cant predict for sure its activity; it is a strong function of time. -tThe spectrum analyzer and scope will look at the same signal in different ways: analog spectrum analyzers have a seep time and settling time determining the frequency and aperture of visit each frequency. If you have a spectrumn analyzer used for compliance tests, probably the CISPR filter is on. Scopes on the other hand (digital scopes) undersample the signal, whether it is called real-time or not. Memory and displey refresh rate does not allow scopes to display and process all data points of high-frequency signals. Real-time scopes do it for a given time window, but it is usually way less than the time constant of a CISPR filter on the spectrum analyzer. - connection to the source makes a big difference. I assume when you calibrated the reading with a sine wave, a coaxial cable with coax connectors at both ends was used. Presumably the product does not have a coaxial connector on the Vcc plane, so you have to make your own connection or use a hand-held probe. This is very extra noise usually gets in the path, and the scope reading becomes unrealistically high. I have found no active scope probes so far, which would give a correct reading in a noisy environment. We hopefully should not see noise on the Vcc planes more than a few hundred mV. In contrast, many scope probes can pick up spikes as big as volts from the environment. If you want to measure noise levels below 100mVpp, double-shielded coax is necessary in noisy environments. Here the 'noisy environment' refers to the close vicinity of the point you test. The simplest test is: take your present probe, and hook up a good double-shielded coax to the same points. Check both readings on the same scope at the same time, and compare. I hope this helps. Best regards, Istvan Novak SUN Microsystems From: Charles Grasso cgrassospri...@earthlink.net To: Ken Javor ken.ja...@emccompliance.com; Emc-Pstc emc-p...@ieee.org Sent: Saturday, July 26, 2003 9:11 PM Subject: RE: Q on Correlation of Votage ripple with a Spectrum Analyser Hi all, Actually I was using a good ole Spectrim Analyser so I sidestepped the windowing issue/software issues altogether. What I was(am)trying to do was match the max voltage as measured on a scope with the value as measured on a SA. I first calibrated myslef using a known source - a sine wave. The amplitudes fell in just as theory predicted. Encouraged, I then probed the Vcc plane on a product I was working on and was not so happy!! Any ideas? -Original Message- From: Ken Javor [mailto:ken.ja...@emccompliance.com] Sent: Saturday, July 26, 2003 4:19 PM To: Charles Grasso; Emc-Pstc Subject: Re: Q on Correlation of Votage ripple with a Spectrum Analyser I presented a paper on that very subject about a decade ago at one of the EMC TD magazine EMC symposia. I used a Fluke Scopemeter and some FFT software that came with it. The Fluke interfaced to the PC through an optically isolated RS-232 protocol. It worked quite well from a pre-compliance or troubleshooting point-of-view. You could use time windowing to separate the signals deriving from leading and falling edges from the signals deriving from the pulse itself. I used LISNMATE and LISNMARK mode separation devices to show that the rising/falling edge signals were common mode, while the pulse itself generated differential mode signals. From: Charles Grasso cgrassospri...@earthlink.net Reply-To: Charles Grasso cgrassospri...@earthlink.net Date: Sat, 26 Jul 2003 16:18:36 -0700 To: Emc-Pstc emc-p...@ieee.org Subject: Q on Correlation of Votage ripple with a Spectrum Analyser Hi All, Has anyone tried correlating the voltage ripple as seen on a scope with the amplitudes measured on a Spectrum Analyser? I tried doing that the other day with ..umm. minimal success. I think that due to the comples convoltions that would have to occur when FFT'ing an irregular voltage shape. Charles Grasso Echostar Communications. --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ 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: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web
Re: Q on Correlation of Votage ripple with a Spectrum Analyser
I read in !emc-pstc that Charles Grasso cgrassospri...@earthlink.net wrote (in ekeeipjkkmpklafoobmcaelkcfaa.cgrassospri...@earthlink.net) about 'Q on Correlation of Votage ripple with a Spectrum Analyser' on Sat, 26 Jul 2003: I first calibrated myslef using a known source - a sine wave. Really? You calibrated yourself? The amplitudes fell in just as theory predicted. Encouraged, I then probed the Vcc plane on a product I was working on and was not so happy!! You really must write more clearly, and give much more information, if you want useful answers. What did you find that made you unhappy? Maybe we can tell you where it came from. -- Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk Interested in professional sound reinforcement and distribution? Then go to http://www.isce.org.uk PLEASE do NOT copy news posts to me by E-MAIL! This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ 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: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
Re: Q on Correlation of Votage ripple with a Spectrum Analyser
I read in !emc-pstc that Charles Grasso cgrassospri...@earthlink.net wrote (in ekeeipjkkmpklafoobmcoelhcfaa.cgrassospri...@earthlink.net) about 'Q on Correlation of Votage ripple with a Spectrum Analyser' on Sat, 26 Jul 2003: Has anyone tried correlating the voltage ripple as seen on a scope with the amplitudes measured on a Spectrum Analyser? More information, please. What voltage is it that has ripple on it? Do you mean the ripple voltage across a rectifier filter capacitor? I tried doing that the other day with ..umm. minimal success. I think that due to the comples convoltions that would have to occur when FFT'ing an irregular voltage shape. It's really no more difficult to FFT one waveform than another. The FFT doesn't 'know' whether the waveform looks regular or irregular to a human. If it is power supply ripple that you are considering, the waveform is approximately a sawtooth, but the short rising branch is a small section of the crest of a sine wave. If the ripple voltage is very small compared with the d.c. voltage and the ESR of the filter capacitor is very low, the spectrum of the ripple is often quite close to that of a sawtooth. -- Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk Interested in professional sound reinforcement and distribution? Then go to http://www.isce.org.uk PLEASE do NOT copy news posts to me by E-MAIL! This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ 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: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
Re: Q on Correlation of Votage ripple with a Spectrum Analyser
Charles, Instead of using the 1GHz single-ended scope probe, have you tried to connect the same coax cable that you made with the seriers 50 ohms for the SA to connect to the scope? With a 50-ohm input impedance setting on the scope, the loading of the planes would be exactly the same. As I mentioned earlier, we checked many scope probes, from Tektronix, Agilent, LeCroy, including some of the new 4-6GHz bandwidth single-ended and differential probes. They all pick up more or less noise from the environment, and not necessarily through the input connection pins or wires. You can take your scope probe, with its input pins open or shorted, and take the infinite persistance scope reading, while you move the probe close to a high-power, high-speed computer board (no connection, just put the probe an inch close). Or, just simply take a desktop light with a transformer in its base, and flip the switch a few times. You will be amased how much noise these probes can pick up through the unshielded (or poorly shielded) front-end amplifier and through the cable connecting to the scope. Best regards, Istvan From: Charles Grasso cgrassospri...@earthlink.net To: Ken Javor ken.ja...@emccompliance.com; istvan novak istvan.no...@worldnet.att.net; Emc-Pstc emc-p...@ieee.org Sent: Sunday, July 27, 2003 4:26 PM Subject: RE: Q on Correlation of Votage ripple with a Spectrum Analyser This is getting pretty intense for a Sunday!! Both Mr. Javor and Mr Novak make excellent observations. Both center on the method of measurment as a point of concern. To measure the voltage ripple I used a high badwidth (1GHz) sigle ended probe with very short leads. In order to establish the error margin, I used the null experiment technique. (I don't have a diff probe with sufficient bandwidth to hand). I then used a piece of coax with very short leads (just like Mr Javor recommends) and a 50ohm resistor in series with the signal to feed the same voltage ripple to a SA. Clearly there is a voltage division here but thats easily accounted for. My concern is that the impedance of the planes is so much lower than the 50ohm input of the SA and I wanted to match that as much as possible. I belive I have taken care of as much of the measurement problem as possible. Still, the maximum voltages between the two measurments do not come close at all. -Original Message- From: owner-emc-p...@majordomo.ieee.org [mailto:owner-emc-p...@majordomo.ieee.org]On Behalf Of Ken Javor Sent: Sunday, July 27, 2003 10:27 AM To: istvan novak; Charles Grasso; Emc-Pstc Subject: Re: Q on Correlation of Votage ripple with a Spectrum Analyser Mr. Novak makes some excellent points. I was under the assumption that the phrase voltage ripple implied conducted emission measurements at a LISN port. Hence my comments on mode separation. Across a spectrum of even 30 MHz, any normal scope probe I know of (1 or 10 M Ohm in parallel with 5/10/20 pF) will present a varying load to the measured waveform. I know there are some broadband active differential probes with low shunt capacity, but I have no experience using them. The scope should be made to look like a flat 50 Ohm load to help correlate to a spectrum analyzer. Instead of using long leads to make measurements from one point of the Vcc plane to another, I would make coaxial measurements from one point on the Vcc plane to chassis ground or the reference/image plane if one exists, and then make the same measurement at another point on the Vcc plane, and compare the waveforms, perhaps using a built-in math function if your scope has that. Of course this technique requires the reference/image plane/chassis ground to be an equipotential plane... By coaxial measurement I mean using coax rather than a scope probe, and terminating the shield at the reference point, while extending the center conductor just far enough to connect to the point of interest on the Vcc plane. Theoretically, an even better technique would be to have a place on the board where the power plane reference was available circumferentially around a Vcc via, and connect the coax shield to the reference plane and the center conductor to the Vcc contact. Given a 50 Ohm load at the other end (with a blocking cap to protect it), and an FFT capability with enough memory, I believe you could achieve correlation with the individual spectral components measured by a spectrum analyzer (which of course would also need a blocking cap with this config). From: istvan novak istvan.no...@worldnet.att.net Date: Sun, 27 Jul 2003 11:02:02 -0400 To: Charles Grasso cgrassospri...@earthlink.net, Ken Javor ken.ja...@emccompliance.com, Emc-Pstc emc-p...@ieee.org Subject: Re: Q on Correlation of Votage ripple with a Spectrum Analyser Charles, Doing this kind of correlation is very difficult for the following reasons: - unless you measure a very simple and dummy system, hardware
Re: Q on Correlation of Votage ripple with a Spectrum Analyser
Then the answer is the difference between the sum of all spectral components measured with the scope vs. the individual components themselves. If you want correlation you have to go the FFT route. From a strictly EMC point-of-view only the spectral components matter, the only point of measuring with a scope is if you don't have an analyzer and are going to perform an FFT anyway. From: Charles Grasso cgrassospri...@earthlink.net Date: Sun, 27 Jul 2003 13:26:40 -0700 To: Ken Javor ken.ja...@emccompliance.com, istvan novak istvan.no...@worldnet.att.net, Emc-Pstc emc-p...@ieee.org Subject: RE: Q on Correlation of Votage ripple with a Spectrum Analyser This is getting pretty intense for a Sunday!! Both Mr. Javor and Mr Novak make excellent observations. Both center on the method of measurment as a point of concern. To measure the voltage ripple I used a high badwidth (1GHz) sigle ended probe with very short leads. In order to establish the error margin, I used the null experiment technique. (I don't have a diff probe with sufficient bandwidth to hand). I then used a piece of coax with very short leads (just like Mr Javor recommends) and a 50ohm resistor in series with the signal to feed the same voltage ripple to a SA. Clearly there is a voltage division here but thats easily accounted for. My concern is that the impedance of the planes is so much lower than the 50ohm input of the SA and I wanted to match that as much as possible. I belive I have taken care of as much of the measurement problem as possible. Still, the maximum voltages between the two measurments do not come close at all. -Original Message- From: owner-emc-p...@majordomo.ieee.org [mailto:owner-emc-p...@majordomo.ieee.org]On Behalf Of Ken Javor Sent: Sunday, July 27, 2003 10:27 AM To: istvan novak; Charles Grasso; Emc-Pstc Subject: Re: Q on Correlation of Votage ripple with a Spectrum Analyser Mr. Novak makes some excellent points. I was under the assumption that the phrase voltage ripple implied conducted emission measurements at a LISN port. Hence my comments on mode separation. Across a spectrum of even 30 MHz, any normal scope probe I know of (1 or 10 M Ohm in parallel with 5/10/20 pF) will present a varying load to the measured waveform. I know there are some broadband active differential probes with low shunt capacity, but I have no experience using them. The scope should be made to look like a flat 50 Ohm load to help correlate to a spectrum analyzer. Instead of using long leads to make measurements from one point of the Vcc plane to another, I would make coaxial measurements from one point on the Vcc plane to chassis ground or the reference/image plane if one exists, and then make the same measurement at another point on the Vcc plane, and compare the waveforms, perhaps using a built-in math function if your scope has that. Of course this technique requires the reference/image plane/chassis ground to be an equipotential plane... By coaxial measurement I mean using coax rather than a scope probe, and terminating the shield at the reference point, while extending the center conductor just far enough to connect to the point of interest on the Vcc plane. Theoretically, an even better technique would be to have a place on the board where the power plane reference was available circumferentially around a Vcc via, and connect the coax shield to the reference plane and the center conductor to the Vcc contact. Given a 50 Ohm load at the other end (with a blocking cap to protect it), and an FFT capability with enough memory, I believe you could achieve correlation with the individual spectral components measured by a spectrum analyzer (which of course would also need a blocking cap with this config). From: istvan novak istvan.no...@worldnet.att.net Date: Sun, 27 Jul 2003 11:02:02 -0400 To: Charles Grasso cgrassospri...@earthlink.net, Ken Javor ken.ja...@emccompliance.com, Emc-Pstc emc-p...@ieee.org Subject: Re: Q on Correlation of Votage ripple with a Spectrum Analyser Charles, Doing this kind of correlation is very difficult for the following reasons: - unless you measure a very simple and dummy system, hardware today is so complex that you cant predict for sure its activity; it is a strong function of time. -tThe spectrum analyzer and scope will look at the same signal in different ways: analog spectrum analyzers have a seep time and settling time determining the frequency and aperture of visit each frequency. If you have a spectrumn analyzer used for compliance tests, probably the CISPR filter is on. Scopes on the other hand (digital scopes) undersample the signal, whether it is called real-time or not. Memory and displey refresh rate does not allow scopes to display and process all data points of high-frequency signals. Real-time scopes do it for a given time window, but it is usually way less than the
RE: Q on Correlation of Votage ripple with a Spectrum Analyser
This is getting pretty intense for a Sunday!! Both Mr. Javor and Mr Novak make excellent observations. Both center on the method of measurment as a point of concern. To measure the voltage ripple I used a high badwidth (1GHz) sigle ended probe with very short leads. In order to establish the error margin, I used the null experiment technique. (I don't have a diff probe with sufficient bandwidth to hand). I then used a piece of coax with very short leads (just like Mr Javor recommends) and a 50ohm resistor in series with the signal to feed the same voltage ripple to a SA. Clearly there is a voltage division here but thats easily accounted for. My concern is that the impedance of the planes is so much lower than the 50ohm input of the SA and I wanted to match that as much as possible. I belive I have taken care of as much of the measurement problem as possible. Still, the maximum voltages between the two measurments do not come close at all. From: owner-emc-p...@majordomo.ieee.org [mailto:owner-emc-p...@majordomo.ieee.org]On Behalf Of Ken Javor Sent: Sunday, July 27, 2003 10:27 AM To: istvan novak; Charles Grasso; Emc-Pstc Subject: Re: Q on Correlation of Votage ripple with a Spectrum Analyser Mr. Novak makes some excellent points. I was under the assumption that the phrase voltage ripple implied conducted emission measurements at a LISN port. Hence my comments on mode separation. Across a spectrum of even 30 MHz, any normal scope probe I know of (1 or 10 M Ohm in parallel with 5/10/20 pF) will present a varying load to the measured waveform. I know there are some broadband active differential probes with low shunt capacity, but I have no experience using them. The scope should be made to look like a flat 50 Ohm load to help correlate to a spectrum analyzer. Instead of using long leads to make measurements from one point of the Vcc plane to another, I would make coaxial measurements from one point on the Vcc plane to chassis ground or the reference/image plane if one exists, and then make the same measurement at another point on the Vcc plane, and compare the waveforms, perhaps using a built-in math function if your scope has that. Of course this technique requires the reference/image plane/chassis ground to be an equipotential plane... By coaxial measurement I mean using coax rather than a scope probe, and terminating the shield at the reference point, while extending the center conductor just far enough to connect to the point of interest on the Vcc plane. Theoretically, an even better technique would be to have a place on the board where the power plane reference was available circumferentially around a Vcc via, and connect the coax shield to the reference plane and the center conductor to the Vcc contact. Given a 50 Ohm load at the other end (with a blocking cap to protect it), and an FFT capability with enough memory, I believe you could achieve correlation with the individual spectral components measured by a spectrum analyzer (which of course would also need a blocking cap with this config). From: istvan novak istvan.no...@worldnet.att.net Date: Sun, 27 Jul 2003 11:02:02 -0400 To: Charles Grasso cgrassospri...@earthlink.net, Ken Javor ken.ja...@emccompliance.com, Emc-Pstc emc-p...@ieee.org Subject: Re: Q on Correlation of Votage ripple with a Spectrum Analyser Charles, Doing this kind of correlation is very difficult for the following reasons: - unless you measure a very simple and dummy system, hardware today is so complex that you cant predict for sure its activity; it is a strong function of time. -tThe spectrum analyzer and scope will look at the same signal in different ways: analog spectrum analyzers have a seep time and settling time determining the frequency and aperture of visit each frequency. If you have a spectrumn analyzer used for compliance tests, probably the CISPR filter is on. Scopes on the other hand (digital scopes) undersample the signal, whether it is called real-time or not. Memory and displey refresh rate does not allow scopes to display and process all data points of high-frequency signals. Real-time scopes do it for a given time window, but it is usually way less than the time constant of a CISPR filter on the spectrum analyzer. - connection to the source makes a big difference. I assume when you calibrated the reading with a sine wave, a coaxial cable with coax connectors at both ends was used. Presumably the product does not have a coaxial connector on the Vcc plane, so you have to make your own connection or use a hand-held probe. This is very extra noise usually gets in the path, and the scope reading becomes unrealistically high. I have found no active scope probes so far, which would give a correct reading in a noisy environment. We hopefully should not see noise on the Vcc planes more than a few hundred mV. In contrast, many scope probes can pick up spikes as big as volts from the environment. If
RE: Q on Correlation of Votage ripple with a Spectrum Analyser
Hi all, Actually I was using a good ole Spectrim Analyser so I sidestepped the windowing issue/software issues altogether. What I was(am)trying to do was match the max voltage as measured on a scope with the value as measured on a SA. I first calibrated myslef using a known source - a sine wave. The amplitudes fell in just as theory predicted. Encouraged, I then probed the Vcc plane on a product I was working on and was not so happy!! Any ideas? From: Ken Javor [mailto:ken.ja...@emccompliance.com] Sent: Saturday, July 26, 2003 4:19 PM To: Charles Grasso; Emc-Pstc Subject: Re: Q on Correlation of Votage ripple with a Spectrum Analyser I presented a paper on that very subject about a decade ago at one of the EMC TD magazine EMC symposia. I used a Fluke Scopemeter and some FFT software that came with it. The Fluke interfaced to the PC through an optically isolated RS-232 protocol. It worked quite well from a pre-compliance or troubleshooting point-of-view. You could use time windowing to separate the signals deriving from leading and falling edges from the signals deriving from the pulse itself. I used LISNMATE and LISNMARK mode separation devices to show that the rising/falling edge signals were common mode, while the pulse itself generated differential mode signals. From: Charles Grasso cgrassospri...@earthlink.net Reply-To: Charles Grasso cgrassospri...@earthlink.net Date: Sat, 26 Jul 2003 16:18:36 -0700 To: Emc-Pstc emc-p...@ieee.org Subject: Q on Correlation of Votage ripple with a Spectrum Analyser Hi All, Has anyone tried correlating the voltage ripple as seen on a scope with the amplitudes measured on a Spectrum Analyser? I tried doing that the other day with ..umm. minimal success. I think that due to the comples convoltions that would have to occur when FFT'ing an irregular voltage shape. Charles Grasso Echostar Communications. --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ 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: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ 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: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
RE: Q on Correlation of Votage ripple with a Spectrum Analyser
Charles Grasso wrote: What I was(am)trying to do was match the max voltage as measured on a scope with the value as measured on a SA. Try zero span on the SA. Compare THAT with the scope. Cortland This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ 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: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
Re: Q on Correlation of Votage ripple with a Spectrum Analyser
I presented a paper on that very subject about a decade ago at one of the EMC TD magazine EMC symposia. I used a Fluke Scopemeter and some FFT software that came with it. The Fluke interfaced to the PC through an optically isolated RS-232 protocol. It worked quite well from a pre-compliance or troubleshooting point-of-view. You could use time windowing to separate the signals deriving from leading and falling edges from the signals deriving from the pulse itself. I used LISNMATE and LISNMARK mode separation devices to show that the rising/falling edge signals were common mode, while the pulse itself generated differential mode signals. From: Charles Grasso cgrassospri...@earthlink.net Reply-To: Charles Grasso cgrassospri...@earthlink.net Date: Sat, 26 Jul 2003 16:18:36 -0700 To: Emc-Pstc emc-p...@ieee.org Subject: Q on Correlation of Votage ripple with a Spectrum Analyser Hi All, Has anyone tried correlating the voltage ripple as seen on a scope with the amplitudes measured on a Spectrum Analyser? I tried doing that the other day with ..umm. minimal success. I think that due to the comples convoltions that would have to occur when FFT'ing an irregular voltage shape. Charles Grasso Echostar Communications. --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ 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: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ 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: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
Q on Correlation of Votage ripple with a Spectrum Analyser
Hi All, Has anyone tried correlating the voltage ripple as seen on a scope with the amplitudes measured on a Spectrum Analyser? I tried doing that the other day with ..umm. minimal success. I think that due to the comples convoltions that would have to occur when FFT'ing an irregular voltage shape. Charles Grasso Echostar Communications. This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ 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: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
