Re: Suppression of radiated emissions and spread spectrum clocking
This is a multi-part message in MIME format. Isn't it fun, that's why I am not sure if being called an EMC expert is a good thing. Now to the point: 1) Pulling back the Vcc plane away from the edge of the ground plane changes the distribution of the EM field at the edge. While the strength at the edge decreases, it can indeed increase radiation in another direction. There have been papers published on that, e.g. search for Gisin and Tanner in the IEEE EMC Symposia or Transactions records. However, what is the practical application and usefulness? While the radiation pattern is changed, one should notice that on the typical PCBs none of the mechanisms of radiations (edge-fringing or Patch-antenna) are not efficient radiators, and in my experience the direct radiation off the PCB edge or off the (undercut) Vcc plane is typically not a problem, because Vcc planes are typically at an electrically very small distance to the ground planes (even in a not-so-good PCB stackup), and the field falls-off quickly with the distance from the edge of the Vcc plane (wherever it is! ). Also, in many cases, the Vcc planes are in between other plane (ground) layers, which again changes the field distribution and coupling and radiation mechanisms (there is no patch-antenna in the PCB if it is sandwitched in the inner layers). The typical practical problem with the edge-radiation that I have seen is not the direct radiation, rather it is when the PCB-edge is close to a piece of metal that is electrically larger, e.g. card-cage in a chassis. The field at the edge in that case feeds the larger antenna (piece of metal), which then radiates as a secondary-radiator much more efficiently. Another similar situation is in the areas of the PCB-moats. The Vcc that is extended all the way to the ground plane edge (of the moat) can very efficiently couple the EM field into the moat, which then easily guides it and couples to the I/O across the moat, e.g. a transformer. In such a case, the transformer and/or common-mode choke across the moat do not suppress the common-mode energy coupled to them. The papers I have seen only concentrate on the redirection of energy, but fail to recognize that the real practical issue is typycally not the direct radiation but the secondary radiation. I'd say, the expert who is stating that it is uselles is definitelly wrong - sometimes it is very benefitial as described above (and witnessed in extensive practice). It is also true that sometimes it does not matter. The expert who said it CAN definitely help is right - it depends on the circumstances. It is typically easy to pull the Vcc planes back and stitch the “ground” planes together at the edges, which eliminates the issue with emission coming from secondary radiators - so why not do it? However, the energy is still going somewhere inside the PCB stackup, so the signal routing, PCB-stackup, bypassing etc. can/should/must be used to minimize the levels and the coupling with the PCB structures that can radiate or carry it to the I/O sections. 2) Spread-spectrum is frequency modulation of the clock signal. If one looks in any textbook on the topic (or remembers from the school), in FM the side-bands receive power from the carrier, so that the deeper the modulation is the power (hence the amplitude) of the carrier is lower and the power (the amplitude) of the side-bands is higher – the total power in the signal is constant. The ideal SS technique, from the point of view of minimizing the radiated emission maximum amplitude) is such that the level at the carrier frequency and the level at the sidebands are equal. Any deeper modulation and the sidebands are higher than in the equilibrium, and any lower modulation and the carrier is higher (thus they are not optimal). Also, the waveform of the modulating signal can make large difference, check Lexmark web page on that. I'd say the expert was wrong on the second one. Again - see the math of how FM works. Cheers, Neven Conformity and SI List I attended an electronic conference where a well-known EMI/EMC speaker/author stated that the 20 H rule is useless for suppression of radiated emissions. On the other hand, another well known EMI/EMC speaker/author states that it can definitely help. What is the opinion of conformity (SI) members and what empirical results have been obtained? Second question. A well-known EMI speaker/author also states that spread spectrum clocking (SSC) does not lower peak radiated emissions. Rather the harmonics of SSC are spread out where the Quasi Peak Detector Spectrum Analyzer (QPDSA) is missing (incorrect frequency band) their peak value readings, i.e. fpeak is outside the frequency band that SPDSA is set for monitoring. Thanks. Robert Hanson - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website:
Re: Suppression of radiated emissions and spread spectrum clocking
Cortland, The Ansoft URL's pdf comes up nil. Going to the \thewave\ index doesn't list that .pdf Would you send me a copy? - Robert - On Sun, 27 Nov 2005 20:47:48 -0500 Cortland Richmond 72146@compuserve.com wrote: Ken Javor wrote I didn't know what the 20 H rule was either, just assumed everyone else did... See http://www.qsl.net/wb6tpu/si-list4/0391.html page 9 of http://www.ansoft.com/thewave/Shielding_and_Decoupling.pdf http://factaee.elfak.ni.ac.yu/fu2k12/fu04.html and others (search on 20H rule.) RF between two equally sized planes does radiate from the slot formed at the edge - but RF on a small plane over a larger one also radiates from the slot around the smaller. We don't want to put patch antennas on a board while trying to avoid other problems. Thus the controversy. Cortland KA5S - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list.Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to emc-p...@ieee.org Instructions: http://listserv.ieee.org/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas emcp...@ptcnh.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher: j.bac...@ieee.org David Heald:emc-p...@daveheald.com All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
Re: Suppression of radiated emissions and spread spectrum clocking
Thank you, now that I understand that 20 H means the power plane outside edge is set back from the ground plane edge a distance 20 times the separation between the two planes, it is possible to opine on the subject. Looking at the fringing field set up by potentials and gradients between the two planes, if you follow the 20 H rule the radiation efficiency will be much less, the quasi-static and induction fields will be there, but much less of the energy will break off and radiated as a traveling electromagnetic wave. From: Cortland Richmond 72146@compuserve.com Date: Sun, 27 Nov 2005 20:47:48 -0500 To: ieee pstc list emc-p...@ieee.org Subject: Re: Suppression of radiated emissions and spread spectrum clocking Ken Javor wrote I didn't know what the 20 H rule was either, just assumed everyone else did... See http://www.qsl.net/wb6tpu/si-list4/0391.html page 9 of http://www.ansoft.com/thewave/Shielding_and_Decoupling.pdf http://factaee.elfak.ni.ac.yu/fu2k12/fu04.html and others (search on 20H rule.) RF between two equally sized planes does radiate from the slot formed at the edge - but RF on a small plane over a larger one also radiates from the slot around the smaller. We don't want to put patch antennas on a board while trying to avoid other problems. Thus the controversy. Cortland KA5S - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list.Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to emc-p...@ieee.org Instructions: http://listserv.ieee.org/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas emcp...@ptcnh.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher: j.bac...@ieee.org David Heald:emc-p...@daveheald.com All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list.Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to emc-p...@ieee.org Instructions: http://listserv.ieee.org/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas emcp...@ptcnh.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher: j.bac...@ieee.org David Heald:emc-p...@daveheald.com All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
Re: Suppression of radiated emissions and spread spectrum clocking
Ken Javor wrote I didn't know what the 20 H rule was either, just assumed everyone else did... See http://www.qsl.net/wb6tpu/si-list4/0391.html page 9 of http://www.ansoft.com/thewave/Shielding_and_Decoupling.pdf http://factaee.elfak.ni.ac.yu/fu2k12/fu04.html and others (search on 20H rule.) RF between two equally sized planes does radiate from the slot formed at the edge - but RF on a small plane over a larger one also radiates from the slot around the smaller. We don't want to put patch antennas on a board while trying to avoid other problems. Thus the controversy. Cortland KA5S - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list.Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to emc-p...@ieee.org Instructions: http://listserv.ieee.org/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas emcp...@ptcnh.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher: j.bac...@ieee.org David Heald:emc-p...@daveheald.com All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
Re: Suppression of radiated emissions and spread spectrum clocking
I didn't know what the 20 H rule was either, just assumed everyone else did... From: John Woodgate j...@jmwa.demon.co.uk Date: Sun, 27 Nov 2005 18:49:13 + To: emc-p...@ieee.org Subject: Re: Suppression of radiated emissions and spread spectrum clocking americo...@aol.com wrote (in 26c.a77014.30bb4...@aol.com) about 'Suppression of radiated emissions and spread spectrum clocking', on Sun, 27 Nov 2005: Conformity and SI List I attended an electronic conference where a well-known EMI/EMC speaker/author stated that the 20 H rule is useless for suppression of radiated emissions. On the other hand, another well known EMI/EMC speaker/author states that it can definitely help. What is the opinion of conformity (SI) members and what empirical results have been obtained? What is 'the 20 H rule'? -- Regards, John Woodgate, OOO - Own Opinions Only. Deadlines are 90% of deadliness. http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list.Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to emc-p...@ieee.org Instructions: http://listserv.ieee.org/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas emcp...@ptcnh.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher: j.bac...@ieee.org David Heald:emc-p...@daveheald.com All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list.Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to emc-p...@ieee.org Instructions: http://listserv.ieee.org/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas emcp...@ptcnh.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher: j.bac...@ieee.org David Heald:emc-p...@daveheald.com All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
Re: Suppression of radiated emissions and spread spectrum clocking
americo...@aol.com wrote (in 26c.a77014.30bb4...@aol.com) about 'Suppression of radiated emissions and spread spectrum clocking', on Sun, 27 Nov 2005: Conformity and SI List I attended an electronic conference where a well-known EMI/EMC speaker/author stated that the 20 H rule is useless for suppression of radiated emissions. On the other hand, another well known EMI/EMC speaker/author states that it can definitely help. What is the opinion of conformity (SI) members and what empirical results have been obtained? What is 'the 20 H rule'? -- Regards, John Woodgate, OOO - Own Opinions Only. Deadlines are 90% of deadliness. http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list.Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to emc-p...@ieee.org Instructions: http://listserv.ieee.org/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas emcp...@ptcnh.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher: j.bac...@ieee.org David Heald:emc-p...@daveheald.com All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
Re: Suppression of radiated emissions and spread spectrum clocking
Reply to second question only. In a limited sense it is correct that a spread spectrum clock doesn't decrease emissions, it spreads them out (in the frequency domain). The spread spectrum clock decreases the amount of time a signal is within the pass-band of the receiver, making it appear a broadband signal which is properly attenuated or filtered by a quasi-peak detector. As long as the resolution bandwidth (120 kHz) is a proper simulation of the spectrum allocation for a radio broadcast protected by the RE limit, all is well. If however the protected broadcast has much wider spectral content than the measurement bandwidth, such as occurs with a television broadcast (4 MHz), then the dithering results in shifting the spectrum around within the pass band of the victim receiver and it is still a narrowband signal captured continuously by the victim. In that case the quasi-peak detector does not properly address the nuisance value of the interference. No direct experience with the following, but it makes sense intuitively. Have heard anecdotes that clock dithering can actually increase TVI over a cw interference signal. Essentially clock dithering that does not push spectral content out of the pass-band introduces frequency modulation in-band, and it is often the case that a modulated signal causes more interference than a purely cw tone. From: americo...@aol.com List-Post: emc-pstc@listserv.ieee.org Date: Sun, 27 Nov 2005 12:37:23 EST To: emc-p...@ieee.org Subject: Suppression of radiated emissions and spread spectrum clocking Conformity and SI List I attended an electronic conference where a well-known EMI/EMC speaker/author stated that the 20 H rule is useless for suppression of radiated emissions. On the other hand, another well known EMI/EMC speaker/author states that it can definitely help. What is the opinion of conformity (SI) members and what empirical results have been obtained? Second question. A well-known EMI speaker/author also states that spread spectrum clocking (SSC) does not lower peak radiated emissions. Rather the harmonics of SSC are spread out where the Quasi Peak Detector Spectrum Analyzer (QPDSA) is missing (incorrect frequency band) their peak value readings, i.e. fpeak is outside the frequency band that SPDSA is set for monitoring. Thanks. Robert Hanson - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list.Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to emc-p...@ieee.org Instructions: http://listserv.ieee.org/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas emcp...@ptcnh.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher: j.bac...@ieee.org David Heald: emc-p...@daveheald.com All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc From: americo...@aol.com List-Post: emc-pstc@listserv.ieee.org Date: Sun, 27 Nov 2005 12:35:46 EST To: emc-p...@ptcnh.net Cc: americomsemin...@aol.com Subject: Suppression of radiated emissions and spread spectrum clocking Conformity and SI List I attended an electronic conference where a well-known EMI/EMC speaker/author stated that the 20 H rule is useless for suppression of radiated emissions. On the other hand, another well known EMI/EMC speaker/author states that it can definitely help. What is the opinion of conformity (SI) members and what empirical results have been obtained? Second question. A well-known EMI speaker/author also states that spread spectrum clocking (SSC) does not lower peak radiated emissions. Rather the harmonics of SSC are spread out where the Quasi Peak Detector Spectrum Analyzer (QPDSA) is missing (incorrect frequency band) their peak value readings, i.e. fpeak is outside the frequency band that SPDSA is set for monitoring. Thanks. Robert Hanson - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list.Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to emc-p...@ieee.org Instructions: http://listserv.ieee.org/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas emcp...@ptcnh.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher: j.bac...@ieee.org David Heald: emc-p...@daveheald.com All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc - This message
