Re: [PSES] Spread-Spectrum Clock Question
Hello Neven, It is always interesting to read the SSCG posts. You never really know who is listening. Here are some comments on recent posts. To get the best correlation between a receiver in peak detector or QP mode set a spectrum analyzer RBW to 100kHz which is a 3dB bandwidth. The 120kHz receiver bandwidth for the CISPR 16 specification is a 6dB bandwidth which is pretty close to the same thing. The video bandwidth should be set to 300kHz or greater. At these settings the QP amplitude and peak detector amplitudes should be the same. This excludes the uncertainties of the detectors themselves which is usually +/- a couple of tenths of a dB. This is for modulation frequencies >30kHz. A modulation frequency in the 3Hz to 100Hz range is a totally different animal. The QP detector will reach the same amplitude as a peak detector if in the energy is in the band for at least 1ms. Even if the signal is in the band less than 1ms, the signal has to stay out of the band for a significant part of the 550ms fall time of the QP detector. If the signal comes back in the band for a significant time less than 550ms then the QP will be very close to the peak value. I agree with Ken that I have never seen a modulation frequency that low. Most of the SSCG modulation profiles are periodic around 32kHz which means that if you narrow the RBW to 10kHz or less you should see series of harmonics that are there continuously. This is an often misunderstood aspect of SSCG. If the modulation profile is periodic, these sub-harmonics are stationary and not "sliding" in and out of the band as often stated. The band of the discrete harmonics will be the 2.5% wide series the greatest amplitudes at each multiple of the odd clock frequencies. When you make the RBW 100kHz then you are adding 3 of the harmonics together and the amplitude should go up about 9.5dB. Some SSCG modulation profiles are not perfectly periodic so it may be difficult to see the individual harmonics at the narrow RBW. One thing that is a subject for discussion is that the spectrum is sweeping so therefore there is energy at every frequency within the 2.5% of the modulation harmonics. If this were true then spectrum analyzer would not show the gaps between 32kHz intervals modulation harmonics. What is true is that two sine waves added together with slightly different frequencies will be in phase part of the time and out of phase part of the time. If one looks at the IF output of a receiver then you will see a amplitude varying at the modulation rate due to the harmonics adding together. The Lexmark modulation profile attempts to make each of the 32kHz harmonics be as close as possible to the same amplitude so that the energy is uniformly distributed giving the lowest amplitude possible. The 32kHz modulation frequency was chosen to be the best frequency possible to be compatible with AM, FM and analog TV modulation schemes. The primary goal of SSCG was to reduce the cost of compliance while not causing interference when possible. As stated in the posts, humans do not hear here 32kHz. A dog could hear it but there are not many receivers that have that kind of audio bandwidth. A number of studies were performed on analog receivers was well as some wide band digital receivers. The results were basically the same that the interference potential was not that different for SSCG than other interference sources. If one were to design a receiver whose sole purpose what to detect a wide band 32kHz FM transmission then SSCG would be a possible interference. All the communications schemes that we have looked at are very compatible with SSCG and keep interference at a manageable level. This is why the FCC, CISPR and Radiocommunications Agency have not taken any actions against SSCG. One could always argue that any energy in any communications bands will decrease the performance of the channel which is true for any source. One fact is true that SSCG or similar methods have been implemented in the billions of quantities and to date only a very very few minor interference issues have been found. For additional reading please see the following publications. Article Title Investigation into the interference potential of spread-spectrum clock generation to broadband digital communications Publication TitleElectromagnetic Compatibility, IEEE Transactions on Posted Online Date 25 Feb 2003 AuthorsHardin, K.; Oglesbee, R.A.; Fisher, F.; Article Title A study of the interference potential of spread spectrum clock generation techniques Publication TitleElectromagnetic Compatibility, 1995. Symposium Record. 1995 IEEE International Symposium on Posted Online Date 6 Aug 2002 AuthorsHardin, K.B.; Fessler, J.T.; Bush, D.R.; Article Title Design considerations of phase-locked loop systems for spread spectrum clock generation compatibility Publication Title
Re: [PSES] Spread-Spectrum Clock Question
My instruments are lying to me!!! Or I have used different settings to look at different SSC signals. And like the proverbial bumblebee that was not told he cannot fly because theory says so, these stupid instruments did not read the textbooks. I gotta get a cheaper instrument to see reality. Those R&S boat anchors are ignoring the textbooks. I'm not disagreeing, as much as being disagreeable, ;-), The automotive tests ask for their own criteria in testing. And I remember the automotive days - fondly! Where, for the first time, I saw those lying instruments show me a QP higher than the Peak. And the instruments were CISPR 16 compliant! Again it was an instrument setting issue - the difference in the spec's that were being tested to! Changing the RBW will give information about the modulation technique, triangle, herseys kiss, digital steps, etc. and will also show different amplitudes depending on conditions. I "assumed" the usual for me. The usual or common, but that depends on what industry is being considered, frequency sweep over time, kinda like dialing a receiver over a frequency range only to find that this SSC noise pops in now and then, to provide a brief moment of interference. The "protected" folks seem to care only about interference in the time domain. They say don't make a chirp in my broadcast! And that happens at any moment in time when the SSC crosses over the licence holders frequency. The automotive guys might be concerned with not deploying the airbags when the SSC crosses over the trigger frequency, or some such situation. Or the sarcastic remark we'd make is that it is broadcasting on all frequencies for everyone's listening enjoyment! From: Ken Wyatt To: neve...@comcast.net Cc: EMC-PSTC@LISTSERV.IEEE.ORG Sent: Friday, February 10, 2012 2:31 PM Subject: Re: [PSES] Spread-Spectrum Clock Question Hi Neven, I can also confirm that you should see a decrease in amplitude in peak mode for SS clocking. I use my simple handheld Thurlby Thander PSA2701T peak-reading spectrum analyzer to demo this during my EMC seminars. ___ Kenneth Wyatt Wyatt Technical Services LLC Woodland Park, CO Email Me! | Web Site | Blog Subscribe to Newsletter Connect with me on LinkedIn On Feb 10, 2012, at 12:16 PM, Neven Pischl wrote: I would like to thank sincerely to all who responded, I appreciate it. I am not going to react to any discussions on whether it is cheating or not :), it was not anywhere in my mind when I posted the question and I hope this topic does not degrade :). > >But, I'd like to summarize a little: > >1. I do care about the Pk measurements, not only about QP and Avg, because >that is in the specs I am dealing with (some automotive emission requirements) > >2. I found out, as I suspected and was confirmed in some replies, that if I >change the modulating frequency up to over 20 kHz, then I see reduction with >100/120kHz RBW also, not only with 1kHz RBW. Hence, there is an effect of the >modulating frequency and the RBW combination on the measured Pk results. > >3. SSC by using frequency modulation of the clock actually does reduce the >peak value. I have seen some replies saying it does not, and over many years I >have come across people who said the same. However, if you look in any >textbook on FM, you can see that - in the frequency domain - FM causes the >power of the carrier to be distributed into the side-bands, with the total >power the same with or without the modulation. Therefore, because the power >power stays the same, it must be that each of the components in the spectrum, >i.e. the carrier and the two side-lobes, must have a lower amplitude than the >unmodulated carrier. Please, do not confuse the individual amplitudes of >individual spectral components with the amplitude of the signal in the >time-domain, which indeed stays the same. > >In case of a digital pulses, the "carrier" is the fundamental as well as each >of the harmonics. > >Because of the above, and if you look with the infinitely small RBW (i.e. do >the math), SSC does indeed reduce the peak value of each harmonic (and >fundamental). BTW, the "speed" or modulating freqeuncy does not have a bearing >on the level of reduction of the peak values, in such an ideal case. Only the >modulatioin index (similar to "depth") is important, as it defines how much >power of the carrier is put into the side-lobes. > >The issue I was facing was that under the test conditions of the EMC >specification I have to use a specified "wide" RBW. Under that condition, the >modulating frequency is important too - as I found out. > > >Best regards to all, > >Neven- >--
Re: [PSES] Spread-Spectrum Clock Question
Hi Neven, I can also confirm that you should see a decrease in amplitude in peak mode for SS clocking. I use my simple handheld Thurlby Thander PSA2701T peak-reading spectrum analyzer to demo this during my EMC seminars. ___ Kenneth Wyatt Wyatt Technical Services LLC Woodland Park, CO Email Me! | Web Site | Blog Subscribe to Newsletter Connect with me on LinkedIn On Feb 10, 2012, at 12:16 PM, Neven Pischl wrote: > I would like to thank sincerely to all who responded, I appreciate it. I am > not going to react to any discussions on whether it is cheating or not :), it > was not anywhere in my mind when I posted the question and I hope this topic > does not degrade :). > > But, I'd like to summarize a little: > > 1. I do care about the Pk measurements, not only about QP and Avg, because > that is in the specs I am dealing with (some automotive emission requirements) > > 2. I found out, as I suspected and was confirmed in some replies, that if I > change the modulating frequency up to over 20 kHz, then I see reduction with > 100/120kHz RBW also, not only with 1kHz RBW. Hence, there is an effect of the > modulating frequency and the RBW combination on the measured Pk results. > > 3. SSC by using frequency modulation of the clock actually does reduce the > peak value. I have seen some replies saying it does not, and over many years > I have come across people who said the same. However, if you look in any > textbook on FM, you can see that - in the frequency domain - FM causes the > power of the carrier to be distributed into the side-bands, with the total > power the same with or without the modulation. Therefore, because the power > power stays the same, it must be that each of the components in the spectrum, > i.e. the carrier and the two side-lobes, must have a lower amplitude than the > unmodulated carrier. Please, do not confuse the individual amplitudes of > individual spectral components with the amplitude of the signal in the > time-domain, which indeed stays the same. > > In case of a digital pulses, the "carrier" is the fundamental as well as each > of the harmonics. > > Because of the above, and if you look with the infinitely small RBW (i.e. do > the math), SSC does indeed reduce the peak value of each harmonic (and > fundamental). BTW, the "speed" or modulating freqeuncy does not have a > bearing on the level of reduction of the peak values, in such an ideal case. > Only the modulatioin index (similar to "depth") is important, as it defines > how much power of the carrier is put into the side-lobes. > > The issue I was facing was that under the test conditions of the EMC > specification I have to use a specified "wide" RBW. Under that condition, the > modulating frequency is important too - as I found out. > > > Best regards to all, > > Neven > - > > This message is from the IEEE Product Safety Engineering Society emc-pstc > discussion list. To post a message to the list, send your e-mail to > > All emc-pstc postings are archived and searchable on the web at: > http://www.ieee-pses.org/emc-pstc.html > Attachments are not permitted but the IEEE PSES Online Communities site at > http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used > formats), large files, etc. > Website: http://www.ieee-pses.org/ > Instructions: http://listserv.ieee.org/request/user-guide.html > List rules: http://www.ieee-pses.org/listrules.html > For help, send mail to the list administrators: > Scott Douglas > Mike Cantwell > For policy questions, send mail to: > Jim Bacher > David Heald - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas Mike Cantwell For policy questions, send mail to: Jim Bacher: David Heald:
Re: [PSES] Spread-Spectrum Clock Question
I would like to thank sincerely to all who responded, I appreciate it. I am not going to react to any discussions on whether it is cheating or not :), it was not anywhere in my mind when I posted the question and I hope this topic does not degrade :). But, I'd like to summarize a little: 1. I do care about the Pk measurements, not only about QP and Avg, because that is in the specs I am dealing with (some automotive emission requirements) 2. I found out, as I suspected and was confirmed in some replies, that if I change the modulating frequency up to over 20 kHz, then I see reduction with 100/120kHz RBW also, not only with 1kHz RBW . Hence, there is an effect of the modulating frequency and the RBW combination on the measured Pk results. 3. SSC by using frequency modulation of the clock actually does reduce the peak value. I have seen some replies saying it does not, and over many years I have come across people who said the same. However, if you look in any textbook on FM, you can see that - in the frequency domain - FM causes the power of the carrier to be distributed into the side-bands, with the total power the same with or without the modulation. Therefore, because the power power stays the same, it must be that each of the components in the spectrum, i.e. the carrier and the two side-lobes, must have a lower amplitude than the unmodulated carrier. Please, do not confuse the individual amplitudes of individual spectral components with the amplitude of the signal in the time-domain, which indeed stays the same. In case of a digital pulses, the "carrier" is the fundamental as well as each of the harmonics. Because of the above, and if you look with the infinitely small RBW (i.e. do the math), SSC does indeed reduce the peak value of each harmonic (and fundamental). BTW, the "speed" or modulating freqeuncy does not have a bearing on the level of reduction of the peak values, in such an ideal case. Only the modulatioin index (similar to "depth") is important, as it defines how much power of the carrier is put into the side-lobes. The issue I was facing was that under the test conditions of the EMC specification I have to use a specified "wide" RBW . Under that condition, the modulating frequency is important too - as I found out. Best regards to all, Neven - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas Mike Cantwell For policy questions, send mail to: Jim Bacher: David Heald:
Re: [PSES] Spread-Spectrum Clock Question
Probably explains why my wifi link is slower than advertised!!! It is the error correction time... From: Ed Price To: emc-p...@ieee.org Sent: Friday, February 10, 2012 2:29 AM Subject: RE: [PSES] Spread-Spectrum Clock Question And we’ve been arguing about it ever since. The use of a QP detector and the 120 kHz defined RBW makes sense if you are trying to test to demonstrate minimum harm to a communications channel using AM modulation. Thus, if you do something to your clock that results in an indicated signal reduction, then this is a valid thing, because you will also reduce your interfering potential to that AM communication channel. But what happens when you have a digital bit-stream? Your dithered clock jumps around, and at one point in time, sits right in the passband of that communication channel. So let’s say it sits there for 10 microseconds. If that channel is running a 100 MB data rate, then you have just turned about a thousand 1’s and 0’s into 1’s. Error correction and/or redundant transmission might keep things going, but you will definitely degrade the performance. Calling a dithered clock a “spread spectrum” signal is just not right; the energy is not spread, it’s all there, somewhere, and then all there again, somewhere else. Ed Price El Cajon, CA USA From:Bill Owsley [mailto:wdows...@yahoo.com] Sent: Thursday, February 09, 2012 8:59 PM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] Spread-Spectrum Clock Question yeah! what John says. He now owns the consulting business started by Don Bush, and worked with all those guys long ago.| ps. I don't recall ever meeting John. But I did work with the guys listed on the patent, when Lexmark was IBM. I recall the big surprise for me with this SSCG was we (Boca) thought that the FCC would never agree to this kind of "cheating" on emissions and when the FCC came out with their ruling that it was okay, we were all in disbelief!!! From:John Barnes To: neve...@comcast.net; EMC-PSTC@LISTSERV.IEEE.ORG Sent: Thursday, February 9, 2012 10:07 PM Subject: Re: Spread-Spectrum Clock Question Neven, I've worked with the EMC Engineers at Lexmark who invented the spread-spectrum clock generator (SSCG), since before they started its development. I used SSCG in a number of products that I designed at Lexmark. Below is my understanding of how SSCG works, based on numerous discussions with the *real* experts. Consider an ordinary clock generator, that creates a continuous near-trapezoidal waveform switching between Alow and Ahigh volts at frequency f1 Hertz. Depending on the duty cycle, rise time, fall time, shape of the rising edge, and shape of the falling edge, the frequency spectrum of this clock will have: * A DC bias, A0 volts, somewhere between Alow and Ahigh volts. * A fundamental of A1 volts at f1 (and -f1) Hertz. * A second harmonic of A2 volts at f2 = 2*f1 (and -f2) Hertz. * A third harmonic of A3 volts at f3 = 3*f1 (and -f3) Hertz. ... * An Nth harmonic of AN volts at fN =N*f1 (and -fN) Hertz. ... If we tune over frequency with a peak/quasipeak/average detector having a bandpass filter with bandwidth BW < f1 Hertz, we will see steady spikes at frequencies f1, f2, f3, etc. with amplitudes A1, A2, A3, etc. volts, each having a width BW Hertz and a shape reflecting the filter's s21 shape factor. (If BW > f1 Hertz, at frequency f Hertz we will see a composite of all of the spikes between f-BW/2 and f+BW/2 Hertz, attenuated by the filter's shape factor, which is much messier to deal with conceptually.) A given detector has a finite risetime for a signal that appears within its filter's bandwidth, and an associated falltime when *no* signal is seen within its filter's bandwidth. For a peak detector, this risetime is very short, and the falltime essentially infinite. For a quasipeak detector, the risetime is somewhat longer, and both the risetime and the falltime are finite, as specified by some standard-- or by the parts chosen and how the detector is built. The quasipeak detector imitates the response of human ears and eyes, where a single brief stimulus doesn't affect us much, but an on-going stimulus-- even at a much lower level-- bugs the dickens out of us (Chinese water torture). An average detector has a risetime and a falltime that extend over several (many) cycles of our clock, so it measures the average value. With a continuous clock at frequency f1, a peak, quasipeak, and average detector will all measure close to: * A1 volts at f1 Hertz. * A2 volts at f2 = 2*f1 Hertz. * A3 volts at f3 = 3*f1 Hertz. ... * AN volts at fN =N*f1 Hertz. ... Now let us vary (dither) the clock frequency between f1- and f1+ Hertz, at some reasonably-fast rate, without changing the rising and falling edges of the clock (i.e. spread-spectrum clock generator (SSCG)): *
Re: [PSES] Spread-Spectrum Clock Question
And we've been arguing about it ever since. The use of a QP detector and the 120 kHz defined RBW makes sense if you are trying to test to demonstrate minimum harm to a communications channel using AM modulation. Thus, if you do something to your clock that results in an indicated signal reduction, then this is a valid thing, because you will also reduce your interfering potential to that AM communication channel. But what happens when you have a digital bit-stream? Your dithered clock jumps around, and at one point in time, sits right in the passband of that communication channel. So let's say it sits there for 10 microseconds. If that channel is running a 100 MB data rate, then you have just turned about a thousand 1's and 0's into 1's. Error correction and/or redundant transmission might keep things going, but you will definitely degrade the performance. Calling a dithered clock a "spread spectrum" signal is just not right; the energy is not spread, it's all there, somewhere, and then all there again, somewhere else. Ed Price El Cajon, CA USA From: Bill Owsley [mailto:wdows...@yahoo.com] Sent: Thursday, February 09, 2012 8:59 PM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] Spread-Spectrum Clock Question yeah! what John says. He now owns the consulting business started by Don Bush, and worked with all those guys long ago.| ps. I don't recall ever meeting John. But I did work with the guys listed on the patent, when Lexmark was IBM. I recall the big surprise for me with this SSCG was we (Boca) thought that the FCC would never agree to this kind of "cheating" on emissions and when the FCC came out with their ruling that it was okay, we were all in disbelief!!! _ From: John Barnes To: neve...@comcast.net; EMC-PSTC@LISTSERV.IEEE.ORG Sent: Thursday, February 9, 2012 10:07 PM Subject: Re: Spread-Spectrum Clock Question Neven, I've worked with the EMC Engineers at Lexmark who invented the spread-spectrum clock generator (SSCG), since before they started its development. I used SSCG in a number of products that I designed at Lexmark. Below is my understanding of how SSCG works, based on numerous discussions with the *real* experts. Consider an ordinary clock generator, that creates a continuous near-trapezoidal waveform switching between Alow and Ahigh volts at frequency f1 Hertz. Depending on the duty cycle, rise time, fall time, shape of the rising edge, and shape of the falling edge, the frequency spectrum of this clock will have: * A DC bias, A0 volts, somewhere between Alow and Ahigh volts. * A fundamental of A1 volts at f1 (and -f1) Hertz. * A second harmonic of A2 volts at f2 = 2*f1 (and -f2) Hertz. * A third harmonic of A3 volts at f3 = 3*f1 (and -f3) Hertz. ... * An Nth harmonic of AN volts at fN =N*f1 (and -fN) Hertz. ... If we tune over frequency with a peak/quasipeak/average detector having a bandpass filter with bandwidth BW < f1 Hertz, we will see steady spikes at frequencies f1, f2, f3, etc. with amplitudes A1, A2, A3, etc. volts, each having a width BW Hertz and a shape reflecting the filter's s21 shape factor. (If BW > f1 Hertz, at frequency f Hertz we will see a composite of all of the spikes between f-BW/2 and f+BW/2 Hertz, attenuated by the filter's shape factor, which is much messier to deal with conceptually.) A given detector has a finite risetime for a signal that appears within its filter's bandwidth, and an associated falltime when *no* signal is seen within its filter's bandwidth. For a peak detector, this risetime is very short, and the falltime essentially infinite. For a quasipeak detector, the risetime is somewhat longer, and both the risetime and the falltime are finite, as specified by some standard-- or by the parts chosen and how the detector is built. The quasipeak detector imitates the response of human ears and eyes, where a single brief stimulus doesn't affect us much, but an on-going stimulus-- even at a much lower level-- bugs the dickens out of us (Chinese water torture). An average detector has a risetime and a falltime that extend over several (many) cycles of our clock, so it measures the average value. With a continuous clock at frequency f1, a peak, quasipeak, and average detector will all measure close to: * A1 volts at f1 Hertz. * A2 volts at f2 = 2*f1 Hertz. * A3 volts at f3 = 3*f1 Hertz. ... * AN volts at fN =N*f1 Hertz. ... Now let us vary (dither) the clock frequency between f1- and f1+ Hertz, at some reasonably-fast rate, without changing the rising and falling edges of the clock (i.e. spread-spectrum clock generator (SSCG)): * The DC bias, A0 volts, stays about the same. * The fundamental is still about A1 volts, but it varies between f1- and f1+ Hertz. * The second harmonic is still about A2 volts, but it varies between f2- = 2*f1- and f2+ = 2*f1+ Hertz. * Th
Re: [PSES] Spread-Spectrum Clock Question
The common spreading freq at the time meant that the interference passed thru the sensitive band at a frequency higher than our human visual and auditory perceptions, (It could be detected by comparing side by side, two systems, one with and one without) but for those of us that opinionated that is was " cheating", we worked to find the beat frequencies and showed that. Our position did not prevail, nor did our purist view of thou shalt not interfere, QP or peak! Besides, it was new, and since our frequency identification for further QP measurements involved peak measurements, this gave us more/freq's info to examine. Those of us on the design side; some counted on the SSCG, and others of us ensured our designs did not need SSCG. We no longer have analog TV in the US where we could place "hum" bars, but if some of this is in the FM band, and computers used work are frequencies comparable to the FM band, it could make a station sound rather weak due to the interference. But only us technical guys would recognize it! And one reason why the SSCG modulation was 50 kHz in general; that beating with the highest audio frequency, was still 25 kHz. Only young girls could hear it and not many of them! From: Ken Javor To: "EMC-PSTC@LISTSERV.IEEE.ORG" Sent: Friday, February 10, 2012 12:16 AM Subject: Re: Spread-Spectrum Clock Question Re: Spread-Spectrum Clock Question Why is a spread-spectrum clock cheating? If the measurement BW is an accurate portrayal of the victims protected by the levied requirement, and if the QP detector is an accurate assessment of the nuisance value of the interfering signal modulation, then why is spreading the spectrum over a range of frequencies cheating, when only one victim frequency can be received by a given listener, and the interference to that one channel has been reduced to the level required by the limit? Now if the victim receiver has a wide enough BW so that the dithered clock harmonics are all in band tot he victim receiver, that could be a problem. That occurs with broadcast television reception, with a 4 MHz BW in the old analog days, and even wider now. Haven’t experienced it myself, but have heard that a dithered clock harmonic can actually cause more of a nuisance to television reception than if the clock was a fixed frequency. But that is a special case. What is the reason for the “cheating” verdict? Ken Javor Phone: (256) 650-5261 From: Bill Owsley Reply-To: Bill Owsley Date: Thu, 9 Feb 2012 20:59:01 -0800 (PST) To: "jrbar...@iglou.com" , "neve...@comcast.net" , "EMC-PSTC@LISTSERV.IEEE.ORG" Subject: Re: Spread-Spectrum Clock Question yeah! what John says. He now owns the consulting business started by Don Bush, and worked with all those guys long ago.| ps. I don't recall ever meeting John. But I did work with the guys listed on the patent, when Lexmark was IBM. I recall the big surprise for me with this SSCG was we (Boca) thought that the FCC would never agree to this kind of "cheating" on emissions and when the FCC came out with their ruling that it was okay, we were all in disbelief!!! From: John Barnes To: neve...@comcast.net; EMC-PSTC@LISTSERV.IEEE.ORG Sent: Thursday, February 9, 2012 10:07 PM Subject: Re: Spread-Spectrum Clock Question Neven, I've worked with the EMC Engineers at Lexmark who invented the spread-spectrum clock generator (SSCG), since before they started its development. I used SSCG in a number of products that I designed at Lexmark. Below is my understanding of how SSCG works, based on numerous discussions with the *real* experts. Consider an ordinary clock generator, that creates a continuous near-trapezoidal waveform switching between Alow and Ahigh volts at frequency f1 Hertz. Depending on the duty cycle, rise time, fall time, shape of the rising edge, and shape of the falling edge, the frequency spectrum of this clock will have: * A DC bias, A0 volts, somewhere between Alow and Ahigh volts. * A fundamental of A1 volts at f1 (and -f1) Hertz. * A second harmonic of A2 volts at f2 = 2*f1 (and -f2) Hertz. * A third harmonic of A3 volts at f3 = 3*f1 (and -f3) Hertz. ... * An Nth harmonic of AN volts at fN =N*f1 (and -fN) Hertz. ... If we tune over frequency with a peak/quasipeak/average detector having a bandpass filter with bandwidth BW < f1 Hertz, we will see steady spikes at frequencies f1, f2, f3, etc. with amplitudes A1, A2, A3, etc. volts, each having a width BW Hertz and a shape reflecting the filter's s21 shape factor. (If BW > f1 Hertz, at frequency f Hertz we will see a composite of all of the spikes between f-BW/2 and f+BW/2 Hertz, attenuated by the filter's shape factor, which is much messier to deal with conceptually.) A given detector has a finite risetime for a signal that appears within its filter's bandwidth, and an associated falltim
Re: [PSES] Spread-Spectrum Clock Question
Why is a spread-spectrum clock cheating? If the measurement BW is an accurate portrayal of the victims protected by the levied requirement, and if the QP detector is an accurate assessment of the nuisance value of the interfering signal modulation, then why is spreading the spectrum over a range of frequencies cheating, when only one victim frequency can be received by a given listener, and the interference to that one channel has been reduced to the level required by the limit? Now if the victim receiver has a wide enough BW so that the dithered clock harmonics are all in band tot he victim receiver, that could be a problem. That occurs with broadcast television reception, with a 4 MHz BW in the old analog days, and even wider now. Haven¹t experienced it myself, but have heard that a dithered clock harmonic can actually cause more of a nuisance to television reception than if the clock was a fixed frequency. But that is a special case. What is the reason for the ³cheating² verdict? Ken Javor Phone: (256) 650-5261 From: Bill Owsley Reply-To: Bill Owsley Date: Thu, 9 Feb 2012 20:59:01 -0800 (PST) To: "jrbar...@iglou.com" , "neve...@comcast.net" , "EMC-PSTC@LISTSERV.IEEE.ORG" Subject: Re: Spread-Spectrum Clock Question yeah! what John says. He now owns the consulting business started by Don Bush, and worked with all those guys long ago.| ps. I don't recall ever meeting John. But I did work with the guys listed on the patent, when Lexmark was IBM. I recall the big surprise for me with this SSCG was we (Boca) thought that the FCC would never agree to this kind of "cheating" on emissions and when the FCC came out with their ruling that it was okay, we were all in disbelief!!! From: John Barnes To: neve...@comcast.net; EMC-PSTC@LISTSERV.IEEE.ORG Sent: Thursday, February 9, 2012 10:07 PM Subject: Re: Spread-Spectrum Clock Question Neven, I've worked with the EMC Engineers at Lexmark who invented the spread-spectrum clock generator (SSCG), since before they started its development. I used SSCG in a number of products that I designed at Lexmark. Below is my understanding of how SSCG works, based on numerous discussions with the *real* experts. Consider an ordinary clock generator, that creates a continuous near-trapezoidal waveform switching between Alow and Ahigh volts at frequency f1 Hertz. Depending on the duty cycle, rise time, fall time, shape of the rising edge, and shape of the falling edge, the frequency spectrum of this clock will have: * A DC bias, A0 volts, somewhere between Alow and Ahigh volts. * A fundamental of A1 volts at f1 (and -f1) Hertz. * A second harmonic of A2 volts at f2 = 2*f1 (and -f2) Hertz. * A third harmonic of A3 volts at f3 = 3*f1 (and -f3) Hertz. ... * An Nth harmonic of AN volts at fN =N*f1 (and -fN) Hertz. ... If we tune over frequency with a peak/quasipeak/average detector having a bandpass filter with bandwidth BW < f1 Hertz, we will see steady spikes at frequencies f1, f2, f3, etc. with amplitudes A1, A2, A3, etc. volts, each having a width BW Hertz and a shape reflecting the filter's s21 shape factor. (If BW > f1 Hertz, at frequency f Hertz we will see a composite of all of the spikes between f-BW/2 and f+BW/2 Hertz, attenuated by the filter's shape factor, which is much messier to deal with conceptually.) A given detector has a finite risetime for a signal that appears within its filter's bandwidth, and an associated falltime when *no* signal is seen within its filter's bandwidth. For a peak detector, this risetime is very short, and the falltime essentially infinite. For a quasipeak detector, the risetime is somewhat longer, and both the risetime and the falltime are finite, as specified by some standard-- or by the parts chosen and how the detector is built. The quasipeak detector imitates the response of human ears and eyes, where a single brief stimulus doesn't affect us much, but an on-going stimulus-- even at a much lower level-- bugs the dickens out of us (Chinese water torture). An average detector has a risetime and a falltime that extend over several (many) cycles of our clock, so it measures the average value. With a continuous clock at frequency f1, a peak, quasipeak, and average detector will all measure close to: * A1 volts at f1 Hertz. * A2 volts at f2 = 2*f1 Hertz. * A3 volts at f3 = 3*f1 Hertz. ... * AN volts at fN =N*f1 Hertz. ... Now let us vary (dither) the clock frequency between f1- and f1+ Hertz, at some reasonably-fast rate, without changing the rising and falling edges of the clock (i.e. spread-spectrum clock generator (SSCG)): * The DC bias, A0 volts, stays about the same. * The fundamental is still about A1 volts, but it varies between f1- and f1+ Hertz. * The second harmonic is still about A2 volts, but it varies between f2- = 2*f1- and f2+ = 2*f1+ Hertz. * The third harmonic is still about A3 volts, but it varies bwteen f3- = 3*f1- and f3+ = 3*f1+ Hertz. ... *
Re: [PSES] Spread-Spectrum Clock Question
yeah! what John says. He now owns the consulting business started by Don Bush, and worked with all those guys long ago.| ps. I don't recall ever meeting John. But I did work with the guys listed on the patent, when Lexmark was IBM. I recall the big surprise for me with this SSCG was we (Boca) thought that the FCC would never agree to this kind of "cheating" on emissions and when the FCC came out with their ruling that it was okay, we were all in disbelief!!! From: John Barnes To: neve...@comcast.net; EMC-PSTC@LISTSERV.IEEE.ORG Sent: Thursday, February 9, 2012 10:07 PM Subject: Re: Spread-Spectrum Clock Question Neven, I've worked with the EMC Engineers at Lexmark who invented the spread-spectrum clock generator (SSCG), since before they started its development. I used SSCG in a number of products that I designed at Lexmark. Below is my understanding of how SSCG works, based on numerous discussions with the *real* experts. Consider an ordinary clock generator, that creates a continuous near-trapezoidal waveform switching between Alow and Ahigh volts at frequency f1 Hertz. Depending on the duty cycle, rise time, fall time, shape of the rising edge, and shape of the falling edge, the frequency spectrum of this clock will have: * A DC bias, A0 volts, somewhere between Alow and Ahigh volts. * A fundamental of A1 volts at f1 (and -f1) Hertz. * A second harmonic of A2 volts at f2 = 2*f1 (and -f2) Hertz. * A third harmonic of A3 volts at f3 = 3*f1 (and -f3) Hertz. ... * An Nth harmonic of AN volts at fN =N*f1 (and -fN) Hertz. ... If we tune over frequency with a peak/quasipeak/average detector having a bandpass filter with bandwidth BW < f1 Hertz, we will see steady spikes at frequencies f1, f2, f3, etc. with amplitudes A1, A2, A3, etc. volts, each having a width BW Hertz and a shape reflecting the filter's s21 shape factor. (If BW > f1 Hertz, at frequency f Hertz we will see a composite of all of the spikes between f-BW/2 and f+BW/2 Hertz, attenuated by the filter's shape factor, which is much messier to deal with conceptually.) A given detector has a finite risetime for a signal that appears within its filter's bandwidth, and an associated falltime when *no* signal is seen within its filter's bandwidth. For a peak detector, this risetime is very short, and the falltime essentially infinite. For a quasipeak detector, the risetime is somewhat longer, and both the risetime and the falltime are finite, as specified by some standard-- or by the parts chosen and how the detector is built. The quasipeak detector imitates the response of human ears and eyes, where a single brief stimulus doesn't affect us much, but an on-going stimulus-- even at a much lower level-- bugs the dickens out of us (Chinese water torture). An average detector has a risetime and a falltime that extend over several (many) cycles of our clock, so it measures the average value. With a continuous clock at frequency f1, a peak, quasipeak, and average detector will all measure close to: * A1 volts at f1 Hertz. * A2 volts at f2 = 2*f1 Hertz. * A3 volts at f3 = 3*f1 Hertz. ... * AN volts at fN =N*f1 Hertz. ... Now let us vary (dither) the clock frequency between f1- and f1+ Hertz, at some reasonably-fast rate, without changing the rising and falling edges of the clock (i.e. spread-spectrum clock generator (SSCG)): * The DC bias, A0 volts, stays about the same. * The fundamental is still about A1 volts, but it varies between f1- and f1+ Hertz. * The second harmonic is still about A2 volts, but it varies between f2- = 2*f1- and f2+ = 2*f1+ Hertz. * The third harmonic is still about A3 volts, but it varies bwteen f3- = 3*f1- and f3+ = 3*f1+ Hertz. ... * The Nth harmonic is still about AN volts, but it varies between fN- = N*f1- and fN+ = N*f1+ Hertz. ... Essentially, over time, the fundamental and its harmonics each occupy a band of frequencies. At high-enough harmonics (f1+ + f1-)/2 N >= - f1+ - f1- these bands will overlap one another, but hopefully the amplitudes of the harmonics have dropped enough that these overlaps won't cause us a problem. Instantaneously, if the clock is at frequency f1x, f1- <= f1x <= f1+, we still have spikes at f1x, 2*f1x, 3*f1x, ..., N*f1x, ... But if we look at the SSCG clock with a detector with a bandpass filter set at frequency f Hertz, within one of these frequency bands, either * The fundamental or harmonic remains within the bandwidth (BW Hertz) of the filter-- and we measure essentially the same amplitude as for a constant clock. OR * The harmonic flits in and out of the filter's bandwidth, thus we see only part of the signal. If we flit through the filter's bandwidth fast enough-- and infrequently enough-- the detector doesn't have time to respond fully, and it *looks like* the fundamental/harmonic is much lower than it really is! Since most of th
Re: [PSES] Spread-Spectrum Clock Question
Neven, you are right on with observations. Except for your expectation. SSC works only with QP measurements. Peak does not show any reduction using the 'normal' measuring techniques.| The modulating frequency I'm familiar with has been 50 kHz.| ps. If implemented using small steps, or something other than the Hersey kiss timing, you may find that the EUT now fails at two frequencies, one at each end of the spread! pps. Those "small steps" can cause all sorts of mischief with PLL's, as they try to correct for a step that may not be small for the control loop. More steps, of a smaller increment is needed, or just use a VCO that will modulate the frequency with the voltage and control that voltage appropriately. The patent on this is held by Lexmark and Keith Hardin, Don Bush (RIP), etc. if I recall correctly. Maybe expired by now. Lexmark was previously one of the printer/keyboard divisions of IBM, and this SSC was used by them to pass EMC, well before the patent was granted. From: Ken Wyatt To: neve...@comcast.net Cc: EMC-PSTC@LISTSERV.IEEE.ORG Sent: Thursday, February 9, 2012 7:50 PM Subject: Re: [PSES] Spread-Spectrum Clock Question Neven, Your suspicion regarding the modulating frequency is on the right track. Normally, the mod freq is set just above the audio range. There's something very wrong if the specs only allow 3 to 100 Hz. Is that a misprint perhaps? You should be able to see the "spread" harmonics easily at 120 kHz RBW. If you don't see that, it's just not being modulated correctly. If you provide the specific SSC chip used, the group could probably help you better. Cheers, Ken ___ Kenneth Wyatt Wyatt Technical Services LLC Woodland Park, CO Email Me! | Web Site | Blog Subscribe to Newsletter Connect with me on LinkedIn On Feb 9, 2012, at 5:32 PM, Neven Pischl wrote: Hello, > >I wonder if anyone can help with a question I have on spread spectrum clock >(SSC). > >I am trying to validate the effectiveness of a SSC chip to reduce emission. I >measure with the peak detector. With the SSC enabled (up to 2.5% >down-spread) I expect the level measured with a spectrum analyzer to go down >at the fundamental and even more at the harmonics, when compared with the peak >levels without the SSC enabled. The problem is that I only see that the >frequency gets spread but the peak value stays exactly the same with and >without the modulation, using 100kHz or 120kHz RBW of the analyzer. This >happens at the fundamental as well as at the harmonics, so there is absolutely >no reduction in the measured peak amplitude, it just looks wider. > >What is interesting is that if I reduce the RBW on the analyzer down to >1-3kHz, I see the expected result on the analyzer. The frequency is spread and >the peak level with the spread is lower when compared with the peak level >measured without the SSC enabled, the same difference as one can see in >various papers and material on SSC, and which I also measured many times in >the past. > >However, that is not the required RBW that I must use between 30MHz and 1GHzs, >so we have a problem. > >I believe there is something in the modulation scheme of the particular IC >that must be changed to make it work when measured with 120 MHz RBW but I am >not sure what. > >Some of the modulation parameters: >Triangular waveform, quasi-linear ramp up and linear ramp down. >Linear is actually represented with “small” discrete steps. >Frequency range of the modulating waveform is very low, it can be adjusted >roughly between 3Hz and about 100 Hz. >T he SSC devices on the market normally use about 30-40 kHz for the modulating >waveform > >I wonder if the modulating frequency has something to do with the observed >lack of amplitude reduction. The equations that describe the emi-reduction do >not contain the modulating frequency as a factor so it "should" not be a >facotr - BUT - maybe I am not taking into account the relation with the >measurement settings and non-ideal world. > >Any sugetions or comments, links to reference article, App Notes, etc?? > >Regards, Neven- > > >This message is from the IEEE Product Safety Engineering Society emc-pstc >discussion list. To post a message to the list, send your e-mail to > >All emc-pstc postings are archived and searchable on the web >at: http://www.ieee-pses.org/emc-pstc.html >Attachments are not permitted but the IEEE PSES Online Communities site >at http://product-compliance.oc.ieee.org/ can be used for graphics (in >well-used formats), large files, etc. >Website: http://www.ieee-pses.org/ >Instructions: http://li
Re: [PSES] Spread-Spectrum Clock Question
Neven, I've worked with the EMC Engineers at Lexmark who invented the spread-spectrum clock generator (SSCG), since before they started its development. I used SSCG in a number of products that I designed at Lexmark. Below is my understanding of how SSCG works, based on numerous discussions with the *real* experts. Consider an ordinary clock generator, that creates a continuous near-trapezoidal waveform switching between Alow and Ahigh volts at frequency f1 Hertz. Depending on the duty cycle, rise time, fall time, shape of the rising edge, and shape of the falling edge, the frequency spectrum of this clock will have: * A DC bias, A0 volts, somewhere between Alow and Ahigh volts. * A fundamental of A1 volts at f1 (and -f1) Hertz. * A second harmonic of A2 volts at f2 = 2*f1 (and -f2) Hertz. * A third harmonic of A3 volts at f3 = 3*f1 (and -f3) Hertz. ... * An Nth harmonic of AN volts at fN =N*f1 (and -fN) Hertz. ... If we tune over frequency with a peak/quasipeak/average detector having a bandpass filter with bandwidth BW < f1 Hertz, we will see steady spikes at frequencies f1, f2, f3, etc. with amplitudes A1, A2, A3, etc. volts, each having a width BW Hertz and a shape reflecting the filter's s21 shape factor. (If BW > f1 Hertz, at frequency f Hertz we will see a composite of all of the spikes between f-BW/2 and f+BW/2 Hertz, attenuated by the filter's shape factor, which is much messier to deal with conceptually.) A given detector has a finite risetime for a signal that appears within its filter's bandwidth, and an associated falltime when *no* signal is seen within its filter's bandwidth. For a peak detector, this risetime is very short, and the falltime essentially infinite. For a quasipeak detector, the risetime is somewhat longer, and both the risetime and the falltime are finite, as specified by some standard-- or by the parts chosen and how the detector is built. The quasipeak detector imitates the response of human ears and eyes, where a single brief stimulus doesn't affect us much, but an on-going stimulus-- even at a much lower level-- bugs the dickens out of us (Chinese water torture). An average detector has a risetime and a falltime that extend over several (many) cycles of our clock, so it measures the average value. With a continuous clock at frequency f1, a peak, quasipeak, and average detector will all measure close to: * A1 volts at f1 Hertz. * A2 volts at f2 = 2*f1 Hertz. * A3 volts at f3 = 3*f1 Hertz. ... * AN volts at fN =N*f1 Hertz. ... Now let us vary (dither) the clock frequency between f1- and f1+ Hertz, at some reasonably-fast rate, without changing the rising and falling edges of the clock (i.e. spread-spectrum clock generator (SSCG)): * The DC bias, A0 volts, stays about the same. * The fundamental is still about A1 volts, but it varies between f1- and f1+ Hertz. * The second harmonic is still about A2 volts, but it varies between f2- = 2*f1- and f2+ = 2*f1+ Hertz. * The third harmonic is still about A3 volts, but it varies bwteen f3- = 3*f1- and f3+ = 3*f1+ Hertz. ... * The Nth harmonic is still about AN volts, but it varies between fN- = N*f1- and fN+ = N*f1+ Hertz. ... Essentially, over time, the fundamental and its harmonics each occupy a band of frequencies. At high-enough harmonics (f1+ + f1-)/2 N >= - f1+ - f1- these bands will overlap one another, but hopefully the amplitudes of the harmonics have dropped enough that these overlaps won't cause us a problem. Instantaneously, if the clock is at frequency f1x, f1- <= f1x <= f1+, we still have spikes at f1x, 2*f1x, 3*f1x, ..., N*f1x, ... But if we look at the SSCG clock with a detector with a bandpass filter set at frequency f Hertz, within one of these frequency bands, either * The fundamental or harmonic remains within the bandwidth (BW Hertz) of the filter-- and we measure essentially the same amplitude as for a constant clock. OR * The harmonic flits in and out of the filter's bandwidth, thus we see only part of the signal. If we flit through the filter's bandwidth fast enough-- and infrequently enough-- the detector doesn't have time to respond fully, and it *looks like* the fundamental/harmonic is much lower than it really is! Since most of the Radiated and Conducted Emission standards specify limits based on quasipeak and average detectors, the spread, modulation frequency, and modulation waveform are all controlled to "fool" the detector as much as possible, without affecting the functionality of the product. In some of the products that my department and I designed at Lexmark, SSCG lowered our *measured* emissions by up to 15dB over ordinary clock generators. To see how well SSCG works on your product, you really need to test it with an appropriate quasipeak detector. The frequency of your modulating waveform sounds way too slow for me. You want to get through the filter's bandwidth
Re: [PSES] Spread-Spectrum Clock Question
There likely isn¹t much change with a peak detector, unless the frequency changes rapidly enough that it is in the pass-band of the receiver for less than 10 us (100 kHz RBW). If you use a QP detector, you will see a big difference. Ken Javor Phone: (256) 650-5261 From: Ken Wyatt Date: Thu, 9 Feb 2012 17:50:40 -0700 To: Cc: Subject: Re: [PSES] Spread-Spectrum Clock Question Neven, Your suspicion regarding the modulating frequency is on the right track. Normally, the mod freq is set just above the audio range. There's something very wrong if the specs only allow 3 to 100 Hz. Is that a misprint perhaps? You should be able to see the "spread" harmonics easily at 120 kHz RBW. If you don't see that, it's just not being modulated correctly. If you provide the specific SSC chip used, the group could probably help you better. Cheers, Ken ___ Kenneth Wyatt Wyatt Technical Services LLC Woodland Park, CO Email Me! <mailto:k...@emc-seminars.com> | Web Site <http://www.emc-seminars.com> | Blog <http://www.emc-seminars.com/Blog/Blog.php> Subscribe to Newsletter <http://www.emc-seminars.com/Newsletter/Newsletter.html> Connect with me on LinkedIn <http://www.linkedin.com/in/kennethwyatt> On Feb 9, 2012, at 5:32 PM, Neven Pischl wrote: > Hello, > > > I wonder if anyone can help with a question I have on spread spectrum clock > (SSC). > > > I am trying to validate the effectiveness of a SSC chip to reduce emission. I > measure with the peak detector. With the SSC enabled (up to 2.5% down-spread) > I expect the level measured with a spectrum analyzer to go down at the > fundamental and even more at the harmonics, when compared with the peak levels > without the SSC enabled. The problem is that I only see that the frequency > gets spread but the peak value stays exactly the same with and without the > modulation, using 100kHz or 120kHz RBW of the analyzer. This happens at the > fundamental as well as at the harmonics, so there is absolutely no reduction > in the measured peak amplitude, it just looks wider. > > What is interesting is that if I reduce the RBW on the analyzer down to > 1-3kHz, I see the expected result on the analyzer. The frequency is spread and > the peak level with the spread is lower when compared with the peak level > measured without the SSC enabled, the same difference as one can see in > various papers and material on SSC, and which I also measured many times in > the past. > > However, that is not the required RBW that I must use between 30MHz and 1GHzs, > so we have a problem. > > I believe there is something in the modulation scheme of the particular IC > that must be changed to make it work when measured with 120 MHz RBW but I am > not sure what. > > Some of the modulation parameters: > Triangular waveform, quasi-linear ramp up and linear ramp down. > Linear is actually represented with ³small² discrete steps. > Frequency range of the modulating waveform is very low, it can be adjusted > roughly between 3Hz and about 100 Hz. > The SSC devices on the market normally use about 30-40 kHz for the modulating > waveform > > I wonder if the modulating frequency has something to do with the observed > lack of amplitude reduction. The equations that describe the emi-reduction do > not contain the modulating frequency as a factor so it "should" not be a > facotr - BUT - maybe I am not taking into account the relation with the > measurement settings and non-ideal world. > > Any sugetions or comments, links to reference article, App Notes, etc?? > > Regards, Neven > - > > This message is from the IEEE Product Safety Engineering Society emc-pstc > discussion list. To post a message to the list, send your e-mail to > > All emc-pstc postings are archived and searchable on the web at: > http://www.ieee-pses.org/emc-pstc.html > Attachments are not permitted but the IEEE PSES Online Communities site at > http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used > formats), large files, etc. > Website: http://www.ieee-pses.org/ > Instructions: http://listserv.ieee.org/request/user-guide.html > List rules: http://www.ieee-pses.org/listrules.html > For help, send mail to the list administrators: > Scott Douglas > Mike Cantwell > For policy questions, send mail to: > Jim Bacher > David Heald - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachme
Re: [PSES] Spread-Spectrum Clock Question
Neven, Your suspicion regarding the modulating frequency is on the right track. Normally, the mod freq is set just above the audio range. There's something very wrong if the specs only allow 3 to 100 Hz. Is that a misprint perhaps? You should be able to see the "spread" harmonics easily at 120 kHz RBW. If you don't see that, it's just not being modulated correctly. If you provide the specific SSC chip used, the group could probably help you better. Cheers, Ken ___ Kenneth Wyatt Wyatt Technical Services LLC Woodland Park, CO Email Me! | Web Site | Blog Subscribe to Newsletter Connect with me on LinkedIn On Feb 9, 2012, at 5:32 PM, Neven Pischl wrote: > Hello, > > I wonder if anyone can help with a question I have on spread spectrum clock > (SSC). > > I am trying to validate the effectiveness of a SSC chip to reduce emission. I > measure with the peak detector. With the SSC enabled (up to 2.5% down-spread) > I expect the level measured with a spectrum analyzer to go down at the > fundamental and even more at the harmonics, when compared with the peak > levels without the SSC enabled. The problem is that I only see that the > frequency gets spread but the peak value stays exactly the same with and > without the modulation, using 100kHz or 120kHz RBW of the analyzer. This > happens at the fundamental as well as at the harmonics, so there is > absolutely no reduction in the measured peak amplitude, it just looks wider. > > What is interesting is that if I reduce the RBW on the analyzer down to > 1-3kHz, I see the expected result on the analyzer. The frequency is spread > and the peak level with the spread is lower when compared with the peak level > measured without the SSC enabled, the same difference as one can see in > various papers and material on SSC, and which I also measured many times in > the past. > > However, that is not the required RBW that I must use between 30MHz and > 1GHzs, so we have a problem. > > I believe there is something in the modulation scheme of the particular IC > that must be changed to make it work when measured with 120 MHz RBW but I am > not sure what. > > Some of the modulation parameters: > Triangular waveform, quasi-linear ramp up and linear ramp down. > Linear is actually represented with “small” discrete steps. > Frequency range of the modulating waveform is very low, it can be adjusted > roughly between 3Hz and about 100 Hz. > The SSC devices on the market normally use about 30-40 kHz for the modulating > waveform > > I wonder if the modulating frequency has something to do with the observed > lack of amplitude reduction. The equations that describe the emi-reduction do > not contain the modulating frequency as a factor so it "should" not be a > facotr - BUT - maybe I am not taking into account the relation with the > measurement settings and non-ideal world. > > Any sugetions or comments, links to reference article, App Notes, etc?? > > Regards, Neven > - > > This message is from the IEEE Product Safety Engineering Society emc-pstc > discussion list. To post a message to the list, send your e-mail to > > All emc-pstc postings are archived and searchable on the web at: > http://www.ieee-pses.org/emc-pstc.html > Attachments are not permitted but the IEEE PSES Online Communities site at > http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used > formats), large files, etc. > Website: http://www.ieee-pses.org/ > Instructions: http://listserv.ieee.org/request/user-guide.html > List rules: http://www.ieee-pses.org/listrules.html > For help, send mail to the list administrators: > Scott Douglas > Mike Cantwell > For policy questions, send mail to: > Jim Bacher > David Heald - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas Mike Cantwell For policy questions, send mail to: Jim Bacher: David Heald:
[PSES] Spread-Spectrum Clock Question
Hello, I wonder if anyone can help with a question I have on spread spectrum clock (SSC). I am trying to validate the effectiveness of a SSC chip to reduce emission. I measure with the peak detector. With the SSC enabled (up to 2.5% down-spread) I expect the level measured with a spectrum analyzer to go down at the fundamental and even more at the harmonics, when compared with the peak levels without the SSC enabled. The problem is that I only see that the frequency gets spread but the peak value stays exactly the same with and without the modulation, using 100kHz or 120kHz RBW of the analyzer. This happens at the fundamental as well as at the harmonics, so there is absolutely no reduction in the measured peak amplitude, it just looks wider. What is interesting is that if I reduce the RBW on the analyzer down to 1-3kHz, I see the expected result on the analyzer. The frequency is spread and the peak level with the spread is lower when compared with the peak level measured without the SSC enabled, the same difference as one can see in various papers and material on SSC, and which I also measured many times in the past. However, that is not the required RBW that I must use between 30MHz and 1GHzs, so we have a problem. I believe there is something in the modulation scheme of the particular IC that must be changed to make it work when measured with 120 MHz RBW but I am not sure what. Some of the modulation parameters: Triangular waveform, quasi-linear ramp up and linear ramp down. Linear is actually represented with “small” discrete steps. Frequency range of the modulating waveform is very low, it can be adjusted roughly between 3Hz and about 100 Hz. T he SSC devices on the market normally use about 30-40 kHz for the modulating waveform I wonder if the modulating frequency has something to do with the observed lack of amplitude reduction. The equations that describe the emi-reduction do not contain the modulating frequency as a factor so it "should" not be a facotr - BUT - maybe I am not taking into account the relation with the measurement settings and non-ideal world. Any sugetions or comments, links to reference article, App Notes, etc?? Regards, Neven - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas Mike Cantwell For policy questions, send mail to: Jim Bacher: David Heald:
