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 <ken.ja...@emccompliance.com> To: "EMC-PSTC@LISTSERV.IEEE.ORG" <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 <wdows...@yahoo.com> Reply-To: Bill Owsley <wdows...@yahoo.com> Date: Thu, 9 Feb 2012 20:59:01 -0800 (PST) To: "jrbar...@iglou.com" <jrbar...@iglou.com>, "neve...@comcast.net" <neve...@comcast.net>, "EMC-PSTC@LISTSERV.IEEE.ORG" <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 <jrbar...@iglou.com> 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 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 as fast as you can, to keep the quasipeak detector from seeing your signal. You might try to increase the size of your steps, so that at a frequency that is causing you grief, no two adjacent steps of the clock frequency are within the 120kHz bandwidth of the quasipeak detector. John Barnes KS4GL, PE, NCE, NCT, ESDC Eng, ESDC Tech, PSE, Master EMC Design Eng, SM IEEE dBi Corporation http://www.dbicorporation.com/ - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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