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 Title Electromagnetic Compatibility, IEEE Transactions on Posted Online Date 25 Feb 2003 Authors Hardin, K.; Oglesbee, R.A.; Fisher, F.; Article Title A study of the interference potential of spread spectrum clock generation techniques Publication Title Electromagnetic Compatibility, 1995. Symposium Record. 1995 IEEE International Symposium on Posted Online Date 6 Aug 2002 Authors Hardin, 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 Electromagnetic Compatibility, 1997. IEEE 1997 International Symposium on Posted Online Date 6 Aug 2002 Authors Hardin, K.B.; Fessler, J.T.; Webb, N.L.; Berry, J.B.; Cable, A.L.; Pulley, M.J.; Article Title Spread spectrum clock generation for the reduction of radiated emissions Publication Title Electromagnetic Compatibility, 1994. Symposium Record. Compatibility in the Loop., IEEE International Symposium on Posted Online Date 6 Aug 2002 Authors Hardin, K.B.; Fessler, J.T.; Bush, D.R.; I hope this has been helpful. Regards, Keith - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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