Mr. George's comment is a very high level discussion which does not address the important technical detail I mentioned earlier. A clock dithered more than 120 kHz will appear to wander in and out of the CISPR 22 receiver passband, and the measured amplitude of the peak while in the pass band will be an average related to how long it spends in the passband compared to the filter time constant. But if the frequency where the measurement is made is used for commercial purposes by TV broadcasts with a 4 MHz channel bandwidth, the dithering may not take the signal outside the bandpass at all, and the dithering may actually cause more interference effect than if the signal was purely cw (this last purely conjecture on my part, but very easy to measure per one of my earlier communications).
---------- >From: "George, David L" <[email protected]> >To: "'Ken Javor'" <[email protected]> >Cc: Product Safety Technical Committee <[email protected]> >Subject: RE: Dithering >Date: Fri, Jun 22, 2001, 10:17 AM > > What is "Dithering"? > > The dictionary defines dithering as agitating or in a state of indecision. > Dithering is a method for distributing energy across a wider spectrum. > Another and more accurate term for dithering is spread spectrum. In a > spread spectrum application the clock frequency or carrier is not fixed. It > varies according to a pseudorandom sequence. Various modulation schemes are > possible. > > There are many uses for this technology. One application is the intentional > radiation for communication purposes. It was first used in the military for > highly confidential applications. Thanks to the development of inexpensive, > reliable semiconductors, this technology has spread to commercial > applications such as cordless telephones, burglar alarms and wireless > networks (WLL and WLANS). The second use of this technology is in digital > equipment as a clock. In this application, the radiation is unintentional > and of much lower in intensity. > > There is really nothing wrong with using this technology since it is already > used by other industries without interference problems. For example, many > people have said CISPR22 already has very stringent limits. Complaints of > interference from IT equipment are almost non-existent. This spread > spectrum method of clock distribution was not developed to reduce the amount > of apparent radiation. It was deliberately chosen because it increases the > signal to noise ratio allowing greater processing speeds. We can take a > lesson from technology in present use. Spread spectrum intentional > radiators produce far greater field strengths (orders of magnitude) than > unintentional radiators such as IT equipment. More importantly the FCC in > USA, the DoC in Canada, ETSI and other national authorities all developed > procedures and legislation to allow its use in portions of the spectrum. In > most countries the regulations allow unlicensed operation of spread spectrum > at far greater magnitudes than unintentional radiators. Since this > technology has been in use for some time and since there have been almost no > reports of interference from these and higher magnitudes, it is very likely > the very low levels from IT equipment will not increase the number of > interference complaints. > > The technology can be used with confidence and many computer manufactures > are already using it. > David George, PE > Unisys Corp. > 2476 Swedesford Road > Malvern, PA 19355 > Tel: 610-648-3653 > Fax: 610-695-4700 > Email: [email protected] > > -----Original Message----- > From: Ken Javor [mailto:[email protected]] > Sent: Sunday, June 17, 2001 1:30 PM > To: Rich Nute > Cc: Product Safety Technical Committee > Subject: Re: Dithering > > > > All I know about others' experience is that I have heard there were problems > on this forum. But this is extremely easy to check out. I used to teach > EMI testing seminars. In order to minimize test equipment needs, I would > use a small TV receiver as an EUT, and feed it a signal on channel 3 or 4 > from a VCR. I used a bulk current injection clamp to drive currents onto > the interconnect coax. All it takes is an rf signal generator, especially > if you use an in-line attenuator out of the VCR to reduce the TV's incoming > signal to an MDS type level. Anyway, I would show that cw didn't affect > much, but AM tore up reception. I also mapped the IF filter pass band by > sweeping the signals source and recording the TOS level (threshold of > susceptibility). You could do exactly the same, but investigating the > effects of FM rather than AM. > > > on 6/15/01 12:54 PM, Rich Nute at [email protected] wrote: > >> >> >> >> Hi Ken: >> >> >>> In conclusion, dithering will help you meet a test requirement, but it > might >>> not actually reduce potential interferences. >> >> I have come to the same qualitative conclusion, >> and the EMC experts with whom I have discussed >> this concept agree. >> >> But, is there any evidence, anecdotal, qualitative, >> or quantitative, that interference is not reduced? >> >> In my home, AM radio, even for local stations, is >> useless due to interference (or is it due to poor >> AM receiver design?). >> >> >> Best regards, >> Rich >> >> >> > > > ------------------------------------------- > This message is from the IEEE EMC Society Product Safety > Technical Committee emc-pstc discussion list. > > Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ > > To cancel your subscription, send mail to: > [email protected] > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Michael Garretson: [email protected] > Dave Heald [email protected] > > For policy questions, send mail to: > Richard Nute: [email protected] > Jim Bacher: [email protected] > > All emc-pstc postings are archived and searchable on the web at: > http://www.rcic.com/ click on "Virtual Conference Hall," > ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: [email protected] Dave Heald [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.rcic.com/ click on "Virtual Conference Hall,"
