Lacey & others, Interesting. But with only the 2nd harmonic being your problem, it's possible the dithered clock's energy envelope was mostly or all still within the 120 kHz passband of the EMI receiver, depending upon the amount of dithering. Was this the case?
I'm curious about the digital TV receiver report. I understand that the entire 6 MHz of a digital TV channel's frequency allocation appears to be filled with content as opposed to the old analog TV systems which basically had audio & video sub-carriers. This suggests the digital receiver front end designs are different (more vulnerable?). Hopefully, there are others on this forum that can explain. Jack Cook, Xerox Corp -----Original Message----- From: Lacey,Scott [mailto:sla...@foxboro.com] Sent: Friday, March 17, 2000 8:19 AM To: 'Robert Macy' Cc: 'emc-p...@ieee.org' Subject: RE: Dithered clocks and EMC - BEWARE Robert, and the group, Although I have only limited experience (1 instance) with dithered clocks, I thought that I might share that experience with the group. I had a product that was failing radiated emissions at one particular frequency with a vertical antenna orientation. I tracked the problem down to a particular cable and circuit card. Use of a near field probe identified the oscillator package as the source of the emissions (the second harmonic). The device was located near the card edge, insufficient decoupling, etc. All the textbook layout errors. A check of the oscillator specifications showed that it was a standard TTL device with a fanout of 10 inputs. I had already found some fixes that would reduce emissions, clamp-on ferrites, additional shielding, etc., but thought it made more sense to reduce the emissions at the source. I suggested to the design engineer that a low-power TTL oscillator, with a fanout of 2 inputs, would reduce currents through the offending etches. Someone else suggested a dithered clock device instead. When we tested the dithered clock, emissions were actually worse. We had simply spread the problem over a wider spectrum. I have heard some success stories for these devices, but results in this case were disappointing. I still think a lot depends on the layout. I strongly suspect that real-world disruption to nearby devices is generally going to be worse, since the idea behind these devices is to "fool" quasi-peak measurements in order to pass. Caveat Emptor! Scott Lacey -----Original Message----- From: Robert Macy [SMTP:m...@california.com] Sent: Friday, March 17, 2000 9:38 AM To: emc-p...@ieee.org Subject: Fw: Dithered clocks and EMC - BEWARE Of interest, so I forward this to the group: - Robert - Robert A. Macy, PE m...@california.com 408 286 3985 fx 408 297 9121 AJM International Electronics Consultants 619 North First St, San Jose, CA 95112 -----Original Message----- From: John Woodgate <j...@jmwa.demon.co.uk> Newsgroups: sci.engr.electrical.compliance,sci.electronics.design Date: Tuesday, March 14, 2000 11:55 PM Subject: Dithered clocks and EMC - BEWARE >About a year ago we had a thread on this subject, concluding that some >research was needed to see whether dithered clocks were better or worse >in terms of conforming to EMC requirements. > >I learned very recently that some reliable but as-yet unpublished >research has found that digital TV receivers are some 40 dB (!!) more >sensitive to dithered clock emissions than to unmodulated carriers. This >is likely to lead to changes in EMC limits within maybe as little as >three years, since the authorities certainly don't want to be deluged >with complaints of interference from people who have just opted for >digital TV. > >So, if you are thinking of using a dithered clock, think again! >-- >Regards, John Woodgate, OOO - Own Opinions Only. Phone +44 (0)1268 747839 >Fax +44 (0)1268 777124. http://www.jmwa.demon.co.uk >I wanted to make a fully-automated nuclear-powered trawler, >but it went into spontaneous fishing. ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. 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