Reply to second question only. In a limited sense it is correct that a spread spectrum clock doesn't decrease emissions, it spreads them out (in the frequency domain). The spread spectrum clock decreases the amount of time a signal is within the pass-band of the receiver, making it appear a broadband signal which is properly attenuated or filtered by a quasi-peak detector. As long as the resolution bandwidth (120 kHz) is a proper simulation of the spectrum allocation for a radio broadcast protected by the RE limit, all is well. If however the protected broadcast has much wider spectral content than the measurement bandwidth, such as occurs with a television broadcast (4 MHz), then the dithering results in shifting the spectrum around within the pass band of the victim receiver and it is still a narrowband signal captured continuously by the victim. In that case the quasi-peak detector does not properly address the nuisance value of the interference.
No direct experience with the following, but it makes sense intuitively. Have heard anecdotes that clock dithering can actually increase TVI over a cw interference signal. Essentially clock dithering that does not push spectral content out of the pass-band introduces frequency modulation in-band, and it is often the case that a modulated signal causes more interference than a purely cw tone. From: [email protected] List-Post: [email protected] Date: Sun, 27 Nov 2005 12:37:23 EST To: [email protected] Subject: Suppression of radiated emissions and spread spectrum clocking Conformity and SI List I attended an electronic conference where a well-known EMI/EMC speaker/author stated that the 20 H rule is useless for suppression of radiated emissions. On the other hand, another well known EMI/EMC speaker/author states that it can definitely help. What is the opinion of conformity (SI) members and what empirical results have been obtained? Second question. A well-known EMI speaker/author also states that spread spectrum clocking (SSC) does not lower peak radiated emissions. Rather the harmonics of SSC are spread out where the Quasi Peak Detector Spectrum Analyzer (QPDSA) is missing (incorrect frequency band) their peak value readings, i.e. fpeak is outside the frequency band that SPDSA is set for monitoring. Thanks. Robert Hanson - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc From: [email protected] List-Post: [email protected] Date: Sun, 27 Nov 2005 12:35:46 EST To: [email protected] Cc: [email protected] Subject: Suppression of radiated emissions and spread spectrum clocking Conformity and SI List I attended an electronic conference where a well-known EMI/EMC speaker/author stated that the 20 H rule is useless for suppression of radiated emissions. On the other hand, another well known EMI/EMC speaker/author states that it can definitely help. What is the opinion of conformity (SI) members and what empirical results have been obtained? Second question. A well-known EMI speaker/author also states that spread spectrum clocking (SSC) does not lower peak radiated emissions. Rather the harmonics of SSC are spread out where the Quasi Peak Detector Spectrum Analyzer (QPDSA) is missing (incorrect frequency band) their peak value readings, i.e. fpeak is outside the frequency band that SPDSA is set for monitoring. Thanks. Robert Hanson - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
