The MIL-STD-461 EMI specs have a different mask for broadband sources than for narrow band sources. The spread spectrum clocks merely take advantage of that. I am not familiar with the FCC spec, but I suppose they probably have a similar handling of narrow band versus broad band emissions.
If you do not like it, do not blame the technology, blame the people who wrote the specs. For some applications, particularly narrow band applications where the discrete spur would otherwise fall inside your passband, a spread spectrum clock is actually a good thing. For other applications, YMMV. Another reason why it is popular with manufacturers is that it is typically harder to identify the source of a broadband interference compared to narrowband, so when a customer has a problem with his or her wireless device, if he or she cannot readily identify the source of the interference, the customer is more likely to blame the device than an unidentified source of interference. In a society where profit is king, this is like music to the ears of management and the finance guys. It bugs me to no end that as a power supply design engineer, I came up with the idea in 1986-1987 and even breadboarded a "random frequency converter" (as I called it then) by using a pseudo-random generator (shift register and a few gates) driving a DAC driving a current source driving the clock of a PWM chip in a power converter. The corporation that I worked for at the time had no interest in even checking if it was patentable (they did not have a patent department). It did exactly what it was supposed to do, reduce peak emissions by 10-20dB depending on the depth of modulation and the particular test (I was more interested in MIL-STD-461 than FCC at the time). I still have the breadboard... Didier KO4BB > -----Original Message----- > From: time-nuts-boun...@febo.com > [mailto:time-nuts-boun...@febo.com] On Behalf Of David Forbes > Sent: Thursday, November 26, 2009 11:03 AM > To: Discussion of precise time and frequency measurement > Subject: Re: [time-nuts] Spread Spectrum Spam! > > > > >Hmph!... Spread Spectrum clocks do *NOT* make the shielding any > >easier, it's just a fudge for the accountants who won't fund a proper > >job in the first place. It only "fools" the QP detector in > a measuring > >receiver into showing a lower value, it does not "Fix" the problem. > > And I thought I was the only person in the world who noticed > that bit of subterfuge. It actually makes EMI *worse*, > artificially raising the test limit by smearing the signal to > get past the FCC's spectrum analyzer-defined peak limit. > > The Part 15 limits for such things as the FM broadcast band, > on the other hand, are defined by field strength. That's the > only cheat-proof way to specify emitter power testing. > > -- > > --David Forbes, Tucson, AZ > http://www.cathodecorner.com/ > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com To unsubscribe, > go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.