[time-nuts] Spread Spectrum Spam!

[Didier Juges]

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-bounces at febo.com 
> [mailto:time-nuts-bounces at 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/
> 
> 
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