[time-nuts] Suspected Spam: External clock for Analog to Digital Converter in GPSRx front-end Rx front-end

John Miles jmiles at pop.net
Fri Mar 2 14:13:56 UTC 2007


Possibly, if the ready-made spread-spectrum clocks provide enough jitter for
him.  It shouldn't be hard to do it to an existing clock, though.

The article by David Chu on time-interval averaging in the June '74 HP
Journal issue talks a little about why phase modulation is preferable to FM
when you want to deliberately add jitter to a carrier.  A frequency
modulator imposes a rolloff characteristic that shapes the spectral
characteristics of the noise source in ways that you might not want, while
making it harder to avoid changing the carrier's average frequency as well
as its instantaneous phase.  He also points out that it's easy to add PM
jitter by applying noise to a varactor-tuned tank circuit across the signal
path.  It all seems obvious and straightforward enough.

Remember that the computer guys don't care what their average frequency is
to any real degree of precision, because everything else is ultimately
slaved off the main CPU clock.  If you use an FM technique and the
modulating function isn't absolutely symmetrical, or the modulator response
itself is the least bit nonlinear, you will have to use a separate
low-bandwidth PLL to keep the mean carrier frequency where you want it, I'm
thinking.  Way too complicated, considering the advantages of the PM
technique.

-- john, K5FX


> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]On
> Behalf Of Hal Murray
> Sent: Friday, March 02, 2007 1:17 AM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Suspected Spam: External clock for Analog to
> Digital Converter in GPSRx front-end Rx front-end
>
>
>
> > How about using one of the "spread-spectrum" microprocessor clock
> > generator IC's? These add a pseudo-random jitter to the clock signal
> > so the interference from the equipment is slightly spread causing the
> > equipment to pass EMC testing by causing an apparent reduction in the
> > peak due to the bandwidth of the test receiver.
>
> I thought it was FM by a sawtooth, or something simple like that.
>
> I could easily have a fuzzy memory and/or there could be several
> techniques
> in use.
>
> --
> These are my opinions, not necessarily my employer's.  I hate spam.
>
>
>
>
> _______________________________________________
> time-nuts mailing list
> time-nuts at febo.com
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>





More information about the Time-nuts_lists.febo.com mailing list