[time-nuts] Noise of digital frequency circuits (was: Programmable clock for BFO use....noise)

Attila Kinali attila at kinali.ch
Wed Oct 10 11:04:02 EDT 2018

On Mon, 17 Sep 2018 00:37:43 -0500
Dana Whitlow <k8yumdoober at gmail.com> wrote:

> For example, take the case of 10 MHz starting frequency; the phase noise
> several MHz out
> is likely to be nil.  But divide the 10 MHz down to, say, 1 Hz; then
> there's likely to be quite a
> lot of phase noise within "folding range" of many Nyquist bands about 1 Hz.

For most low-noise systems, the white noise floor is dominated by
the thermal (Johnson) noise due to the 50 Ohm source impedance.
Although, one could say this noise is very low, it is wrong to
assume it can be ignored. Jitter, for a low-noise 10MHz system,
is dominated by the white noise and very little of the contribution
is due to flicker noise (unless you go for very long integration times).

> This, again, is why I wonder so much about our efforts in re-synthesizing
> higher frequencies from
> the 1PPS from GPS receivers.  I don't know much the architecture of GPS
> receivers, but it seems
> to me it would sure be nice if there were some convenient way to extract a
> clean signal at the
> chipping rate, for use in generating standard reference frequencies.

There are systems that do that, but one has to use signals from
multiple satellites to get the noise down. But for proper combination
of multiple signals one has to calculate a fix. Hence it is easier
to just check the reference oscillators phase against the fix.
And synthesizing from PPS is exactly this process of referencing
the 10MHz oscillator to the calculated fixes.

			Attila Kinali

Science is made up of so many things that appear obvious 
after they are explained. -- Pardot Kynes

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