[time-nuts] Programmable clock for BFO use....noise

Scott Stobbe scott.j.stobbe at gmail.com
Sat Sep 15 22:05:12 EDT 2018


That is fascinating. So, the 1PPS line on a GPSDO (a divide by 10Meg in
many cases) is 70 dB worse than the traditional 20log(N) PN scaling?

On Sat, Sep 15, 2018 at 11:40 AM Richard (Rick) Karlquist <
richard at karlquist.com> wrote:

> Another great posting from Attila that keeps the S/N ratio
> on this list high.
>
> On 9/15/2018 3:26 AM, Attila Kinali wrote:
>
> > possible logic family for the task. Otherwise the harmonics of the
> > switching of the FF will down-mix high frequency white noise down
> > to the signal band (this is the reason for the 10*log(N) noise scaling
> > of digital divider that Egan[1] and Calosso/Rubiola[2] and a few others
> > mentioned).
>
> Wow, I never knew this in 45 years of designing synthesizers!
> I do remember that some of the frequency counter engineers at HP
> talked about noise aliasing.  I think this is another way of
> describing the same problem.
>
> About 10 years ago, the frequency synthesizer chip vendors started
> talking about a Figure of Merit (FOM) that predicted phase noise floor,
> and it also included the 10 LOG N noise scaling.  An application
> engineer at ADI told me this was a characteristic of the sampling phase
> detector that all these chips used.  But I always wondered if the
> frequency divider could come into play.  The way FOM is defined,
> it doesn't distinguish between phase detector and divider noise.
>
> At Agilent, we used to make a lot of lab demos using a Centellax
> (now Microsemi AKA Microchip) frequency divider that could divide by any
> number between 8 and 511 up to 10 GHz.  It was absolutely fabulous for
> dividing 10 GHz down to 2.5 GHz.  But 20 LOG N quit working if I tried
> to divide down to 50 MHz.  Now you have explained it.
> >
> > If you divide by something that is not a power of 2, then it is important
> > that each stage produces an output waveform with a 50% duty cycle.
> Otherwise
> > flicker noise which has been up-mixed by a previous stage, will be
> down-mixed
> > into the signal band, increasing the close-in phase-noise.
>
> Wow, another thing I never knew.  The conventional wisdom was to
> divide by any number (even or odd) and then follow that divider
> with a divide by 2 flip flop to get 50%.  Now, that is in question.
> The now correct answer is to us a variable modulus prescaler to
> divide by P and P+1, controlled by a toggle flip flop to make
> half the divisions at P and half at P+1.
>
> Does anyone else have experience with these issues?
>
> Rick N6RK
>
>
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