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

Bob kb8tq kb8tq at n1k.org
Sat Sep 15 22:57:31 EDT 2018


Hi

Most of the traditional rules about phase noise apply out to 10 or 20% of the “carrier”
frequency. If the carrier is 1Hz, then you are talking about the traditional definitions holding 
out to 0.1 or 0.2 Hz relative to carrier. That’s *deep* in the 1/F noise part of the divider’s 
“noise curve”.

Since the ADEV of the 1 PPS is typically no worse than the ADEV of the 10 MHz, it would be
hard to come up with a model where the 1 PPS has picked up a lot of extra noise.

Bob

> On Sep 15, 2018, at 9:05 PM, Scott Stobbe <scott.j.stobbe at gmail.com> wrote:
> 
> 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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