[time-nuts] Re: Tuning a GPSDO loop for optimal disturbance handling

[Bob kb8tq]

Hi

DVM’s show up from a lot of places. Indeed some are nutty when 
shipped from here to there. The shopping process is always going
to be “delivered price” based. 

The reason for looking at the dac output is: The frequency just jumped
2x10^-8 … was it from the dac or internal to the TCXO ? The oscillator 
drifted 5x10^-9 it one minute, was it the DAC / Vref? …

Phase Noise:

The “quick / simple” way to do phase noise is with a single mixer setup.
You run both ports at “max” ( so 7 dbm on a 7 dbm mixer) and lock them
in quadrature. A fairly simple audio amp based on any of a number of 
op-amps boosts the output to something an audio spectrum analyzer 
or sound card can “see”. You would like an opamp with something in the
1 nv/Hz vicinity in terms of noise. 

Once you get the setup going, it’s just a matter of calibrating things. There
are a variety of app notes and papers on that part. It normally involves
unlocking the loop and measuring the phase slope of the beat note with
whatever you are using to look at noise. ( The op amp “preamp” normally 
gets switched out for this step).

Not crazy expensive or hard to wire up. I’ve built the circuit a lot of times
using “dead bug” sort of construction. They all seem to work. 

On a typical GPSDO design, you are looking at noise inside about 10 KHz. 
Sure there could be issues anywhere, but the most likely stuff is below the 
max limit on the typical sound card. Low end wise, it would be nice to get 
to 1 Hz. That isn’t going to be easy with the typical sound card. 

Yes that glosses over all the “joy” of tracking down ground loops and other
local noise sources. You are measuring a *very* low level signal. Quiet 
supplies and good grounding are part of this. 

Bob


> On Mar 6, 2022, at 11:37 AM, Erik Kaashoek <erik at kaashoek.com> wrote:
> 
> Bob,
> Shipping will be a bit of an issue as I'm not in the USA.
> But the most sensitive voltmeter may be the VC-TCXO so some SW was added to the GPSDO/Timer/Counter to sweep the dac's while measuring a 10 MHz signal from a Rb.
> This showed the high dac steps are all monotonic and almost equal size.
> For the low dac a single step is to small too accurately measure the frequency difference but again the steps seem to be monotonic.
> 
> I'm aware of different way's to measure phase noise. Is there a method particularly good for GPSDO phase noise measurement?
> Erik.
> 
> On 5-3-2022 20:50, Bob kb8tq wrote:
>> Hi
>> 
>> One suggestion on the frequency plot:
>> 
>> TimeLab as you have it set does a “per pixel” process on its charts. You have
>> a limited screen resolution and a ton of data. You have to do something. There
>> is no way to display it all.
>> 
>> It’s a good idea to zoom in to some of the “problem regions” and see
>> what is really going on. This reduces the number of data points that
>> map to one pixel and potentially changes what you see a bit. The closer you
>> get to one data point per pixel, the closer you are to “reality” without any
>> processing potentially getting in the way.
>> 
>> On the DVM, eBay (with some time spent shopping) will sell you a working
>> 6 1/2 digit device for < $100 if you are patient. If you need it now, it’s still
>> likely to be below $200. Yes it will be a big clunky box that takes up bench
>> space. It will be HPIB and not serial i/o. The display might be a bit fun in
>> bright lighting.
>> 
>> While you are shopping, I would get set up for phase noise as well. Often
>> it can spot issues that are tough to find with a second to second data stream.
>> The setup does not have to be very fancy or terribly expensive.
>> 
>> Bob
>> 
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