[time-nuts] Phase measurement of my GPSDO

[Bob kb8tq]

Hi

The quick way to do this is with a single mixer. Take something like an old
10811 and use the coarse tune to set it high in frequency by 5 to 10 Hz. 

Then feed it into an RPD-1 mixer and pull out the 5 to 10 Hz audio tone. 
That tone is the *difference* between the 10811 and your device under test. 
If the DUT moves 1 Hz, the audio tone changes by 1 Hz. 

If you measured the 10 MHz on the DUT, that 1 Hz would be a very small shift
( 0.1 ppm ). At 10 Hz it’s a 10% change. You have “amplified” the change
in frequency by the ratio of 10 MHz to 10 Hz ( so a million X increase ). 

*IF* you could tack that on to the ADEV plot of your 5335 ( no, it’s not that 
simple) your 7x10^-10 at 1 second would become more 7x10^-16 at 1 
second. 

The reason its not quite that simple is that the input circuit on the counter 
really does not handle a 10 Hz audio tone as well as it handles a 10 MHz
RF signal. Instead of getting 9 digits a second, you probably will get three
*good* digits a second and another 6 digits of noise. 

The good news is that an op amp used as a preamp ( to get you up to maybe
32 V p-p rather than a volt or so) and another op amp or three as limiters will
get you up around 6 or 7 good digits. Toss in a cap or two as a high pass 
and low pass filter ( DC offsets can be a problem ….) and you have a working
device that gets into the parts in 10^-13 with your 5335. 

It all can be done with point to point wiring. No need for a PCB layout. Be 
careful that the +/- 18V supplies to the op amp *both* go on and off at the 
same time …. 

Bob

> On Apr 3, 2020, at 5:13 AM, Tobias Pluess <tpluess at ieee.org> wrote:
> 
> hi John
> 
> yes I know the DMTD method, and indeed I am planing to build my own DMTD
> system, something similar to the "Small DMTD system" published by Riley (
> https://www.wriley.com/A Small DMTD System.pdf).
> However I am unsure whether that will help much in this case, because all
> what the DMTD does is to mix the 10MHz signals down to some 1Hz Signal or
> so which can be measured more easily, and I already have 1Hz signals (the
> 1PPS) which I am comparing.
> Or do you suggest to use the DMTD and use a higher frequency at its
> outputs, say 10Hz or so, and then average for 10 samples  to increase the
> resolution?
> 
> Thanks
> Tobias
> HB9FSX
> 
> 
> On Fri, Apr 3, 2020 at 12:53 AM John Miles <john at miles.io> wrote:
> 
>>> b) if I want to measure 1e-11 or even 1e-12 at 1sec - what resolution
>> does
>>> my counter need? If the above was true, I would expect that a 1ps
>>> resolution (and an even better stability!) was required to measure ADEV
>> of
>>> 1e-12, The fact that the (as far as I know) world's most recent,
>>> rocket-science grade counter (some Keysight stuff) has "only" 20ps of
>>> resolution, but people are still able to measure even 1e-14 shows that my
>>> assumption is wrong. So how are the measurement resolution and the ADEV
>>> related to each other? I plan to build my own TIC based on a TDC7200,
>> which
>>> would offer some 55ps of resolution, but how low could I go with that?
>> 
>> That sounds like a simple question but it's not.  There are a few
>> different approaches to look into:
>> 
>> 1) Use averaging with your existing counter.  Some counters can yield
>> readings in the 1E-12 region at t=1s even though their single-shot jitter
>> is much worse than that.  They do this by averaging  hundreds or thousands
>> of samples for each reading they report.  Whether (and when) this is
>> acceptable is a complex topic in itself, too much so to explain quickly.
>> Search for information on the effects of averaging and dead time on Allan
>> deviation to find the entrance to this fork of the rabbit hole.
>> 
>> 2) Search for the term 'DMTD' and read about that.
>> 
>> 3) Search for 'direct digital phase measurement' and read about that.
>> 
>> 4) Search for 'tight PLL' and read about that.
>> 
>> Basically, while some counters can perform averaging on a post-detection
>> basis, that's like using the tone control on a radio to reduce static and
>> QRM.  It works, sort of, but it's too late in the signal chain at that
>> point to do the job right.  You really want to limit the bandwidth before
>> the signal is captured, but since that's almost never practical at RF, the
>> next best thing to do is limit the bandwidth before the signal is
>> "demodulated" (i.e., counted.)
>> 
>> Hence items 2, 3, and 4 above.  They either limit the measurement
>> bandwidth prior to detection, lower the frequency itself to keep the
>> counter's inherent jitter from dominating the measurement, or both.  You'll
>> have to use one of these methods, or another technique along the same
>> lines, if you want to measure the short-term stability of a good oscillator
>> or GPSDO.
>> 
>> -- john, KE5FX
>> 
>> 
>> 
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