[time-nuts] Phase measurement of my GPSDO

[Dana Whitlow]

Caution: opamps make terrible limiters- their overload behavior is
generally ugly
and unpredictable.  It's much better to use a genuine level comparator, and
wire it
up so that it has a modest amount of hysteresis.

Dana


On Fri, Apr 3, 2020 at 6:45 AM Bob kb8tq <kb8tq at n1k.org> wrote:

> 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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