[time-nuts] Phase measurement of my GPSDO

Fri Apr 3 15:26:57 UTC 2020

Jup, some of them even have phase reversal when they are overloaded, so it
is perhaps not a good idea in general, but I think there are opamps which
are specified for this.

Tobias

On Fri, Apr 3, 2020 at 3:30 PM Dana Whitlow <k8yumdoober at gmail.com> wrote:

> 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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