[time-nuts] Phase measurement of my GPSDO

Fri Apr 3 23:38:53 UTC 2020

Hi Bruce

I have some TUF-1 mixers in my junk box as well as some JFET OpAmps AD8626.
So, if I connect the OpAmps appropriately with some diode limiters as you
suggest, would you say this would give an acceptable DMTD system?
If so it sounds like something that can easily be built on a breadbord or
in manhattan style, as Bob already mentioned. That would be really cool.
I think a while ago I asked a question which goes in a similar direction -
which mixers are better as phase detectors (to build a PLL for phase noise
measurement) and which ones should be used as actual mixers (like in this
case).

Tobias
HB9FSX

On Fri., 3 Apr. 2020, 23:09 Bruce Griffiths, <bruce.griffiths at xtra.co.nz>
wrote:

> One can merely add diodes to the opamp feedback network form a feedback
> limiter and maintain the opamp outputs within the range for which the opamp
> is well behaved whilst maintaining the increase in slew rate for the output.
>
> Bruce
> > On 04 April 2020 at 04:26 Tobias Pluess <tpluess at ieee.org> wrote:
> >
> >
> > 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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