[time-nuts] Phase measurement of my GPSDO

Fri Apr 3 15:59:37 UTC 2020

Hi John

Yes, I totally agree with you and I also understand the difference.
But what I still don't understand is the following:
Obviously, my 5335A is not accurate/precise enough to measure below 1e-9
for short tau. Currently I am comparing the 1PPS signals, but when I change
that and use the DMTD method, I will still compare some 1Hz signals, and
the counter is still not able to resolve stuff that is lower than 1e-9. So
why would the DMTD work better?
I totally see that the error is somehow multiplied, but if my GPSDO is good
(which I hope it is :-)) the error will still be very small - perhaps in
the 1e-9 or 1e-10 region, so too low for my 5335A. Not?

Tobias

On Fri, Apr 3, 2020 at 5:34 PM John Ackermann N8UR <jra at febo.com> wrote:

> I think the difference is between *mixing* or *dividing* down to a low
> frequency.
>
> When you divide, you divide the noise along with the carrier frequency.
>
> When you mix, you "translate" the noise.  If the signal bounces around
> 0.1 Hz at 10 MHz (awful, I know), when you divide to 1 PPS the noise is
> also divided by 1e7 so the ratio remains the same.
>
> But if you mix via a 9.999 999 MHz local oscillator, now your output at
> 1 Hz still has 0.1 Hz of noise on it.  i.e., it's the same absolute
> value of noise as you started with.  So you measure that absolute value
> but don't compare it to the mixed down 1 Hz frequency, compare it to the
> original 10 MHz frequency.  It's basically an error multiplier.
>
> John
> ----
>
> On 4/3/20 11:25 AM, Tobias Pluess wrote:
> > Hi again Bob,
> >
> > yes you describe a simple DMTD measurement. But could you tell me what
> the
> > difference is between that and comparing the 1PPS pulses?
> > I mean, I could set the 10811 high in frequency by just 1Hz, and then it
> > would result in two 1Hz signals which are then compared.
> > Which is essentially the same as comparing two 1PPS signals, isn't it?
> > Ok there is a minor difference: since the 1PPS signals are divided down
> > from 10MHz, their noise is also divided down, which is not the case for
> the
> > DMTD.
> > However, in the end I am comparing signals in the 1Hz to 5Hz or 10Hz
> > region, and apparently, the 5335A is not suitable for those, at least not
> > with the desired stability, is it?
> >
> >
> > Tobias
> >
> > On Fri, Apr 3, 2020 at 1:45 PM 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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