[time-nuts] Phase measurement of my GPSDO

[ew]

If you have an extra 10811 use it as an offset all mine tune 20 Hz + and I have used it as offset at 5 MHz , 10 Hz at 10 MHz all will do.
Bert Kehren
In a message dated 4/13/2020 5:07:38 PM Eastern Standard Time, tpluess at ieee.org writes:

Hi Bob
awesome, thanks! of course it is 1e6, not 1e7, I got a mistake :-)

Maybe I have some good OpAmps for this purpose in my box. I will try it! of
course I saw that my setup was not ideal as there was a bit of noise on the
signals which I guess does lead to some jitter in the trigger circuit and
therefore decreases my measurement noise floor.

Can you say something about how it would be done using a TIC?
I don't have two identically good counters, but the HP 5335A could be used
as TIC, couldn't it.

And the offset source I used is not directly the HP 10811, but the HP 8663A
Signal generator internally uses a 10811 as reference source. But I didn't
wait for days for it to warm up properly. (Should I?)

> Fun !!!
Yea, of course! :-)
I already implemented the ADEV, MDEV and TDEV calculations in Matlab by
myself. I use TimeLab to see what numbers I should expect, and then I want
to compute it all myself in Matlab because I want to see how it actually
works. ;-)


Best
Tobias



On Mon, Apr 13, 2020 at 10:50 PM Bob kb8tq <kb8tq at n1k.org> wrote:

> Hi
>
> Ok, first the math:
>
> If your offset oscillator is 10 Hz high at 10 MHz, you have a:
>
> 10,000,000 / 10 = 1,000,000 : 1 multiplier in front of the DMTD
>
> You get to add a 6 to what Time Lab shows you.
>
> If you are getting an ADEV at 1 second of 1x10^-4 then that multiplier
> gets you to 1x10^-10
>
> So, what’s going on?
>
> You can’t feed the mixer outputs straight into a counter. The counter
> front
> end does not handle LF audio sine waves very well. You need to do an
> op-amp based limiter. A pair of OP-37’s in each leg ( or something
> similar)
> should do the trick.
>
> Second, the offset source needs to be pretty good. A 10811 tuned high with
> both the mechanical trim and the EFC is a pretty good choice to start out.
>
> If you only have one counter, simply ignore the second channel. You are now
> running a single mixer. It still works as a comparison between the offset
> oscillator
> and your DUT.
>
> If you want to do it properly as a DMTD, then you set up two counters. One
> to measure mixer A and the other to measure mixer B.  Set them both up to
> measure frequency. Time tag the data files so you know which reading
> matches up with which.
>
> Fun !!!
>
> Bob
>
> > On Apr 13, 2020, at 3:18 PM, Tobias Pluess <tpluess at ieee.org> wrote:
> >
> > Hi again Bob
> >
> > I tried to do some measurements with a DMTD!
> > In my junk box I found a little PCB from earlier experiments on that
> topic,
> > with a power splitter and two SRA-3H mixers, it was even already wired
> for
> > the DMTD configuration. So I gave it a try!
> > As "transfer oscillator" I used my HP 8663A signal generator, and set it
> > high in frequency by 10 Hz. To the two mixers, I connected the two 10MHz
> > signals and at the mixer outputs, I put a little lowpass filter with
> 100Hz
> > corner frequency.
> > The output signals from the two SRA-3 mixers are almost 0.5Vpp, so I
> tried
> > to feed them directly into the HP 5335A TIC and used the TI mode to
> measure
> > the delay between the two signals.
> > This gives 10 readings/sec, which I try to process with TimeLab.
> > It does give some interesting graphs, but I don't know yet how to
> correctly
> > set up TimeLab for this kind of measurement. I.e. now, I get an ADEV in
> the
> > order of 1e-4 (at tau=1sec) to 1e-5 (at tau=500sec). So does that mean I
> > simply need to multiply this with 1e-7 to get the *real* ADEV at 10MHz?
> > this would mean that my real ADEV is in the range of 1e-11 to 1e-12,
> which
> > is indeed my target value, BUT I expect that things are not that simple.
> > (i.e. what if I didn't set the transfer oscillator high by +10Hz but only
> > by 9.9Hz for example).
> > Can you give some hints on that?
> > Of course I also did the noise floor test (i.e. I fed the 10MHz signal
> into
> > a power splitter and connected the two outputs to my DMTD with two
> > different lenghts of cables. This gave results starting at 1e-4 going
> down
> > to 1e-7, maybe it would have gone even lower but I measured only for a
> > couple of minutes.)
> >
> > Can you give some hints on that?
> >
> > Best
> > Tobias
> > HB9FSX
> >
> >
> >
> >
> > 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
> >>>
> >>
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