[time-nuts] Phase measurement of my GPSDO

Mon Apr 13 22:11:08 UTC 2020

Hi Bob

Riley suggests to use a single TIC

http://wriley.com/A%20Small%20DMTD%20System.pdf

when you look at the block diagram Fig. 4, you can see that one TIC allows
to compare two oscillators.
I don't know exactly how, though :-)

OK and I see your point on the 8663. I will try to use another reference!
I definitely didn't keep mine on for a long time. I didn't use the signal
generator for a while now, so it was unplugged for a few months. I assume
that's far from optimal for the 10811's stability.

Tobias

On Mon., 13 Apr. 2020, 23:53 Bob kb8tq, <kb8tq at n1k.org> wrote:

> Hi
>
> > On Apr 13, 2020, at 5:06 PM, Tobias Pluess <tpluess at ieee.org> wrote:
> >
> > 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
>
> You need something that is quiet (like the OP-37) and has a pretty good
> slew
> rate. Past that, there are a lot of candidates. The TI OPA-228 family is
> one.
>
> > 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.
>
> Typically a good limiter takes you from 3 or 4 digits up to 6 or 7 good
> digits.
> Net result is a measurement that’s good in the vicinity of parts in 10^-13
>
> >
> > 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.
>
> The standard way of doing the test is to run two counters / two TIC/s /
> two whatever’s.
> I know of no practical way to do it with a single 5335.
>
> >
> > 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?)
>
> The 8663 synthesizer adds a *lot* of crud to the 10811. Regardless of how
> you
> use the 10811, it needs to be on for a while. How long very much depends
> on
> just how long it’s been off. Best to keep it on all the time.
>
> >
> >> 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. ;-)
>
> Be careful any time you code this stuff for the first time. It’s amazingly
> easy
> ( = I’ve done it ….) to make minor errors. That’s in no way to suggest that
> you should not code it up yourself. I generally do it in Excel or in C.
>
> Bob
>
> >
> >
> > 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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