[time-nuts] Phase measurement of my GPSDO

Tue Apr 14 06:23:05 UTC 2020

Hey Bob

ok now I see your point! you talk about the phase spillovers. Timelab and
also Stable32 can correct for them, so it shouldn't be a problem, right?

But I agree, if you cannot correct for the spillovers it becomes even more
difficult.

Tobias

On Tue., 14 Apr. 2020, 01:38 Bob kb8tq, <kb8tq at n1k.org> wrote:

> Hi
>
> The gotcha with using a conventional counter (as opposed to a time tagger)
> is that you never know when things are going to “slip” past each other.
> When they
> do you get a major burp in your data. Bill’s setup is running a time
> tagger ….
>
> ( = It runs an internal time count, each edge gets “labeled” with a
> precise time
> stamp that is good to nanoseconds or picoseconds. A Time Interval Counter
> simply measures the time between edges. That sounds like the same thing,
> but
> it’s not quite ….)
>
>
> > On Apr 13, 2020, at 6:11 PM, Tobias Pluess <tpluess at ieee.org> wrote:
> >
> > 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 :-)
>
> The gotcha with using a conventional counter (as opposed to a time tagger)
> is that you never know when things are going to “slip” past each other.
> When they
> do you get a major burp in your data. Bill’s setup is running a time
> tagger ….
>
> >
> > 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.
>
> Best approach is to mount your reference off on it’s own and just power
> it. That way
> you don’t wear out all the guts of a fancy piece of gear.
>
> Bob
>
> >
> >
> > 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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