[time-nuts] Phase measurement of my GPSDO

Tue Apr 14 12:11:01 UTC 2020

G'day

OK I see I must do it with time tagging :-)
Is it correct that the time tagging just spits out the time (in ns, for
example) when the rising edge on the A or B input occured? and then, you
calculate the phase by subtracting the time tags for the A channel from the
time tags for the B channel?
Riley also says that DMTD works better with time tagging, so I am not
surprised that you recommend it as well. However I hoped that some simple
measurements (only to get a ballpark figure) would be possible with my
current setup.
In fact, since I read the Riley paper about the DMTD system he built, I
have had my own design on my bucket list since quite a while. I planned to
make my own time-tag counter with two TDC7200 as interpolators, to get ps
resolution, very similar to the TAPR TICC.

Tobias

On Tue, Apr 14, 2020 at 1:48 PM Bob kb8tq <kb8tq at n1k.org> wrote:

> Hi
>
> If the phase slips are “well behaved” they can be handled. The problem
> with a dual channel setup is that they are often not well behaved.  The
> period is 100 ns so a frequency drift of 1 ppb will put you in trouble in
> under 2 minutes.
>
> The only real answer is to do it properly and time tag the two outputs.
> Any other approach will get you yelling and screaming at the test set.
> Playing with two counters and not time tagging is in the “yelling and
> screaming” category as well.
>
> Get a TAPPR TICC if you really want to do a DMTD.
>
> Of course you *could* just use a single mixer. That works fine with the
> counter you already have. It will give you an A to B test just like a
> DMTD. The only limitation is the need to tune at least one of the
> oscillators
> in each pair.
>
> There is no requirement that you tune only one. If both are tunable,
> you could tune one to the high end of its range and the other to the low
> end.
> With most OCXO’s, there is plenty of tune range.
>
> Bob
>
> > On Apr 14, 2020, at 2:23 AM, Tobias Pluess <tpluess at ieee.org> wrote:
> >
> > 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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