[time-nuts] Phase measurement of my GPSDO

Tue Apr 14 12:35:46 UTC 2020

Hi

yes sure I agree that it is hard to beat. But from a
commercial perspective, any hobby is more or less nonsense - the same is
true for my own GPSDO. I just could have bought one and would be finished.
But making my own is more interesting ;-) I'll see whether I buy a TAPR
TICC. Maybe it would come in handy to have something I can compare with.

Tobias

On Tue., 14 Apr. 2020, 14:27 Bob kb8tq, <kb8tq at n1k.org> wrote:

> Hi
>
> Indeed all a time tagger does is spit out picoseconds since some arbitrary
> start point. Some run on and on forever ( counting up to pretty big numbers
> in the process). Others roll over at a pre-defined point. You then massage
> the data to take those out.
>
> I’d suggest that the “software/ firmware included” and “fully debugged”
> nature
> of the TAPR TICC make it pretty hard to beat unless you are planning to
> build a
> couple dozen ….
>
>
> Bob
>
> > On Apr 14, 2020, at 8:11 AM, Tobias Pluess <tpluess at ieee.org> wrote:
> >
> > 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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