[time-nuts] Phase measurement of my GPSDO

Tue Apr 14 12:26:45 UTC 2020

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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>>>>>>>> 
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