[time-nuts] Phase measurement of my GPSDO
Bob kb8tq
kb8tq at n1k.org
Tue Apr 14 17:10:17 UTC 2020
Hi
Have you tried it with the latest firmware update?
I’ve never seen the problem here.
Bob
> On Apr 14, 2020, at 1:05 PM, Taka Kamiya via time-nuts <time-nuts at lists.febo.com> wrote:
>
> Bob, Tobias, et al
>
> TICC (TAPR) isn't problem free either. It has a tendency to get TimeLab confused on data from port A and port B. The data stream has identifier on them but TimeLab discards it. Then it expects A and B comes alternately. I communicated with both developers but for time being, the solution is to record the data and inspect.
>
> ---------------------------------------
> (Mr.) Taka Kamiya
> KB4EMF / ex JF2DKG
>
>
> On Tuesday, April 14, 2020, 7:48:34 AM EDT, 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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