[time-nuts] Phase measurement of my GPSDO

Mon Apr 13 23:37:33 UTC 2020

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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