[time-nuts] Phase measurement of my GPSDO

Mon Apr 13 21:53:07 UTC 2020

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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