[time-nuts] Phase measurement of my GPSDO

[Tobias Pluess]

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
>>
>