[time-nuts] Phase measurement of my GPSDO

[Bob kb8tq]

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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