[time-nuts] Low cost alternate to Dual Mixer/DMTD
bruce.griffiths at xtra.co.nz
Sat Oct 3 01:00:38 UTC 2009
> ws Reply to Bruce
>> You also need to measure the EFC slope at the operating point as the EFC
>> transfer characteristic can be highly nonlinear.
> Yes there is lots of things that can be done wrong but
> Another one of this configuration's many advantages is that the operating
> range of both the EFC and the Phase detector is very small,
> typical under a millivolt, so nonlinearly is NOT a problem.
> To calibrate end to end so that everything is included, The DUT can be changed by a
> small known offset, of say 1e-8 and measure the voltage change at the DVM/ADC output.
> Mine is calibrated for 1 mV per 1e-10 at the EFC,
> That calibration is linear over > than a 1 Hz (1e-7) offset range.
Whilst that may be true for your OCXO, this is certainly not true for
every ocxo one may wish to measure.
Since mixer nonlinearity near zero phase isnt an issue with a quiet ocxo
pair, a traditional diode double balanced mixer phase detector would be
appropriate as it hs lower noise than any other mixer/phase detector.
>> Also need to ensure that injection locking doesn't occur
>> through injection via the EFC input.
> Yes, One of the reasons for the isolations transformers (and lots of bypass caps).
The isolation tranformers are not useful for blocking RF, nor in general
is an arbitrary bypass capacitor array.
> If there is ANY ground noise between the Oscillators it can effect the EFC voltage.
> Like all low level uV signal measurement and control, a lot of attention HAS to be paid
> to insure there is no added noise or errors. This takes good analog and digital understanding
> of possible noise sources. Differential input and output amps go a long way to insure
> there is no ground loops, offset voltages or noise coupled errors.
These measures have little or no effect on RF injection via the EFC input.
Effective filtering of the phase detector output and use of a high
isolation mixer low noise is required.
>>> Maybe due to the fact that the Osc are locked.
>> That is the worst possible case for injection locking.
> Possible, but not exactly what I've seen in this configuration.
> The effect of coupling between Oscillators is very phase sensitive,
> and can be positive, neg or null as their relative phase shifts.
> By adjusting the phase in the way I noted any changing effects
> can be easily seen when there is ANY interaction between Osc.
I'd still prefer to compare results with and without the isolation
>>> This is verified by adding a slow low level freq modulation on the DUT
>> Probably not a good test for injection locking as a small shift in
>> frequency from equality rapidly attenuates the effective injection
>> locking signal.
> Another one of the advantages of this circuit is that there is NO shift in freq
> from equaqlity (If that means what I think it is). In any case,
> I have found this to be a good test because ANY coupling of any type between
> Oscillators causes a nonlinear transfer function, as a function of voltage and/or freq.
> So by checking that the transfer function between the DUT EFC input
> and the fast ADC output is linear and freq independent over a wide range of signals,
> non coupling is assured down to the level of the noise.
> Another check I did was to unlock the two Osc and add an freq offset,
> to see if that caused ANY effect at all on the other Osc.
If there is another contributor to phase locking then the full frequency
instability wont be reflected by the EFC input fluctuations.
>> Better test is to insert a very high reverse isolation amplifier between
>> each ocxo and the mixer and see if that makes any difference.
> Maybe so, and I did try to do that per your early suggestion, but the test was unsuccessful
> because of my poor isolation/buffer amps. It's something I'll get back to when
> I've lower the noise more to see if there is anything below the present noise level.
> From: "Bruce Griffiths" <bruce.griffiths at xtra.co.nz>
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