[time-nuts] Averaging effects

Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Dec 27 22:49:03 UTC 2010


Then add an isolation amplifier with at least 40dB reverse isolation 
between each OCXO and the measurement system and repeat.
If the shielding is adequate (and coupling via power supplies or other 
stray paths is sufficiently small) any injection locking effects should 
be significantly reduced.

Bruce

Tom Van Baak wrote:
> Hi Magnus,
>
> Another test that's really fun to do is to measure what effect
> DUT frequency differences have on stability measurements.
>
> Let me explain. Say you measure the ADEV of DUT1 using
> DUT2 as the reference. Theoretically this measurement is
> independent of the relative frequency offset between the two
> independent frequency standards. This because, ADEV is
> immune to any constant phase or frequency offset.
>
> But in the real world, as you have found, and as I have seen,
> you can sometimes see unexpected biases or perturbations
> depending on the value of the frequency difference.
>
> This affects the quality of the measurement -- and unless you
> know ahead of time how susceptible or immune your system
> is to this, the fidelity of your results are in question. And by
> system, I mean the sum total of both DUT and the instrument
> used to measure.
>
> Call it injection locking, or frequency pulling, or some sort of
> measurement system artifact involving interpolators or DDS
> or ADC quantization, or analog or digital filtering, or output
> resolution, or whatever. At a high level it doesn't matter what
> the actual cause is -- you just want to know if it exists and if
> so, what its magnitude is.
>
> So here's how to see this with your own eyes. Start with two
> free-running, slightly drifting BVA. Find the one that is, say,
> drifting up in frequency and calculate the approximate drift
> rate per hour. Then set its frequency low by about that same
> amount.
>
> Now run your measurement system continuously for 2 hours.
> The relative frequency difference between the two will very
> gradually go from some negative value, through zero, up to
> some positive value. All the while you're collecting data. The
> more data the better; the more gradual the better.
>
> Using DAVAR, or by binning the samples and using ADEV
> (or even just STDEV) you can then clearly see if there are
> any gradual changes, or asymmetry, or quantized spikes in
> the measured stability as a function of frequency difference.
>
> /tvb
>
>





More information about the time-nuts mailing list