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 _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
