Phasemeter for frequency ringdown measurements

Projects & Applications
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I’ve been thinking about the possibility of using the moku phasemeter to track the exponential ringdown (of frequency) of a system. So far, I’ve been doing this in the least sophisticated way imaginable - I have knowledge of the starting frequency, so I simply run the Spectrum Analyzer with a 1 or 2 kHz span (so I have sufficient frequency resolution), acquire data, find the peak frequency, and then set the center frequency for the next iteration of the SA measurement to be the position of the last found peak. This sort of works because I didn’t need to know the instantaneous frequency at particularly high sampling frequency.

I am wondering if others have used the phasemeter instrument for similar applications? In principle, as long as one can arrange for the phasemeter to be locked to the frequency at the start of the ringdown, and provided the fastest rate of change of frequency is within the phasemeter bandwidth, I can track all parameters (amplitude, frequency and phase). The trouble is that the signal whose frequency I am trying to track has a few spurious peaks, and the phasemeter appears to lock onto these frequencies as the main signal frequency rings down. Is there some clever way to bypass this limitation? A bunch of software notches won’t do the job, as I guess the phasemeter will lose lock whenever the spurious spectral peaks (but also the signal frequency) get notched out. Do I have to do this programmatically, forcing the phasemeter to re-acquire lock every time the detected frequency stays constant for > some threshold time?

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Hi Gautum,
Interesting problem! I agree, that the phasemeter should be able to do this if you can solve the problems with locking to the spurious peaks either by some sort of (gentle) filtering or fiddling with the phasemeter bandwidth. With Mokus, there are often multiple ways to make the same measurement, so…crazy idea warning for the suggestions below.
Depending on the dynamics of the ringdown, you might want to try the Moku’s Time and Frequency Analyser. It can measure period of a signal by e.g. setting Interval A to start at Event A and stop at Event A. You can use the built in data logger to log this, or you could write the result out as a voltage (using the third tab “Output”) to monitor the period live on say, an oscilloscope or data logger in the next slot. You might have dead/noisy times in your measurement as the signal frequency passes through the frequencies of the spurious peaks, but assuming that your ring down signal amplitude is bigger than the spurious signals, this might work. At least there is no lock to drop so it should recover on its own. You could also try notching the bigger spurs out with the FIR filter instrument.

Or if you are super keen and smarter than me, maybe try training the Moku Neural Network instrument to remove the spurious peaks before feeding into the phasemeter. This would be super cool and would love to hear about it if you get it to work. Good luck!