Source of propagation delay?

[Davor]

With exponential decay (and linear time) you may start observing signal at any point of time until it sinks under noise, and observe the same decay shape. With enormous gain of a preamp this is precisely what you get. Same mechanism applies to sinusoidal exponentially decaying signal, and again, with sufficient gain you'll be able to see exponential gain further on in time, just as you did.

[VinceC]

I have been examining the circuit operation of my Surf Pi 1.2 to better understand some of the issues associated with the design. The attached are screenshots from my scope that shows the trace of the voltage at the coil simultaneously with the output of the preamp. There are a couple of items that I noted.

1) It appears that using the output of the preamp (red trace) to select the value of the damping resistor (which is what I did) may not be the best approach. The coil voltage measurement (yellow trace) clearly shows significant ringing.

2) The output of the preamp (red trace) does not settle until ~25us but the coil (yellow trace) would appear to settle in <10us (assuming it was critically damped). Why the huge differences in time? Is it the opamp which is taking that long to recover? That does not make sense. Thoughts?

Vince