Because the Analog Discovery 2 makes doing impedance spectroscopy so easy, I decided to do a quick check of my inductors to plot their impedance, checking the series resistance in the process. This was just going to be a short interruption to my day of working on my book, but it ended up taking up most of the day, because I got interested in seeing whether I could determine the characteristics of the scope inputs that were limiting the performance at higher frequencies.
The data looks fine up to 1MHz, but above that is a resonant peak, probably from the capacitance of the oscilloscope and the wiring to it.
I tried modeling the oscilloscope inputs as capacitors, but that resulted in way too sharp a spike at the resonance to match the data, so I tried a resistor in series with a capacitor. Initially, I tried modeling both channels identically, but I got better fits when I used a different model for each channel:
It might be interesting to swap which channel is connected to which device, and see whether the R+C models still fit well, but I’ve not got the time for that tonight. I did have some earlier data (from playing with resistor sizes) and I fit the oscilloscope models to it: