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I feed the RFfrom a signal generator through a resistor, then read the
voltage across the trap as it is tuned. The resistor value depends on your
test equipment. If you have a 50 ohm source, start with maybe 450 ohms to
give a ten to one ratio. The voltage drops as you approach the resonant
frequency. I've used this method for almost 50 years to identify the SRF
frequency of inductors, as well.
You can also use this method to pre tune an RF circuit before powering it
up. It puts you close to the ideal settings. I'm a retired Broadcast
Engineer.