This circuit uses a double balanced mixer/oscillator in the Philips SA612. The tuned components, L1 is wound
on a 1/4 inch diameter with 6 turns of 22AWG wire. This set up is for 100MHz operation but can be scaled
up to 200MHz. The 30pf tank capacitor can be replaced by a small trimmer to extend the measurement range.
No anti-image filter is used on the input, since the measured frequency will be roughly known and you want
to know if a signal is present or not.
The dual gate mosfet amplifies the IF signal and also amplifies the sum signal, which the low frequency scope
will not see and is filtered out. Gate 2 of Q1 is connected to a trimmer potentiometer and sets the stage
gain at a fixed level. If calibration can't be done because you don't have access to a 100MHz scope, then the
probe can still be used to measure relative levels of signals.