From the 2023 Dayton Hamvention, I came home with an ICM Frequency Meter for $5.

At it's heart, it is a crystal oscillator which generates a 12th or 24th harmonic to beat with transmitters to be measured in the 150 or 450 MHz bands.  Oscilloscope measurements and simulations are in the slides below.

A brief dabble into Frequency Marker Harmonic Generators.