For several decades, IFM (Instantaneous Frequency Measurement) systems have relied on the delay line discriminator circuit.
You can assemble something fairly simply from components that you can buy from MiniCircuits.
First of all put your signal through a limiting amplifier then drive a power divider with the output, one leg of the power divider then feeds the RF port of a schottky diode ring mixer and the other leg feeds the LO port of the mixer through a suitable delay line (some semirigid cable should work well). The mixer functions as a phase detector and the IF output of the mixer will, when filtered suitably, give an S shaped frequency vs voltage curve.
If you look on the Triquint website you can find some classic papers from Watkins-Johnson that give information on using mixers as phase detectors, MiniCircuits have an app note on this subject too I believe.