LC Oscillator Gain Compression
You have to control the amplitude of oscillations by feed back loop in order to keep the oscillator in a linear range. Yes, you are right: it is not possible to do by manual adjustment.

However: if you use clean PCB design and you select a right MOSFET as an adjustable resistor - it can work fine for gain adjustment.

Your circuit needs few modifications to use MOSFET:







simulate this circuit &ndash; Schematic created using CircuitLab

P/N for Q1 and M1 have no sense.

You need to select the high frequency MOSFET with very low output capacitance for M1. The control voltage for its gate is, most likely, in range +2..+4 V.

Generally, I would suggest increasing C1 and decreasing C2 - to get higher Q factor for the resonant circuit. Your transistor seems to have too low transition frequency for this application: 250 MHz min. I suggest 10 times higher frequency than the operating one (90 MHz), this leads to a GHz range. There are plenty of cheap BJTs with 1-2 GHz transition frequency from NXP.

SMD design is more suitable for this application.