Simple circuit to convert a RF frequency (~100MHz) to voltage without using IC
A PLL is the best way, especially since your signal is in the middle of the FM broadcast band.  But if you want the circuit to be passive, you can use a slope detector.  Make a narrowband filter at 100 MHz.  Then use a diode to make an envelope follower (also called an AM detector circuit, like a crystal radio.)  The signal strength will be greater when the signal is closer to 100 MHz.