Here’s a dandy circuit for a VCO and buffer that operates across the entire FM broadcast band (88-108 MHz). I stole the main idea from the local oscillator in a radio shack scanner (pro2004). I like this design because it doesn’t require a tapped coil, it tunes very broadly, it’s stable, and it has a nice, hot output.
C1, C2, C8, C11 .1 uF C3, C4 .001 uF C5, C6, C7 39 pF NPO or silver mica C9 10 pF NPO or silver mica C10 22 pF CR1 ECG616 varactor (tuning) diode CR2 9v Zener L1 5T #20 wire, 1/8 inch I.D., adjust spacing for tuning range Q1 2N3663 Q2 2N3904 R1, R2 47 K R3 22 K R4 15 K R5 1 K R6 390 ohm R7, R8 4.7 K R9 100 ohm VR1 100 K, linear taper
As with all VHF circuits, pay particular attention to construction techniques. I recommend cutting little square islands on one side of a two-sided copper-clad board. Use the remainder of that side as the ground plane, and leave the bottom side to serve as a shield. If you keep all lead lengths short, the circuit is quite stable.
With the parts listed here, the effective frequency range extends well beyond the FM broadcast band in both directions. If a 6V Zener is substituted for CR2, the circuit will run from a 9V battery, with a slightly smaller tuning range. The output is hot enough that the signal can easily travel a city block using just a clip lead for an antenna.