Voltage Controlled Oscillator for Commercial FM Band
VCO (voltage controlled oscillator) is an electronic signal generator which produce a signal that has variable frequency depends on a voltage level at its control input. Many VCO circuits are based on ramp generator to produce a variable frequency output, but they produce a square wave signal. With varactor (voltage controlled variable capacitor), we can produce a high quality sine wave output comparable to standard LC circuit since a varactor is basically similar with the capacitor in LC oscillator circuit.
The oscillator configuration is similar with you might see on old SW transmitter oscillator stage with ordinary variable capacitor (trimmer cap), but with additional varactor in parallel. The 82 pF fixed cap (connected in series with the varactor) is used to prevent the DC current from control signal being shorted to ground through the coil. A control voltage from 3 to 10 volts will adjust this VCO output from 100MHz to 120MHz, but the exact value can be further shifted by the trimmer variable capacitor 3-35 pF.