FM Oscillator Circuit

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.

FM Oscillator Diagram, circuit diagram, circit, radio diagram, radio circuit diagram, Circuit design

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.

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