VFO A VFO (variable frequency oscillator) allows you to operate across a range of frequencies without investing in a lot of crystals and without the complexity of a PLL circuit. There are 4 keys to building a stable VFO: 1. Use a regulator IC or a zener diode to regulate the voltage going to the oscillator. Voltage fluctuations produce frequency drift. 2. Build the circuit carefully. Keep component leads short, do your best soldering work, and make sure all the components are physically secure (no wiggling). 3. Choose your inductor and variable capacitor carefully. Air-core inductors are more stable across a range of temperatures than iron-core inductors. Air-dielectric variable capacitors are more stable than mica compression and other types. 4. Put a buffer amplifier between the VFO's output and the rest of the transmitter. Failure to do this makes the VFO vulnerable to being "pulled" by modulation, by changes in antenna loading, etc. The circuit shown here is a series-tuned Clapp oscillator. Unlike the parallel LC tanks seen in many oscillators, this one has the LC components in series. Similar VFOs have been used successfully in many ham radio transmitters and receivers. This design has not yet been tested by MWA members. Let us know what results you get if you try it. Update: You can see another VFO design in the Poppet schematic. Parts: C1: 4 to 20 pF, air-dielectric variable capacitor       (such as Antique Electronic Supply catalog number C-VT20) C2: 100 pF polystyrene C3: 82 pF polystyrene (for approx. 1640-1700 kHz tuning range)       or 100 pF polystyrene (for 1570-1630 kHz tuning range) C4, C5: 1000 pF C6: .01 uF ceramic disk C7: 50 pF polystyrene D1: 1N914 L1: 47 uH inductor Q1: MPF102 R1: 100 K, 1/4 watt R2: 470 ohm, 1/4 watt RFC: 1 millihenry choke VR1: any 8 or 8.2 volt, 1/2 watt zener diode (such as NTE 5016A) With special thanks to the late Doug DeMaw, who taught so much to so many.

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