Varactor Voltage Controlled VFO

An alternative to finding and mounting a venier drive for the tuning capacitor, this VVC tuned circuit can make life easier.

The MV2109, MV2104 or similiar diodes are recommended. Build the circuit underneath the board directly to the ground plane near the VFO circuit.

Use one tuning diode (MV2), R1, and C1 as stand-offs (one end soldered to ground) to mount the transistor and choke. The other tuning diode connects to the "WIDE" copper pad.

The 2N3904 is used as a temperature compensation diode for the VVC diodes.

R2 (470 ohm) is used as a current limiter to protect the transistor and diodes. R2 can be raised in value, but do not lower the value. R3 (3.3k) can be changed to any value.

Frequency changes slower at lower voltages, so using a high value for R3 raises the low voltage the VVC's see, and provides a smoother frequency change over the range of the variable 10 turn resistor (See Chart).

Raising the 5 volt supply to 12 volts will give a greater range. Use a voltage regulator or a very stable supply, as the most minute change in voltage will change the frequency.

The diodes will add about 10 to 20pf to the tuned circuit. To compensate, lower the value of C35 10 to 30pf and recalibrate your VFO frequency.

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Last Update: 11-24-2002
Web Author: David White, WN5Y