Bredhurst Receiving and Transmitting Society

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As was said on the previous page :-

VARICAP OR VARACTOR DIODE TUNING

When DIODES have a reverse voltage applied they exhibit the characteristics of a capacitor. Changing the applied voltage alters the capacitance. Most diodes can be used but varactor diodes (D1 & D2) are specifically manufactured for oscillator and tuned circuit use.

Schematic of a varactor tuned Hartley Oscillator

The Hartly Oscillator, formed from L1 Ct C1a/b to gether with D1 and D2, generate a frequency that is in the RF ranges. Using two varactor diodes D1 and D2 back-to-back in series halves the total varactor capacitance.

This arrangment also eliminates effect of any RF present in a tuned circuit which only drives a single diode as on peaks of the frequency could the bias voltage and the oscillator would then be unstable and may well produce harmonics.

Varying the bias voltage between 1 and 8 volts  to the varactor diodes will give a typical capacitance swing or variation in value (depending on the diodes used) from say 10 pf to say 250pf.

Its is the varying of the capacitance which changes the frequency of the oscillator, , as the applied bias voltage is varied. The applied voltage must be from a well regulated (psu's/zener diodes) stable supply, obviously if the bias voltage drifts (varies) so will the oscillator output.

FREQUENCY MODULATION.

Replacing the bias supply to the varactor diode with a varying voltage from a simple microphone audio amplifier will produce frequency modulation of the oscillator when you speak in the microphone, the audio voltage can be limited by a variable resistor in line to prevent over deviation.