The circuit is a modified Colpitts oscillator, tuned with MV209 varactor diodes. The resonating inductor and the drain choke are selected by a rotary switch. 1N5711 Schottky diode, D, clamps the maximum positive voltage on the gate of oscillator FET, Q1 to just above ground to reduce the distortion in the wave form on the gate of Q1. Buffer Q2 provides about 1.8 volts peak-to-peak at 20 MHz to drive a frequency meter. Since the P-P voltage on the gate of Q1 is too large to be buffered by a 2N5485 without sever distortion -see the pictures further down this page), the signal is reduced by a capactitive voltage divider formed by 1 picofarad capacitor C11 and the input capacitance of Q3. This results in about 650 millivolts on the gate of Q3, and about the same on the source.

Varactor diodes replace the conventional 365-pF tuning capacitors, which reduces size and weight when used as a

Here`s a design circuit for frequency modulator that is completed with tuning circuit. In this circuit is using pair of 1N4007s are used as varactor diodes. I picked 1N4007s not because they are particularly well suited, but simply that I had some on the bench. One of the varactors is directly coupled to the xtal circuit through 1 nF so most of it

The curves for a capacitor are all markedly non-linear and this property can be used as an element in a circuit to modulate, demodulate and multiply frequency. This is non-linear reactance, rather than a resistance, however, and because of this, very little loss occurs. The charge in a capacitor is equal to the capacitance multiplied by the voltag