VHF band frequency modulation circuit using variable capacitance diode

VHF band frequency modulation circuit using variable capacitance diode

Function of the circuit

A frequency modulation FM transmitter that can be used in the FM broadcast band of 76~90MHZ is also commonly called a wireless microphone. The signal is received by an FM broadcast receiver. If a low-frequency signal is input without a microphone, the signal or data can be transmitted wirelessly. With a 60CM antenna, the transmission distance can reach more than 30 meters.

How the Circuit Works

TT1 is used to amplify the signal generated by the electret capacitor microphone to the working voltage of the diode. The 80MHZ frequency band signal is generated by the LC oscillation circuit formed by TT2. As shown in the figure, the oscillation circuit L is composed of winding the wire on a winding frame with a magnetic core. By adjusting the magnetic core, the oscillation frequency can be changed between 76~90MHZ.

The frequency of the resonant circuit is changed by using the varactor diode 1S2236, and FM modulation is performed directly. When the ECM output is 3MV, a modulation degree of ±25KHZ can be obtained.

The FM modulated signal is amplified to about 2.3MV by the high-frequency amplifier stage TT3 and then sent to the antenna.

Notes

FM modulation using the variation of the capacitance between transistor electrodes.

FM modulation circuits using varactor diodes can achieve larger frequency deviations, so they are mostly used in high-frequency circuits.

As is known to all, the inter-electrode capacitance of a transistor changes with the bias voltage, and this can be used to achieve FM modulation.

The circuit in the figure is a Colpitts circuit which is often used as a high-frequency oscillator. The base oscillation frequency depends not only on the coil L, circuit components C2 and C3, but also on the inter-electrode capacitance of the transistor. The total capacity of the oscillation tank is the sum of C2, C3 and the inter-electrode capacitance of the transistor.

Inputting a low-frequency modulation signal from the base changes the emitter current, and the capacitance between the collector and the emitter also changes, so the oscillation frequency also changes.

Although this circuit cannot achieve a large frequency deviation, it does not require other components for FM modulation and can be said to be a cheap modulation circuit. It is most suitable for use as a simple wireless telephone microphone circuit.