An experimental crystal oscillator FM transmitter circuit

An Experimental Crystal Oscillator FM Transmitter Many radio enthusiasts hope to make an FM transmitter, especially in the FM band of 87 to 108 MHz, which can be received by ready-made FM radios, so it is favored by everyone. Examples of FM transmitters are introduced in many publications, but most of them use capacitive three-point circuits and Clapp oscillation circuits. Although this circuit is simple, its frequency stability is not high, especially under amateur conditions, the frequency will change if the circuit board or antenna position is slightly moved. Here the author introduces an FM transmitter that uses a crystal oscillator to stabilize the frequency. The circuit principle is shown in Figure 1. V1 and related resistor-capacitor components form a first-level audio amplifier circuit to provide an audio signal with sufficient strength for the modulation stage. D1 is a varactor diode, and its equivalent capacitance changes as the reverse voltage applied to the two poles changes, thereby causing the center frequency of the oscillator composed of the crystal oscillator and peripheral circuits to change accordingly, achieving the purpose of frequency modulation. The signal output by the oscillator is multiplied and amplified by V3, and then matched and filtered by the tuning transformer before being output. This circuit uses a tuning transformer, so its core needs to be adjusted after fabrication to make it match. The method is to make a simple field strength circuit (as shown in Figure 2), connect it to the output end of the transformer, and adjust the magnetic core until the ammeter indicates the maximum value. The components used in the circuit should try to use components with good high-frequency characteristics. Choose a crystal oscillator with a nominal value between 29 and 36MHz. D1 can use MV2105. The transformer needs to be homemade. You can use the middle periphery of the TV as the frame. Remove the shielding cover and use an enameled wire of about ∮0.2mm to wrap 3 turns of the primary around the frame. Level winding 1 turn. The antenna can be replaced by a 1/4 wavelength soft wire.