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An experimental crystal oscillator FM transmitter

Many radio enthusiasts want to make an FM transmitter, especially in the 87-108MHz FM band, which can be received by off-the-shelf FM radios, which is popular among everyone.
Examples of FM transmitters have been introduced in many publications, but most of them use capacitive three-point circuits and Krato oscillator circuits. Although this circuit is simple, its frequency stability is not high, especially in amateur conditions, the frequency changes when the circuit board or antenna position is slightly moved. Here I introduce a FM transmitter that uses a crystal to stabilize the frequency.
Circuit principle

As shown in Figure 1, a level 1 audio amplifier circuit is formed by V1 and related resistance-capacitance components, which provide a sufficient intensity audio signal for the modulation level. D1 is a varactor diode, and its equivalent capacitance changes with the reverse voltage applied by the two poles, so that the center frequency of the oscillator composed of the crystal oscillator and the peripheral circuit changes accordingly, and the purpose of frequency modulation is achieved. The signal output by the oscillator is frequency-multiplied and amplified by V3, and then output by the tuning transformer after matching and filtering.
This circuit uses a tuned transformer, so the core must be adjusted after production to make it match. The method is to make a simple field strength circuit (as shown in Figure 2), connect it to the output of the transformer, and adjust the magnetic core until the ammeter indicates the maximum value. Use components with good high-frequency characteristics as much as possible for the components used in the circuit. The crystal chooses a crystal with a nominal value between 29 and 36MHz. D1 can use MV2105. The transformer needs to be self-made. The middle of the TV can be used as the skeleton. The shielding cover is removed, and the enameled wire around ∮0.2mm is wound around the skeleton for 3 turns. Level turns 1 turn. The antenna can be replaced by a 1/4 wavelength flexible wire.

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