Working principle of mono frequency modulation circuit

The figure below is a more classic 1.5km single tube frequency modulated circuit. Key components in the circuit are transmitted transistors, multi-use D40, D5O, 2N3866, and the like. The operating current is 60--80mA. But the above triodes is difficult to purchase, and the price is high, and there are many fakes. The author chooses other triode experiments, relatively priced trioders C2053 and C1970 are quite good, and the actual distance communication distance is greater than 1.5km. The author also replaced D40 to ordinary triode 8050, and the working current is 60--80mA, but the emission distance is less than 1.5km. If it is changed to 9018, the working current is smaller, the emission distance is also shorter, the circuit In addition to the transmitting tripod; the parameters of the coil L1 and the capacitor C3 are more important. If it is not properly selected, it does not control the void or the operating frequency exceeds the range of 88--108 MHz. Among them, L1, L2 can be used on a round rod of 3.5 mm in a single layer of about 5 and 10 hinders, and C3 is selected from 5 to 20 pF of porcelain or polyester adjustable capacitance. When actually produced, the capacitor C5 may be omitted, and the L2 can also be replaced with a normal inductor coil of 10-100MH. If the emission distance is tens of meters, then the battery voltage can be selected from 1.5-3V, and the D40 tube is replaced by a cheap 9018, etc., the power consumption will be less, or the "Electronic News" 2000 No. 8 The fifth edition (simple long-distance wireless frequency modulated microphone) is slightly changed. The single tube transmitter introduced in Figure 1 has a simple circuit, the output power is large, and the production is easy to make, but it is inconvenient to send the high frequency cable to transmit the RF signal to the outdoor transmit antenna, which is generally a rod of 0.7--0.9m. The antenna is directly transmitted on C5. Due to the Doppler effect, people move around the antenna, the frequency drifting is very serious, making the sound of the receiver that the radio is normal. The sound is distorted or silent. If the transmitter is used as a wireless microphone, when the antenna is pinched, it can be imagined. Since the FM transmitter uses a capacitor three-point oscillator, the antenna parameter will occur when the antenna parameter will occur, and the running frequency will occur. If it is a single tube self-excited oscillation, the operating current is large, when the work is second to several minutes, the triode The temperature rise causes the pole capacitance to change, and the oscillation frequency is changed (in general, the oscillation frequency is lowered), and sometimes the float is from 0.2 to 1 MHz. It is poorly reliability when used as FM broadcast or long-distance remote alarm, but the components are less, low cost, and easy to debug, suitable for primary enthusiasts.

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