Simple high sensitivity, a transistor amplifier, low-power FM transmitter, FMTransmitter

The radio frequency (RF) oscillator portion of this FM transmitter is connected to a high sensitivity, a broadband audio amplifier and a capacitive microphone, and there is a field effect transistor (Field Effect Transistor, an abbreviation FET) for the RF oscillator. The base is modulated, as shown in the following figure. Be Transistor Q1 constitutes a relatively stable RF oscillator, and its frequency is determined by the coil L1 and the tuning capacitor C4. By setting C4, the desired operating frequency is determined, and the tuning circuit is designed to produce a frequency of 110 MHz in a standard FM broadcast band. Capacitor C2 provides the necessary feedback voltages through the resistor R3 in the Q1 emitter circuit to maintain oscillating conditions. Resistors R1 and R2 provide the correct operation of the emitting junction voltage required, while the capacitance C1 will be applied to all radio frequency signals of the base. The energy storage circuit composed of capacitance C3 is L1 and C4 provides a radio frequency passage while blocking the power supply voltage of the collector of Q1. The audio portion uses a high-sensitivity tolerance microphone (ML) and built-in field effect transistor, and will clearly pick up the low level sound in the spectral range of the speech spectrum. The speech voltage generated by ML in R9 is coupled to the base of the audio amplifier transistor Q2 via capacitor C6. You will notice that the DC bias of the oscillating transistor Q1 is provided by the resistors R1 and R2. A signal voltage generated on R6 is now coupled to the base of Q1 via a resistance R4 from a non-polar capacitor C5. The gain of Q2 is controlled by the ratio of R7 and R6. The DC operating point is set to the maximum change in the collector to the amplified signal. Now, the amplified voice signal is modulated by the oscillating circuit by ft, the oscillation circuit is modulated by a small amount of moving the base portion. Resistance RL0 is coupled to oscillators and audio circuits to prevent feedback and other undesirable effects. After proper assembly, when the receiver is tuned to the transmitter frequency, the quality clear sound should be received. Note that one capacitor can be connected to the base of the transistor Q1 to reduce sensitivity. The circuit made of the included element is preferably working in the high end of the FM band, which is a clear point that is not subject to the FM broadcast station. However, satisfactory performance is obtained in a limited frequency range of 110 MHz or more. Since this is a aviation communication band, be careful. Should not be used near the airport (as shown in the following table below). Be 1. Assembly steps as shown in the figure below. (1) Table plate (1IN = 2.54 cm) of a long x-width × thick = 2in × 1in × 0.1 in. (2) Tight 8 # 16 # busbar on the 8 # screwdriver, and the coil L1 is made. This will make an 8-turn coil that is approximately 0.135 in. Insert it into the appropriate hole and weld it as shown. (3) Insert the fine adjustment capacitor C4 into the hole shown in the figure. You can put this component on any side of the PC board. This is selected by you, you can determine according to the final assembly graphs you need. Here, 3 holes are prepared here, and the two holes are electrically the same. Since some fine-tuning capacitors may have three pins, this design is necessary. Be sure to connect the public pin to the same electrical point, connect the extra pins to other points. (4) If you use a porous plate to make it, it is recommended to start the interpolation device from the lower left corner. Pay attention to the capacitance with polar symbols and the polarity of all semiconductor components. The pin of the mounting element is shown, and the excess wire is cut off. Try to use a certain number of pins or 24 # bus sections as connecting. The broken line on the assembly chart indicates the trace on the bottom surface of the mounting plate. (5) Connect the external pin (CL1) of the battery holder. (6) Repeatedly check the quality of the line and solder joints, avoid crossing, short circuit, and is too close to other components. If the bridging is necessary, you need to put on the insulated sleeve on the pin to avoid any possible short circuits. (7) Tuning an FM receiver to a radio station on a high end (108MHz or higher) on the frequency band. Put the volume and place the receiver to 25 to 50 ft from the circuit. (8) Tandem a milliam table with the battery pins. This can be implemented by unpacking a button of the battery holder, and is connected to the idle contact, as shown in Figure 3. The readings of the table should be 5 ~ 10mA. Starting from one end of C3, a single coil is exposed to a short bare wire. Note that when you leave C3 in one turn, the current indicated on the table will fall or change. (9) In the case where there is a battery, the C4 is slowly turned slowly with an insulated tuning rod until the radio received by the radio is disappeared or switched off in audio feedback. Since the tun is very sensitive, the first discovery signal will be more difficult. Also note that there are several points in the adjustment possible to be wrong, and the signal is very weak and unstable. The correct signal should be relatively strong, stable and clear; and verification can be performed by a transmission distance. (10) Once the expected setting of the C4 is found, the frequency value is used to note. (11) After the operation is verified, it is best to package the circuit board into a plastic box with a battery for tuning and operation. 2. Note One of the things that use this type of device need is the correctness of tuning. Adjustable capacitance C4 is very sensitive, only need to gently move a little bit when changing frequencies, so there is always a tuning rod. If you are not familiar with the device, it is very easy to tune into a wrong signal. This phenomenon is easily happened when the device is close to the monitoring receiver. As mentioned above, if it is a signal, the error signal will be weak, distortion and unstable (it is often misunderstanding as the main signal, and guilty is not good in the device), and the main signal will be strong and stable. Not really true. Before using the device for the desired application, a few tuning experiments should be made. Also, as long as possible, the device should be used in the 108-109 MHz band, which is located on the edge of aviation communication, above the FM broadcast band. When an approximate expectation point is found, the last tuning is performed at the receiver end, and it is clearly available. In most parts, these "high-end" frequencies of the FM broadcast band are clear, allowing uninterrupted use, and the low end of the frequency band changes a little bit from a clear frequency, it will be overwhelmed by the strong broadcast station signal. If it is near the airport or aircraft route, don't use 108 MHz. Most FM radios are easily missed, and 108MHz on the dial is 109MHz. This task can be accomplished by carefully adjusting the "oscillator", and the fine-tuning capacitor is located on the main tuning capacitor. The antenna peak adjustment tablet should now be adjusted to obtain the maximum signal with high frequency. To get the best performance, you will need a high quality receiver with analog script-tuned tuning dial. Digital tuning receivers cannot complete this task well.

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