Amateur FM transmitter circuit is shown in Figure 1.
Figure 1 is a more classic 1.5km single-tube FM transmitter circuit. Key components in the circuit transistor is fired, the use of D40, D50, 2N3866, etc., operating current 60 ~ 80mA. However, the above is difficult to buy into the transistor, and the price is higher, more fake. I use other transistor experiments, relatively easy to purchase the transistor C2053 and the C1970 is very good, the actual line of sight communication distance of more than 1.5km. I have the D40 tube into an ordinary transistor 8050, the working current has 60 ~ 80mA, but the firing distance of less than 1.5km, so if a switch from 9018, operating current is smaller, shorter firing distance. In addition to transmitting transistor circuit outside the coil L1 and capacitor C3 of the parameters of the more important, if you choose incorrectly, you may not afford to vibration or exceed 88 ~ 108MHz frequency range. Where L1, L2 can ? 0.31mm ? 3.5mm magnet wire around the rod 5 turns on a single level and 10 turns around, C3 use 5 ~ 20pF ceramic or polyester dielectric tunable capacitors. Actual production, the capacitor C5 can be omitted, L2 can be replaced by the ordinary 10 ~ 100mH inductors. If the transmission range as long as tens of meters, then the battery voltage can be selected as 1.5 ~ 3V, and D40, etc. 9018 cheap pipe replaced, power consumption will be less, but also refer to "electronic newspaper" No. 8 2000 fifth edition of "Simple Remote wireless FM microphone," a paper after the minor changes.
If its described in short, Figure 1 describes the single-tube transmitter has a simple circuit, the output power, making the characteristics of easy, but the inconvenience to access high-frequency radio signal sent to the outdoor cable to the transmitting antenna, usually is 0.7 ~ 0.9m of the rod antenna directly to make the C5 launch, due to the Doppler effect, moving around in the antenna, frequency drift phenomenon is very serious, so that the receiver would have a normal radio voice distortion or silence. If the transmitter for wireless microphone use, pinching the antenna, the frequency drift can imagine how serious.
Figure 2 2km FM transmitter circuit. The circuit is divided into oscillation, frequency, and power amplification levels. Circuit, V1, C2 ~ C6, R2, R3 and L1 composition capacitor three-point oscillator, the oscillation frequency mainly by the C3, C4 and L1 parameters of the decision, the oscillation frequency is 44 ~ 54MHz, the signal from the L1's center tap at the output that, through the C7 coupled to the V2 amplification, elected by the C8 and L2 44 ~ 54MHz second harmonic signal that the 88 ~ 108MHz, the signal carried by the C9 power amplifier coupled to the V3, V3 transistor in parallel by the three 3DG12 composition, can be expansion of output power. Normal operation of the circuit, the current about 80 ~ 100mA. V3 of the three components can be combined with the appropriate 3DG12 heat sink to prevent overheating. L1 ~ L3 when making use ? 0.31mm enamelled wire on the rod ? 3.5mm single flat around.
Figure 3 shows a practical wireless headset 50m FM transmitter part of the circuit. The circuit is divided into oscillation and signal amplification section. L1, C2 ~ C5, V1 composed of black and white TV tuner local oscillator circuit is similar to the improved capacitance three-point oscillator, frequency stability, long working hours do not run frequently, proved an amateur case, using the improved capacitance-type three-point oscillator fully competent. Electric branding iron I welded directly to the collector a few seconds after V1, the transistor under high temperature, using an ordinary radio receiver is still normal, non-running frequency phenomenon. Oscillator frequency is mainly determined by the L1 and C2, by fine-tuning L1, can cover the range 88 ~ 108MHz. Audio signal by R6, C11 coupled to the base of V1, V1 of the e, b capacitances of the change with the audio voltage changes caused by oscillation frequency, to achieve frequency modulation. L1 ~ L3 of the circuit with ? 0.31mm enamelled wire on the rod ? 3.5mm single flat around. L1-turn by adjusting the distance between the oscillation frequency tuning, and then fine-tune the L2, L3's turn to turn spacing of the resonance in the oscillation frequency, the maximum output power.
Figure 4 below shows the crystal oscillator transmitter circuit. Circuit J, VD1, L1, C3 ~ C5, V1 form crystal oscillator circuit. J, the quartz crystal frequency stability, but also less affected by temperature, it is widely used in cordless phones and AV modulator. V1 is 29 ~ 36MHz crystal oscillator transistor, the emitter output rich in harmonics and amplified by V2, the collector by the C7, L2 constitute a resonant network in the 88 ~ 108MHz frequency signal selected 3 (ie 87 ~ 108MHz the strongest signal), and then amplified by V3, L3, C9 get better after the selected frequency band FM signal. Frequency modulation process is like this, the audio changes in voltage caused by changes in capacitances of VD1, as VD1 J series with the crystal, the crystal oscillation frequency of small changes have taken place, the third harmonic, the deviation is 29 ~ 36MHz crystal deviation of 3 times. Practical applications, in order to obtain the appropriate modulation, frequency deviation can choose larger quartz crystal or ceramic oscillator, the circuit can also be used slightly more complex multiplier circuit 6 to 12. If the input audio signal is weak, can be coupled with a voltage amplifier.
