Single-channel FM transmitter
Figure 1 is a more classic 1.5km single-tube FM transmitter circuit. The key components in the circuit is fired transistor, multi-use D40, D50, 2N3866, etc., working current 60 ~ 80mA. But the more difficult to buy the transistor, and the price is higher, more fakes. I use other transistor experiments, relatively easy to buy the transistor C2053 and the C1970 is very good, the actual line of sight communication distance of more than 1.5km. I have replaced the ordinary triode tube D40 8050, with operating current 60 ~ 80mA, but the firing distance of less than 1.5km, so if the switch from 9018, operating current is smaller, shorter firing distance. In addition to emission triode 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 frequency beyond the 88 ~ 108MHz 范围. Where L1, L2 can ∮ 0.31mm enameled wire around the rod in ∮ 3.5mm on a single level around 5 turns and 10 turns, C3 use 5 ~ 20pF of ceramic or polyester adjustable capacitor. Actual production, the capacitor C5 can be omitted, L2 can be replaced by 10 ~ 100mH inductor coil ordinary. If the firing 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 will be less, also can refer to "electronic newspaper" 8 2000 fifth edition of "Simple remote wireless FM microphone" after the minor changes to the article.
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 connected to the C5 to make launch, due to Doppler effect, moving in the vicinity of the antenna, the phenomenon of frequency drift is very serious, so the normal radio receivers have been the voice distortion or silence. If we use the transmitter for wireless microphone, Hand antenna, the frequency drift can imagine how serious.
Figure 2 FM transmitter for the 2km circuit. The circuit is divided into oscillation, frequency, and power amplifier 3. Circuit, V1, C2 ~ C6, R2, R3 and L1 composition capacitance three-point oscillator, the oscillation frequency mainly by the C3, C4 and L1 of the parameters of the decision, the oscillation frequency is 44 ~ 54MHz, the signal from L1's center tap output Department that, through the C7 coupled to 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 V3, V3 transistor in parallel by the three 3DG12 composition, can be expansion of output power. The circuit normal operation, the current is about 80 ~ 100mA. Composed of three 3DG12 V3 can be combined with the appropriate heat sink to prevent overheating. L1 ~ L3 when making use ∮ 0.31mm enamelled wire rod on the ∮ 3.5mm single flat around.
Figure 3 is a practical 50m FM wireless headphone transmitter circuit. The circuit is divided into oscillation and signal amplification section. L1, C2 ~ C5, V1 composed of black and white TV with high-frequency oscillator circuit similar to the first of the three-point modified capacitance oscillator, frequency stability, long working hours do not run frequency, proved an amateur, it will adopt the improved capacitance-type three-point oscillator completely competent. Electric soldering iron directly branded the author of the collector a few seconds after V1, the transistor under high temperature, with ordinary radio receivers are still normal, non-running frequency of the phenomenon. Oscillator frequency is mainly decided by the L1 and C2, by fine-tuning L1, can cover 88 ~ 108MHz range. Audio signal through 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. The circuit L1 ~ L3 with ∮ 0.31mm enamelled wire rods on the ∮ 3.5mm single flat around. Turn by adjusting the distance between L1 oscillation frequency tuning, and then fine-tune the L2, L3's turn to turn spacing to resonance in the oscillation frequency, the maximum output power.
Figure 4 for the crystal-type transmitter circuit. Circuit J, VD1, L1, C3 ~ C5, V1 form crystal oscillator circuit. As the frequency stability of quartz crystal J good, are 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 in the collector by the C7, L2 constitute a resonant network in the 88 ~ 108MHz 3 octave selected signals (that is 87 ~ 108MHz the strongest signal), then amplified by V3, L3, C9 get better after the election frequency FM band signals. Frequency modulation process is like this, the audio changes in voltage caused by changes in capacitance between VD1 great, because VD1 J series with the crystal, the crystal oscillation frequency of occurrence of small changes, by the third harmonic, the frequency offset is 29 ~ 36MHz crystal offset of 3 times. Practical application, in order to get the right tone system, choose a larger crystal modulation frequency offset or ceramic oscillator, the circuit can also be used a little complicated multiplier circuit 6 to 12. If the input audio signal is weak, can be coupled with a voltage amplifier.
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