88-108mhz FM transmitter circuit production PCB

FM transmitter circuit as shown in drawings that works Resolution is as follows:

1) The high-frequency triode V1 and capacitor C3, C5, C6 to form a three-point oscillator capacitor

2) C4, L form a resonator: resonant frequency is the FM transmitting power of the microphone, according to the parameters of the diagram component transmission frequency can be between 88 ~ 108MHZ, just FM radio receiver covering the frequency by adjusting the value of L (La stretch or compress the coil L) can easily change the firing frequency, to avoid the FM radio. C4 coupled to transmit signals through the antenna to launch out.

3) R4 is a V1 base bias resistor, to provide a transistor base current, so that work in the enlarged area V1.

4) R5 is a dc feedback resistor, transistor operating point stabilizing role.

5) MIC microphone sound signal collected from outside.

6) The resistance of R3 for the MIC to provide a DC bias, R3 of the resistance the greater the sensitivity of the sound collecting microphone weaker, the higher the resistance the smaller the sensitivity of the microphone.

7) The microphone collected audio signals through the exchange of C2 coupling and R2 taken after matching transistor base.

8) circuits in the two diodes D1 and D2 reverse parallel, mainly from a two-way limiter functions, the diode turn-on voltage is only 0.7V, if the signal voltage exceeds 0.7V diode conduction will be streaming, so you can ensure that the voices of the signal amplitude can be limited to between plus or minus 0.7V, the signal will sound too strong too modulation transistor, resulting in sound distortion can not even work properly.
9) CK is an external signal output socket, you can carry-TV headphone jack or headphone jack to listen to such external audio signal source through a dedicated connection line of the introduction of FM transmitter, external sound signal attenuation through R1 and D1, D2, after clipping sent to transistor base to frequency modulation.

10) circuit in the light-emitting diode D3 to indicate the working state, when the FM microphone was electrical work and would light up, R6 is a light-emitting diode current-limiting resistor. C8, C9 is the power supply filter capacitor, because the big capacitor winding fabrication commonly used, so the relatively large equivalent inductance, shunt capacitor C8 can make a small high-frequency power supply resistance.

11) circuit K1 and K2 is a switch, it has three different locations, disconnect the power when appropriated for the far left, far right is the K1, K2 FM microphone connected to make use of the middle position is K1 connected, K2 off do the wireless transponder use, as do the wireless repeater use is the microphone does not work, but the microphone will consume certain amount of quiescent current, it can reduce the power off K2, extend battery life.

By changing the transistor base and the emitter capacitance to achieve between the FM, when the audio voltage signal added to the base on the transistor, the transistor base and the emitter capacitance between the size of the place as the voice of voltage signal synchronization changes, while the launch of the frequency change in transistor to achieve frequency modulation. 

Transmitter circuit 
Transmitter finished map

Transmitter finished PCB diagram 
pcb diagram of the polyline part of the antenna within the system, after the completion of the design do not need to match the antenna of the. simple debugging to work after a! if the distance is not an ideal launch feel may be appropriate to increase the supply voltage. the circuit in 9018 can be withstand voltage of 12V or less; can also add a modulation circuit in the amplification, but also can effectively increase the firing range! FM radio receiver can be used to replace!






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