7 / 15 watt FM Transmitter PLL stereo digital circuit

Main circuit 15 watt Stereo FM transmitter

A very high quality stereo FM transmitter circuit , thoroughly time tested ,highly reliable, crystal controlled PLL based for zero  frequency drift is reviewed below. The main circuit uses a very popular IC -KT0803L ,to develop two channel stereo audio modulated FM signal from76 MHz to 108 MHz covering the FM band of  88 MHz to 107 MHz feeding  to 3 stage buffering before being fed to the final power amplifier using most commonly used 2SC1971 for 7 watt or MOSFET -RD15HVF1 for 15 watt. But unfortunately most commercially available 2SC1971 or  RD15HVF1 are duplicate. Thus if one ventures trying any circuit with this power Transistor/ MOFET usually are unsuccessful. So I strongly recommend to go for readymade units. Aux audio is directly fed at pin 6 & 7 while for mic input it is amplified by LM358 and then its output is fed same 6&7 pins of the IC. Final modulated FM output is available at PN 16 that feeds the 3 stage amplifier. Most of the components are SMD excepting pre-driver and the final power transistor. PCB is grounded on either sides for all unused places.

LCD circuit

The LCD circuit is managed by 3 push buttons S1,S2,S3 only , meant for controlling frequency, audio, and power which are fed to a microcontroller STM103K3 . This programmed microcontroller is used for display that also delivers through I2C protocol control at SDA & SCL parameters to pin 13 &14 of the main circuit through CON6. Power control is made using PWM signal from this microcontroller at pin 1 of CON6 is amplified in the main circuit by first a voltage follower U2B and then an amplifierU2A with the help of LM 358 for controlling the DC bias at the power transistor feed point through an inductor L1 at PWR_DA. Other items in the circuit are very standard such as DC power, tuned LC coils etc. Not very critical tuning is necessary excepting checking the power output through a simple dummy load circuit using a 12v, 0.1A incandescent lamp for maximum glow while carrying out spacing in-between some coils indicating maximum power delivery.

Dummy Load circuit

Main parameters:
1, The input voltage: dc 12 v
2, Maximum input current about 2.5 A at 12 v DC
3, The audio signal: the maximum input Vpp is less than 1 v.
4, Microphone types accepted: Any
cylindrical microphone or condenser microphones.
5, The output power: 0.5 W – 15 W continuously adjustable!
6, The output impedance: 50 Ω for antenna
7, Working frequency range: 76 m – 108 MHZ
8, Frequency step of 0.1 MHz for adjusting
9, The distortion degree: 0.03% ~ 0.1%

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Advisor on electronics electrical hardware

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