Stereo FM transmitter circuit designed with BH1417

The whole circuit is shown in the figure below.

1. Stereo modulation circuit

After the audio signal is input through pins 1 and 22, it is sent to the mixer through the pre-emphasis circuit, limiter circuit and low-energy filter through the cooperation of pins 21, 20, 19, 2, 3, and 4 of BH1417 and the external circuit. The oscillation circuit of the 7.6MHz crystal input from pins 13 and 14 generates a 38kHz subcarrier signal through 200 division. At the same time, the 38kHz subcarrier generates a 19kHz pilot signal after 2 division. The (LR) signal is balanced modulated with the 38kHz subcarrier, and the modulated composite signal is output through pin 5.

2. FM transmitter circuit

The FM transmitter circuit uses a frequency-stable phase-locked loop system. After the frequency codes input from pins 15, 16, 17, and 18 are decoded and phase-detected, the control signal vco of the PLL oscillator is output from pin 7. This vco controls the high-frequency oscillation circuit composed of external discrete components to generate the FM frequency-modulated carrier signal, and performs FM frequency modulation on the composite stereo signal output from pin 5 through a Darlington transistor 2SD2142. The modulated signal is input to BH1417K through pin 9, and the RF signal amplified by the internal RF amplifier is output from pin 11. The output signal can be directly connected to the transmitting antenna for transmission.

3. High frequency oscillation circuit

Pins 5, 7, 9, 10, and 12 of BH1417 cooperate with the discrete components connected to them to form the frequency oscillation and RF modulation part of the FM carrier; Pins 13 and 14 need to be connected to an external 7.6MHz crystal oscillator to provide the stable clock required by the phase detection, stereo signal modulation and other parts inside BH1417. Pins 6 and 8 are the power supply part; Pin 11 and the externally connected components constitute the FM signal transmission part.

Since the clock required for the internal operation of BH1417 comes from a 7.6MHz crystal oscillator, and the operating frequency of the crystal oscillator is generally very stable. The external FM carrier signal and the carrier modulation circuit both use the PLL (phase-locked loop) circuit controlled by vc0 (voltage-controlled oscillator) to work. Therefore, the transmission frequency of the FM transmitter composed of BH1417 is very stable, and there will be no frequency runaway or self-excited oscillation during the transmission process.