Introduction
BH1417 FM wireless transmitter chip, it can work at 87MHz ~ 108MHz frequency band, used in conjunction with a simple peripheral circuit can transmit audio FM signal, it can be a computer sound card, video game, CD, DVD, MP3, mixer, stereo stereo audio signal modulation transmitter transmission, wireless FM stereo transmission can be achieved with ordinary FM stereo receiver. Suitable for the production of stereo wireless speakers, wireless headphones, CD, MP3, DVD, PAD, notebook computers and other wireless audio adapter.
BH1417 principle characteristic
FM transmitter circuit uses PLL system for stable frequency. In part by the high-frequency oscillator, high frequency amplifier and a phase-locked loop frequency synthesizer. FM composed of high-frequency by a varactor diode oscillator, the high-frequency oscillator is a phase locked loop VCO FM stereo composite signal directly through it. High frequency oscillator by a 9-pin external LC circuit and internal circuit, the oscillation signal after the high-frequency amplifier output from the 11-pin, at the same time conveying to the phase-locked loop circuit compares a signal output on pin 7 corrected value of the high frequency oscillator, to ensure that the frequency stability. A but exceeds PLL the frequency set, the pin 7 will the level of the output high; if less than a set frequency, it will output level down; the same time, its level will be the same .
1) of the pre-emphasis circuit, the limiter circuit, the integration of the low-pass filter circuit (LPF), so that the quality of audio signals in discrete circuitry (such as BA1404, NJM2035, etc.) has greatly improved.
2) The phase-locked loop frequency-locked, and integration with the FM transmitter circuit, making the transmitter frequency is very stable.
3) using 4 DIP switch frequency setting, can be set to 14-point, easy to use.
BH1417 the internal structure shown in Figure 1. It consists of 5 parts: audio pre-processing circuit (emphasis, limiter and low pass filter); baseband generator circuit (crystal, divide); PLL circuit (phase detection, frequency-locked); frequency setting circuit (high and low level conversion); FM transmitter circuit. Peripheral circuit DIP switch consisting of the frequency control circuit, a voltage controlled oscillator composed of a carrier generation circuitry, timer, and some of the coupling capacitor.
Application Circuit
Audio input terminal of the limiter circuit design
BH1417 audio input limit of the maximum level, the excessively large input level will damage the chip. In the pre-trial stage, the unpredictability of the input audio level amplitude, in order to protect the chip, you need to input audio signal limiter. The limiter circuit is very simple, the use of variable potentiometer can. The circuit shown in Figure 2, the capacitive function of the audio signal is coupled to the chip, while the DC blocking function.
Voltage-controlled oscillator parameters design
The combined the BH1417 frequency point shows that the frequency variation range of the voltage controlled oscillator must cover all frequency points of the chip. Taking into account the general components of accuracy and level of processing technology, here appropriate to relax the band, in order to ensure that the chip normally locked frequency. Assumes that band: 80MHz ~ 120MHz. The voltage controlled oscillator circuit shown in Figure 3. L adjustable inductance of the common core, the inductance and the nominal value (30 NH ~ 60nH); the capacitance of the varactor diode is changed with the change of the bias voltage, and its limits (7pF to 35pF). In order to ensure the stability of the circuit, C2 and C3 value can not be that much difference here assume that the C2 take 51 pF C3 range to take 7 pF to 35pF. Next, the determine the value of C1. By formula (1), formula (2) shows the inductor L, capacitor C3 are to take the minimum, the maximum oscillation frequency of the voltage controlled oscillator to obtain, on the contrary, obtain the minimum frequency.
Merge (1), (2), we have:
Wherein, C2, C3 connected in series capacitance range: 6.16 pF to 20.76pF, the L, C3 limits substituted into the formula (3), after finishing
Units of the formula C1 pF. Calculated: 45.27 Through the above calculation shows that, not only here just demonstrates calculations ideas and methods of the various components of the value. I hope to provide a theoretical reference for the design of the circuit.
FM transmitter circuit design
Stereo signal input through the 1,22 pin, with 2,3,20,21 several pin external resistive and capacitive combination, the completion of the low-pass filtering of the stereo signal pre-emphasis, and the composite signal after the modulation, the modulation by 5 feet output. 15, 16, 17, 18-pin input frequency code after decoding and phase control signal VCO of the PLL oscillator output by 7 feet. This VCO control an external high-frequency oscillation circuit is composed of discrete components to produce a carrier signal of the FM stereo, and through a Darlington transistor 2SD2142 5 feet output composite stereo signal modulation of the FM frequency. The modulated signal input through 9 feet BH1417, the internal RF amplifier amplified RF signal output by 11 feet. The output signal can be directly connected to the transmitting antenna transmitter input to the RF power amplifier amplified emission to expand transmitting distance. 13-14 feet need an external a 7.6MHz crystal oscillators, to provide a stable clock to the part of the the BH1417 internal phase stereo signal modulation.
BH1417 Wireless transmitter chip In summary, the typical application circuit, as shown in Figure 4. The user can JP1 switch closed and disconnected by changing the dial code to set the transmit frequency (specifically, as shown in Table 1), in order to avoid interference from strong radio stations within the region may exist.
Epilogue
BH1417 in many products can be applied, but its peripheral circuit design and control principle is basically the same, the article describes the design can guarantee the a BH1417 normal work through the practical application of the voltage-controlled oscillator and RF part of the design can be learn from or directly application.
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