FM radio transmission circuit design based on BH1417 chip
Publication date: October 30, 2007 Reference: electronic design applications Author: Shijun Waxing Fan
BH1417 is FM transmitter chip, it can works at 87MHz～108MHz band, used with simple peripheral circuit, to launch the audio FM signal. It can be computer sound card, game consoles, CD, DVD, MP3 and mixer, such as stereo audio signals for stereo modulation transmission. Applicable to the production of stereo wireless speakers, wireless headphones, CD, MP3, DVD, PAD, notebook computers and other wireless audio adapter
The principle characteristics of BH1417
FM transmission circuit uses the stable frequency of phase-locked loop system. This part consists of the high frequency oscillator, the high frequency amplifier and phase-locked loop frequency synthesizer. High frequency oscillator frequency modulation of varactor implementation, the high frequency oscillator VCO is phase-locked loop, composite signal through its direct frequency modulation stereo. Outside of the high frequency oscillator by of 9 pin of LC loop circuits with the internal circuit, oscillating signal output from 11 pins after high frequency amplifier, at the same time, comparing to the phase-locked loop circuit, output a signal from 7 pin, to modify the value of high frequency oscillator, to ensure that the frequency stability.
But more than the frequency of the phase-locked loop setting, 7 pin higher level of output; If less than the set frequency, It will lower level of output; if it at the same time with the set frequency, output will be the same.
1) Will integrate the pre-emphasis circuit, limiter circuit, low pass filter (LPF) d, make the circuit of the quality of the audio signal onto element (such as BA1404, NJM2035, etc.) have a great improvement.
2) Using phase-locked loop frequency lock, integrate with FM transmission circuit, makes the emission frequency is very stable
3) Adopted four yards pull switch to set frequency, frequency can be set 14 points, use very convenient. The internal structure of BH1417 is shown in figure 1. It consists of five parts: audio pretreatment circuit (aggravating, low-pass filtering and limit filtering); the fundamental frequency circuit (crystals, frequency division); Phase-locked loop circuit (phase detection, lock frequency); Frequency (high and low level conversion circuit); FM transmission circuit. Peripheral circuit mainly consists of drawing code switch control circuit; voltage controlled oscillator frequency carrier wave generating circuit, timer and some coupling capacitance.
Audio input limiter circuit design
BH1417 audio input maximum level has limitation; too much input level will damage the chip. In the early stage of the test phase, the input audio level amplitude is unpredictable; in order to protect the chip, need to limit the input audio signal. Limiter circuit is easy, can use variable potentiometer. Circuit shown in figure 2, figure is the role of capacitance in the audio signal is coupled to the chip, at the same time have a blocking function.
The voltage-controlled oscillator design parameters：
Combining BH1417 frequency, voltage controlled oscillator frequency range must cover all frequency points of the chip. Considering the general components of precision and processing level, appropriate here to broaden the spectrum, to ensure that the chip can lock the frequency point normally. Assume that band for
80 MHZ to 120 MHZ. The voltage-controlled oscillator circuit is shown in figure 3. L using common core adjustable inductance, inductance nominal value for nH (30 ~ 60 nH); Varactor capacitance change with the change of the bias voltage, its scope of limit (7 pf ~ 35 pf). In order to guarantee the stability of the circuit, C2 and C3 value difference is not too large, take 51 pF this assumes the C2, C3 scope for 7 ~ 35 pF pF. Below to determine the value of C1. By type (1), (2) clear inductor L, capacitance C3 are the minimum, maximum oscillation frequency of the voltage controlled oscillator, on the contrary, to obtain the minimum frequency.
After which C2 and C3 concatenated capacitance range as follows: 6.16 ~ 20.76 pF pF, will L, C3 limit value generation into the type (3), after finishing
The formula C1 unit is pF. Calculated: 45.27 by the above calculation shows that the value of the various components are not unique, here is a demonstration of ideas and methods of calculation. Hope to provide a theoretical reference for the design of circuits.
FM transmitter circuit design
1,22 stereo signal input pin, these pins with 2,3,20,21 external resistor-capacitor combination, complete low-pass filtering the stereo signal pre-emphasis and modulation signal modulated by a composite pin 5 output. 15,16,17,18 pin input frequency code and phase after decoding from 7 feet out PLL oscillator control signal VCO.
This VCO control external discrete components from the high-frequency oscillation circuit generates the FM carrier signal and 2SD2142 composite stereo signal output by pin 5 FM frequency modulation by a Darlington transistor.
The modulated signal is input to pin 9 by BH1417, through the interior of the RF amplifier of the RF signal from the output pin 11. After the signal output can be connected directly to the transmitting antenna to transmit, or input into the RF transmitter power amplifier amplified to extend the transmission range. pin 13,pin 14 requires an external crystal oscillator 7.6MHz, available to phase stereo signal modulation section BH1417 internal clock needed stability.
To sum up the wireless transmitter chip available BH1417 typical application circuit shown in Figure 4. Users can set the transmission frequency (specifically shown in Table 1) by changing the DIP switch is closed and disconnect JP1 order to avoid interference that may exist in the region of strong radio station.
BH1417 in many products can be applied, but its peripheral circuit design and control principle is basically the same, the design described in this article through the practical application, you can ensure BH1417 work, its voltage-controlled oscillator and RF part of the design can be draw or direct application.