The core device of this circuit is the stereo special chip BA1404. Many FM stereo modules are made by encapsulating BA1404 and peripheral components in a plastic or metal enclosure, only exposing other types of devices that can be converted into electrical energy are called power supply input, audio input, and RF output leads. As long as you understand BA1400, You know what is going on inside the FM stereo module. The audio signal is coupled to the BA1404 via the network composed of R1, R2, R5, C1, C3, C5 (R4, R3, R6, C2, C4, C6). The left (right) channel is amplified, and then balanced modulation is performed. The modulated composite signal is output from the {14} pin of the IC, and the pilot signals on the back and the {13} pins pass through R9, C15, R10, C16, C17 The formed network is mixed, and the mixed signal after mixing enters the {12} pin of the IC, and the frequency of the capacitive three-point oscillator composed of the {8}, {9}, {10} pins of the IC, C20 ~ C22 and L3 is tuned. , The modulated RF signal on the {10} pin of IC is internally amplified and output from the {7} pin, after C18, L2 After frequency selection, it is sent to antenna TX1. To realize FM stereo, BA1404 requires {38} and {6} pins to be externally connected with a 38kHz crystal, but it is really difficult to purchase a 38kHz special crystal in amateur production, so in the absence of this crystal You can refer to the circuit in the dotted line to make a 38kHz oscillator with discrete components, and the 38kHz signal is sent to IC pin {5} via R8 and C10. During production, L1 can be used with radio intermediate frequency transformers TTF-2-1, TTF-2 ï¼2 or TTFï¼2-9, etc. At the same time, pay attention to the connection of the pins, the {3} pin is grounded, the {2} pin is connected to the emitter of V1, and the {1} pin is the feedback and output pin. By adjusting its magnetic The core can obtain a 38kHz signal with a stable frequency and a sufficiently high amplitude. It is particularly worth noting that C8 should choose a polyester Capacitor of 0.033μF, and should not choose a ceramic capacitor, because the stability of the ceramic capacitor is poor and the oscillation frequency is prone to occur. Instability, the phenomenon that the FM stereo works abnormally. Because the high-frequency oscillation of BA1404 is a three-point capacitor Oscillator, so the frequency stability is poor, so this circuit does not use the original high-frequency oscillator, instead of using an improved capacitor three-point oscillator with a stable external frequency, which can meet the requirements of amateur FM broadcasting and FM wireless headphones. For example, the transmission part of the ZN-2001 FM stereo wireless earphone adopts an improved capacitor three-point oscillation circuit. After the stereo composite signal is amplified by the V2 voltage, it is directly added to the V3 base through C26 and R14 to achieve frequency modulation. Its characteristics are based on Users need to use a screwdriver to adjust the inductance of L4 outside the case, so that it can be adjusted freely within the range of 88 ~ 108MHz, avoiding the frequency of local FM radio stations. Another feature of the machine is: there are already left on the circuit board 1 ～ 5W power extension part, for example, the components of this part can be installed during campus broadcasting and can be put into use after debugging. But it is worth noting that if the wireless headset increases the power, the whip antenna on the machine is still used Transmitting, the strong radio frequency signal will cause its own interference, causing sound distortion and There is hum or silence, so the RF signal must be transmitted outdoors through a 50Ω dedicated communication cable. When installing the power expansion part, you can use the RF detector shown in Figure 5 to adjust the resonance state at all levels. Connect the input terminal of the RF detector (the end of the 1kΩ resistor) to the collector of the preamplifier transistor, adjust the inductance coil on the collector to make the voltage at the output of the RF detector the highest, and then follow the same method step by step. Level adjustment, then detect the antenna end, and finally adjust the inductance coils at all levels to make the output voltage the highest, that is completed. Compared with infrared wireless earphones, the host (transmitter) and receiver of FM stereo wireless earphones can be used normally across the wall, while infrared earphones cannot. In addition, ordinary infrared earphones do not have a stereo function, so FM stereo wireless earphones are more suitable, and they are more pleasant to listen to when listening to music; if an outdoor antenna is installed, even weak RF signals can be transmitted far away, so make a good antenna It is much more effective than simply increasing the transmit power. It is troublesome to make a horizontally polarized, omnidirectional antenna, and general FM radio stations also use the horizontally polarized method. In order not to cause interference, the author here introduces the reader to a simple assembly, high efficiency vertical Polarized antenna. Since the headphone cable is vertical when the person uses the headphone cable as a radio antenna to receive sound while moving; the antenna of the car radio is also approximately vertical, so vertical polarization is more suitable for mobile reception. The antenna adopts a 50Ω umbrella antenna dedicated to the communicator. As shown in Figure 6, there are 4 or 7 vibrators on the antenna base, each of which is about 0.75m long, and the vertical one is the main vibrator of the transmitting antenna, which is inclined The three or six downward vibrators form a simulated ground, and the angle between them is uniform. The angle between the main vibrator and the vibrators forming the simulated ground is also fixed as required. The impedance of the entire antenna is 50Ω, 10MHz. The gain in the bandwidth is about 2dB, and the standing wave is less than 1.2; many occasions transmit digital signals, so you can refer to the circuit in Figure 7 and add a few components to realize the wireless digital transmission of the transmitter.
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