High-performance dual-band wireless microphone receiving circuit

Making high-quality wireless microphones is the wish of many readers, but it is not easy to make high-performance receivers under amateur conditions. The author found another way and used the FM radio part of the used car radio and receiver, added an XR1075 audio processor, redesigned the printing board, and produced a stable, reliable, and excellent dual-band wireless microphone transceiver system. The transmitter part adopts BA1404 dedicated FM transmitter integrated circuit. The electromechanical principle of this machine is shown in the figure below. The production process is now introduced as follows. As can be seen from the figure, the circuit consists of four parts, namely the FM head, IF amplification, audio processing and power supply. The basic principle is that the signal induced by the antenna is input from the pin of the FM head, and after internal high amplifier and local oscillator mixing, a 10.7MHz intermediate frequency signal is output from the pin. It is input to T1 from C1 for intermediate frequency preamplification and then filtered out by JT1 at 10. The 7MHz IF signal is sent to the pin of IC1, and is demodulated into an audio signal after internal limiting, amplification, frequency identification and output from the pin. It is coupled to T2 by C8 for amplification to meet the level requirements of IC3. The audio signal is input from the pin of IC3 and output from the pin after digital processing by internal frequency division delay. The pin of IC1 is the 2.25V reference voltage source, and the pin is the tuning indication output, which is used as a noise suppressor in this circuit. The principle is: when the pin has an intermediate frequency signal input of 10.7MHz, the pin is low level, T3 is cut off, and the audio signal of IC3 pin is output without attenuation; otherwise, the pin is high level, and the power supply passes through R11, D2, R13 provides bias to T3, and T3 is saturated, allowing the audio signal to pass through the c and e poles of T3 and enter the ground, thereby achieving the purpose of squelch. L1 and C form a 10.7MHz frequency selection network, which is also the load of the second mid-amplifier of IC1. C2 and C3 are intermediate frequency bypass capacitors. W1 and W2 are the treble and bass gain adjustment potentiometers of the audio processor respectively. The principle can be See related articles in "E-Newsletter". Installation and debugging: Find two broken car radios (the tuner is required to contain an FM head), take out the circuit board and tuner, and solder the 10.7MHz filter JT1 and the intermediate frequency filter (the shape looks like a mid-circuit, but the magnetic cap is ordinary (in light blue), TA7640, FM tuner (see the upper right corner of the attached picture for appearance) and other key components are spare. Make a printed board according to the size of the casing. The printed board should be an epoxy board and should be grounded over a large area. The components can be soldered when ready. T3 and T6 can be installed after the whole machine is debugged. Turn on the power (take one channel as an example), measure the pin voltage of IC1 should be about 6.5V, the FM header pin should be 6V, and the pin voltage should be about 0.15V (measured by an M47 meter, measured by a digital meter, it is 1.9V). Adjust R1 and R10 so that the collector voltages of T1 and T2 are both about 3V. Connect the power amplifier and speaker. If the circuit is normal at this time, the speaker should make a loud "rustling" sound. If there is no rustling sound, use a screwdriver to touch the pins of IC3 and the base of T2 respectively. There should be a humming sound, and then touch the pins of IC1, T2 and IC1. , , pin, and T1 base should have a loud "clucking" sound, otherwise JT1, L1, IC1 and surrounding components should be checked. After everything is normal, rotate the FM head to receive a local FM station, and adjust the L1 core to make the sound quality clear. Then adjust the transmitting and receiving frequency with the microphone (be careful to avoid the local radio frequency), and finally use the microphone to adjust W1 and W2 to familiar music with good sound quality until the sound quality of the receiver is clear and natural. If the microphone is mainly used to pick up human voices, you can Increase the treble appropriately (adjust W2). Adjust the other channel in the same way. At this time, solder T3 and T6, debug the automatic squelch function, and turn off the transmitter. The corresponding TA7640 pin should be high level; it should be low level when it is turned on, otherwise L1 needs to be readjusted. Although this circuit does not use a crystal oscillator for frequency stabilization, due to the high stability of the FM head, the transceiver frequency of this wireless microphone is very stable, and there is never a frequency deviation from summer to winter. The author uses two microphones for guzheng and voice respectively, and subjectively feels that guzheng The strings are very clear and beautiful, and the vocals have obvious sibilance. As long as the quality of the components is guaranteed, this wireless microphone can be comparable to finished machines costing hundreds or thousands of dollars. Of course, interested readers can also add harmonic generators to the IC1 and IC2 outputs to improve the clarity of the sound. This machine uses a 9.6V rechargeable battery as the main power supply, and an external 16V power supply is specially used for battery charging. Due to the current limiting and voltage stabilizing functions of R31 and D4, the external power supply can be connected to the machine for a long time.