[Photo] 1000m crystal frequency stabilized FM stereo transmitter

It has been the wish of many radio enthusiasts to make a FM stereo transmitter with a stable frequency and a long distance. Here we introduce a FM stereo transmitter with crystal frequency stabilization produced by Minshi Technology Development Center. Stable and excellent sound quality, it is very suitable for home wireless audio.

Figure 1 is the working principle diagram of the machine. The circuit is mainly composed of four parts: power supply circuit, stereo encoding circuit, crystal oscillation circuit and radio frequency amplifier circuit. U1 and light-emitting diode LED and related resistance-capacitance components form a power supply circuit. U1 outputs a stable 15V voltage for BA1404 and Q1 and Q2. In addition to being used as a power indicator, the light-emitting diode here also provides a stable 2V for BA1404. Working voltage. R5 is the current-limiting resistance of the LED. C11, C12, C23, C24, C37, C38, and C39 are power supply filter capacitors, and R9, R15, and R18 are power supply decoupling resistors, which can reduce interstage interference caused by the use of the same power supply. BA1404 and peripheral components form a frequency-modulated stereo encoding circuit, which does not use its internal high-frequency oscillation circuit, only uses it as a stereo encoder. R3, R4 and C3 ～ C10 form the FM pre-emphasis network, which can effectively improve the frequency response effect with the de-emphasis network of the receiver. The two audio signals of L and R are input into BA1404 ① and Xia Zha foot after pre-emphasis network. The encoded audio signal is output from Xia Xia foot, and at the same time Xia Xia foot outputs 19kHz pilot signal for receiver synchronization Demodulate the LR signal. Q2 and the surrounding components form a crystal oscillation circuit, whose oscillation frequency is determined by the crystal oscillator JZ2, which is 49.405MHz in this picture. The audio coded signal and pilot signal output by IC1 are amplified by the amplifier circuit formed by Q1 and sent to the crystal oscillation circuit. The Q1 level can effectively increase the modulation frequency deviation. By selecting the varactor diode and crystal oscillator, the modulation frequency deviation can also be effectively increased. Q3 and Q4 form an RF power amplifier circuit, and Q3 also functions as a frequency doubling circuit. By changing CV1, the RF frequency can be multiplied to 98.81MHZ, which happens to fall in the FM broadcast frequency band. Q4 works in category C and has a high launch efficiency. L8 ~ L11 and CV2 ~ CV6 constitute a radio frequency filter and antenna coupling circuit. By adjusting CV2 ~ CV6, high frequency waves can be effectively transmitted to the antenna, reducing harmonic components. In order to reduce the debugging workload of the high-frequency filter circuit under amateur conditions, the filter circuit after L9 can be omitted. The output power of the whole machine is about 1W, and it can be measured in an open area by an outdoor antenna for about one kilometer.

Figure 2 is a diagram of the machine's printed circuit board. L1, L2, L6 use color-coded inductors, L6 should choose a color-coded inductor with a power of not less than 1 / 8W, and other inductance coils are wound on a 3.3mm drill bit using an enameled wire with a diameter of 0.51mm Mark. Q4 can also choose C9018 transistor, if conditions permit, using C2053 can make the RF output power larger. After the welding of the components of the whole machine is completed and the inspection is correct, cooperate with the field strength meter to adjust CV1-CV6 to maximize the RF output power before it can be put into use. The working current of the whole board is about 150 mA. The DC power supply should select a DC adapter with an output current of not less than 500 mA and an input voltage of 18V. If the output power of the working power supply is not enough, it is easy to introduce hum, which will affect the working performance of the transmitter. The appearance of the whole board after assembly is shown in Figure 3.

For more technical information and mail order information related to the product, you can visit the website of Minshi Technology: http: //