[Photo] 1000m FM stereo transmitter (crystal frequency stabilization)


1000m FM stereo transmitter (crystal stabilized frequency)

    It has been the wish of many radio enthusiasts to make a FM stereo transmitter with a stable frequency and a long distance away. Here, we introduce a FM stereo transmitter with crystal frequency stabilization. For home wireless audio, the following 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 amplification circuit; U1 and light-emitting diode LED and related resistance-capacitance components Power supply circuit, where U1 outputs a stable 15V Voltage for BA1404 and Q1Q2 . In addition to being used as a power indicator, the LED also provides a stable operating voltage of about 2V for BA1404 . 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 receiver's de-emphasis network. L, R audio signals through two pre-emphasis network are input BA1404 after 1 and 18 feet, the encoded audio signal output by the pin 14, while the pin 13 output a pilot signal for synchronization of the receiver 19KHZ demodulated signal LR. Q2 constituting the surrounding element and the crystal oscillation circuit, the oscillation frequency determined by the crystal JZ2, this figure is a 49.405MHZ, an audio signal coding and pilot frequency amplifying circuit IC1 output signal Q1 constituted amplified into a 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 multiplier circuit. By changing CV1 , the RF frequency can be multiplied to 98.81 MHZ, just falling into the FM broadcast frequency band. Q4 works in category C and has a high launch efficiency. L8-L11 and CV2-CV6 form 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 using an outdoor antenna for about one kilometer.

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