Implemented by the Min-Micro Technology Research MEC002A efficient wireless sound transmission module is small, the RF output power, the sound sensitivity is high, the output frequency is relatively stable, etc., use it to form the FM transmitter circuit is simple, easy to build, debug convenience, only one level in the following C1971 posed by the RF power amplifier circuit can be realized within three kilometers of the FM transmitter circuit. Circuit structure quite simple, and thus more suitable for the production of amateur radio enthusiasts.
Figure 1 is composed of three kilometers from the MEC002A mono FM transmitter circuit, can be seen, the RF circuit are actually only two, one related by MEC002A and the surrounding radio frequency control devices constitute the oscillator output circuit, another level C1971 and related components from the composition of C RF power amplifiers, mainly supply other components and microphone amplification and filtering purposes. Although MEC002A includes a microphone amplifier circuit, but for some units, the need for additional external microphone instead of using the module inserted within the high-sensitivity microphone amplifier circuit, in order to get a stable operating frequency, the circuit is set up multi-level power supply voltage regulator and filter circuit, where U1 (7812) for the MEC002A module to provide a stable 12V voltage, C12-C15 filter capacitor for the power to make a more stable operating voltage to ensure that the module will not work MEC002A changes in voltage to frequency shifts. R3, R7, and L3 for the interstage decoupling devices, can be effectively overcome using the same power between levels brought about by the mutual interference. Q1 and its peripheral devices constitute a microphone amplifier circuit, coupled to the audio output through C5 W1 to control the microphone volume, microphone volume controlled by the modulation of C8 coupled to MEC002A side (7 feet), the internal high-frequency oscillator circuit of MEC002A the FM. MEC002A the first 6 feet of its internal microphone audio amplifier output pin, the W2 control the sound sensitivity is coupled after the C9 to 7 feet of the internal high-frequency oscillator circuit MEC002A FM, who wish to make MEC002A the internal microphone does not work, only need to be suspended 4 feet, when the need to activate the internal microphone circuit, the 4 foot ground can be realized. CV1 is MEC002A RF output frequency control capacitor, by changing the capacitance, the output can MEC002A in the 68-118MHZ frequency range of changes. Basic coverage of the entire campus radio and FM radio bands. But once the change in output frequency, followed by an adjustment of RF power amplifiers have to follow in order to make the maximum RF output power, firing distances. Therefore proposed that the best time of purchase to specify the desired frequency, so that after the factory amp circuit will be set to maximum gain frequency. MEC002A 3 of the RF output pin to its feet, the FM wave modulated by the output pin through C16, CV2, L1 frequency network formed after the election sent to the Q2 (C1971) C amplifier consisting of base, MEC002A 1 foot for the power supply side, 5 feet for the power ground. Q2 work in the C state, the bias resistor to provide negative bias to its base, the output of the circuit efficiency can Bijia class higher, the prerequisite is required before a sufficient level of drive, work on 12V MEC002A under voltage, its output power is nearly 200mW, can drive the C amplifier. C1971 U1 to provide a stable 15V voltage, L5, L6, CV3, CV4 form frequency selective network, by adjusting the CV3, CV4 Q2 output power can effectively transmitted to the transmitting antenna, the whole work in the current 18V voltage of about to 800mA, RF output power is about 3W. This circuit also provides electricity supply function, D1-D4 form a full-bridge rectifier circuit, C30-C33 for the filter capacitor can reduce the AC ripple caused by electricity. Control the input AC voltage of about 18V. DC voltage for the machine work 18V, when using the 13.5-18V voltage U2 can be removed when not directly connected with a short jumper pin pad 1,3.
图1（点击上图可放大）
图2
图2为整机装配图，图3为该电路的焊接层电路板图，图4为元件装配图，图10为实物装配图。
图3 图4
图5
图6（点击上图可放大）
对于小区广播而言，目前的单声道广播已不能满足广大音乐爱好者的需求，更多的收听者希望收到高品质的调频立体声广播。图6就是在图1的基础上增加了一级由BA1404构成的调频立体声编码电路，使得该电路传送音质得到进一步的改善，它可以传送调频立体声信号。BA1404为一块调频立体声调制发射集成电路，在这里我们舍弃了其高频振荡部份不用，直接取其13脚输出的19KHZ的导频信号和14脚输出的音频信号对MEC002A的调频振荡电路进行调制。1和18脚所接元件构成调频预加重网络，配合接收机的调频去加重网络可有效地得到理想的频响，使接收音质最佳。LED除了提供电源指示外还为BA1404提供稳定的工作电压。两块发射板上还预留了麦克风音量控制电位器和电源开关的接线焊盘，用户可根据原理图自行改造，加上电源开关和麦克风音量控制电位器。
图7
图7为调频立体声三公里板的整机装配图，图8为该电路的焊接层电路板图，图9为元件装配图，图10为实物装配图。
图8 图9
图10
Caution: (1) audio source and the antenna: the larger transmission power of the machine, it generates the field around the shield poor enough to affect the normal operation of electrical appliances, in the course of the above two proposed emission metal box shielding plate, and lead to the outdoor antenna or farther away from the source areas. The circuit board in the actual test, using the whip antenna directly to a launch lock on the circuit board, when using the MP3 player as the source close to the transmitter MP3 player without interference, but the use of player or VCD player as the source close to the The board fired within three meters will not work, other sources not tested; but when using an outdoor antenna will not have the above phenomenon. In addition, the use of computers as the source should also be away from the transmitter, especially for stereo transmitter board, will generate noise. Outdoor antenna is best described using the Web site GP Min real antenna or horn antenna made by "half-wave dipole" antenna, 50 ohm coaxial cable with a radio frequency signal. Using a 1.5 m telescopic antenna than the original 1-meter board supporting the launch rod antenna distance increased significantly. (2) Power: As the emission current board's work more, for higher power requirements, requiring 18V power supply output current at the time of not less than 800 mA, and thus the power output is too small will not properly launch board work, such as laminated with two 9V batteries in series, the battery in a very short period of time will run out of power, DC power adapter is also required when the output current greater than 800 mA, operating voltage must be selected in 13.5 -18V. We use the test power supply with IC TL431 precision voltage reference is made. (3) on the drift of the solution: The three-kilometer board within a few minutes before the start of the temperature increase will be generated with a small range of frequency drift, and finally stabilized at a working frequency, more effective solutions are: A . The circuit CV2, CV4 adjustable into a fixed capacitance ceramic capacitors (high-frequency capacitance with black spots), the capacity for the 20-68P range. (Note: CV2 change the value of an impact on the transmission frequency) B. The heat sink is fixed to the metal chassis, and increase heat dissipation area, so that gives off heat as soon as possible, conditions for the installation of a small heat sink fan. There are two proposals on the three-kilometer board can effectively overcome the frequency drift caused by temperature.
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