3 km stereo transmitter

1. How it works:

Micro-site to provide efficient wireless transmission of MEC002A sound module is compact Xiao, RF output power of big, sound level meter of high sensitivity, output Pinshuaixiangdui Bijiaowending other Youdian, using the FM transmitter Dian Lu Ta constitute simple structure and easily Zhizuo, Tiaoshifangpian, Just then added a composition by the C1971 RF power amplifier circuit can be realized within three kilometers of the FM transmitter. Circuit structure is simple, and therefore more suitable for the production of amateur radio enthusiasts.


Figure 1 is composed of three kilometers from the MEC002A single-channel FM transmitter, you can see, the RF circuit are actually only two, one is MEC002A and their peripheral devices constitute the RF frequency control oscillator output circuit, and the other class C1971 and related components from the composition of C RF power amplifier, the other main power supply and the filter element and the microphone amplification purposes. Although MEC002A includes a microphone amplifier, 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 set up multi-level power supply voltage regulator and filter circuit, where U1 (7809) to provide stability for the MEC002A module 9V voltage, C12-C15 for the power supply filter capacitor, so that a more stable voltage to ensure MEC002A module will not produce voltage changes in the frequency drift. 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 around the device constitutes a microphone amplifier circuit, coupled to the audio output through C5 W1 to control the volume level of the microphone, the microphone volume controlled by the modulation C8 coupled to MEC002A end (7 feet), on the internal high-frequency oscillator circuit MEC002A for FM. The first 6 feet MEC002A internal microphone for audio amplification output pin, through its sound sensor sensitivity W2 control after the C9 coupled to 7 feet on the internal high-frequency oscillation circuit MEC002A FM, who wish to make MEC002A internal microphone does not work, only need to be suspended 4 feet can be, when the need to activate the internal microphone circuit, the 4 foot ground can be realized. CV1 to MEC002A RF output frequency control capacitor, by changing the capacitance, the output can MEC002A in the 68-118MHZ frequency range changes, covering the entire campus radio and FM broadcast bands. However, once the output frequency changes, followed by an RF power amplifiers have to follow the regulation, can the maximum RF output power, firing distances. Therefore proposed that the best time of purchase the designated frequency required for manufacturers to power amp circuit after the frequency set to maximum gain. MEC002A 3 of the RF output pin for pin, after the FM wave modulated by the output pin through C16, CV2, L1 frequency network formed after the election sent by 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, will enable the output of the circuit more efficient Bijia higher class, which provided that the former class have to have enough incentive to power, the work of the 9V MEC002A under voltage, its output power nearly 200mW, can promote the C amplifier. L5, L6, CV3, CV4 constitute a frequency selective network, by adjusting the CV3, CV4 Q2 of the output power can effectively transmitted to the transmitting antenna, the whole work in the 12V supply voltage current is about 300-700mA, 3W RF output power of about . C30-C33 for the filter capacitor can reduce electricity ripple caused by the exchange. Machine working DC voltage of 9-13.5V.

Figure 1 Figure 2 is based on the increased level posed by the BA1404 FM stereo coding circuit, the circuit can be further improved transmission quality, it can transmit FM stereo signal. BA1404 FM stereo modulation as a launch integrated circuits, where we give up some of its high-frequency oscillations do not directly whichever 19KHZ 13-pin output pilot signal and 14 pin output audio signal MEC002A FM modulation oscillator . 1 and 18 feet by then constituted the FM pre-emphasis network components, with the FM receiver to increase the network to effectively be the ideal frequency response, so to receive the best sound quality. In addition to providing power LED also BA1404 instructed to provide a stable voltage. 2 launch board also set aside the microphone volume control potentiometer and power switch of the connection pad, the user can transform itself according to the schematic, with power switch and microphone volume control potentiometer.

2. Caution:

(1) sources and antennas: transmitting power as the larger machine, it generates a field strength sufficient to affect the poor around the electrical shielding of the normal work process, recommended the use of the above two firing board with metal box shield up, and will lead to the outdoor antenna or a place far away from the source. The circuit board in the actual test, the use of outdoor GP antenna, operating voltage 12V, can be launched nearly three kilometers. In addition, the use of computers as the source should also be away from the transmitter, especially for stereo launch board, will produce noise.

(2) Power: As the launch of the work of the current board greater power requirements for higher power output required in the 9-13.5V when the current is not less than 800 mA, thus the power output is too small will launch board does not work, such as the use of laminated two 9V batteries in series, the battery in a very short period of time will run out of power, when using the DC adapter is also required greater than 800 mA of output current and the filter in good condition voltage must be selected among the 9-13.5V, or it may burn the transmitter.

(3) on the drift of the solution: the 3 km board within a few minutes before the start of the working temperature of the increase will be generated with small-scale frequency drift, and finally stabilized at a working frequency, the more effective solution would have: A . The circuit CV2, CV4 variable capacitance capacitors replaced with fixed tiles (with black spots in high-frequency capacitance), capacity 20-68P range. (Note: CV2 change the value of an impact on the emission frequency) B. The heat sink fixed to a metal chassis, the increased heat dissipation area to distribute the heat as soon as possible, conditions for heat sinks to install a small fan. Have two suggestions to overcome the three-kilometer board frequency caused by temperature drift.

3. Assembly and debugging:

When all the components according to assembly drawings and check for correct after welding is completed, connect the outdoor antenna can be transmitters GP commissioning, power has been connected before to ensure that the antenna and connect securely, or it may burn C1971 launch tube. If you do have with frequency display TV Field Test Instruments, debugging is relatively simple and some will be fixed at the required frequency TV Field Test Instruments launch frequency, adjust the maximum indicated CV1 to TV Field Test Instruments, at this time that the output frequency of the MEC002A required operating frequency, then adjust CV3 make TV Field Test Instruments Directive further increase to the maximum so far. At this point in the current meter in series with the power switch position measuring machine operating current should be between 300-700mA. If the machine is less than 150mA operating current, but still receive the local launch of the FM signal that the final amplifier circuit (C1971-level) is not working properly, should be the focus of this part of the circuit checks whether the Weld, and open circuit or short circuit point . In the case of only the most simple multimeter debugging method is to place the power switch in series multimeter, adjust the CV1 and CV3 to operating current in the 300-700mA of the largest you can, then the transmitter operating frequency usually within 88-108MHZ a certain frequency, the frequency uncertainty of the method can be used to debug launch. In addition, within the control of the 10K R13 can effectively control the size of FM stereo sound quality and separation.


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