[Photo] Use MEC002A to make FM (stereo) 3km transmitter

The cell range we refer to here refers to a radius of 1-3 kilometers. Such a FM transceiver system is more suitable for campus or unit broadcasting. This article will elaborate on the cell FM (stereo) transmitter made by the miniature high-efficiency wireless sound transmission module MEC002A produced by Minshi Technology .

The MEC002A miniature high-efficiency wireless sound transmission module developed by Minshi Technology has the advantages of small size, high RF output power, high sound sensitivity, relatively stable output frequency, etc. The FM transmitter circuit structure composed of it is simple, easy to manufacture and debug Convenient, only need to add a level of RF power amplifier circuit composed of C1971 to realize the FM transmission circuit within three kilometers. The circuit structure is quite simple, so it is more suitable for amateur radio production.

FIG. 1 is a three kilometers MEC002A monaural FM transmitter circuit configuration, it can be seen that in fact only two RF circuit, and a related peripheral MEC002A by the oscillation output frequency of the RF circuit configuration of a control device, a further stage It is a Class C RF power amplifier composed of C1971 and related components. The other components are mainly used for power supply, filtering and microphone amplification. Although MEC002A contains a microphone amplifier circuit, for some units, it is necessary to add an external microphone without using the high-sensitivity microphone amplifier circuit in the module. In order to obtain a stable operating frequency, this circuit is provided with multi-stage power supply voltage regulation and filtering Circuit, in which U1 ( 7812 ) provides a stable 12V voltage for the MEC002A module , C12-C15 is the power supply filter capacitor, so that the operating voltage is more stable, to ensure that the MEC002A module will not cause frequency drift due to changes in the operating voltage. R3 , R7 and L3 are decoupling devices between stages, which can effectively overcome the mutual interference caused by the use of the same power supply between the stages. Q1 and its peripheral devices constituting the microphone amplifying circuit, coupled to the audio output through C5 W1 to control the volume of the microphone, the microphone volume controlled MEC002A coupled to the modulation terminal (pin 7) through C8, the internal frequency oscillating circuit MEC002A Perform frequency modulation. After the sixth pin MEC002A amplifying its internal microphone audio output pin, through which the control sound sensor sensitivity W2 coupled by a C9 to 7 feet of internal MEC002A frequency modulated high-frequency oscillation circuit, the internal microphone For MEC002A not working, only Need to suspend its 4 feet, when need to start the internal microphone circuit, it can be achieved by grounding 4 feet. CV1 is MEC002A RF output frequency control capacitor, by varying the capacitance, the output can MEC002A 68-118MHZ frequency within the variation range. Basically covers the entire campus radio and FM radio frequency band. However, once the output frequency changes, the RF power amplifier at the latter stage must also be adjusted to maximize the RF output power and the farthest transmission distance . Therefore, it is recommended to specify the required operating frequency when purchasing, so that the manufacturer can adjust the gain of the power amplifier circuit at the predetermined frequency point to the maximum . The third pin of MEC002A is its RF output pin. The modulated FM wave is output from this pin through the frequency selection network composed of C16 , CV2 and L1 and then sent to the base of the class C amplifier composed of Q2 (C1971) , MEC002A 1 is a power supply terminal pin, pin 5 for the power. Q2 works in the Class C state, and the bias resistor provides a negative bias for its base, which can make the output efficiency of the circuit higher than that of Class A. The prerequisite is that the front stage must have enough excitation power to work at 12V As for the MEC002A under voltage , its output power is nearly 200mW , which can fully promote the Class C amplifier circuit. U1 provides a stable 15V operating voltage for C1971 . L5 , L6 , CV3 and CV4 form a frequency selection network. By adjusting CV3 and CV4 , the output power of Q2 can be effectively transmitted to the transmitting antenna. The working current of the whole machine is about 18V operating voltage. is 800mA, RF output power is about 3W. This circuit also provides the mains power supply function. D1-D4 form a full-bridge rectifier circuit. C30-C33 are filter capacitors, which can reduce the AC ripple caused by the mains. The input AC voltage is controlled at about 18V . The working DC voltage of the whole machine is 18V . When the voltage of 13.5-18V is used, U2 can be eliminated and directly connected to the 1 and 3 pin pads with a jumper .

 

Figure 1 (click on the image to enlarge)

Picture 2

Fig. 2 is the assembly diagram of the whole machine, Fig. 3 is the circuit board diagram of the soldering layer of the circuit, Fig. 4 is the assembly diagram of the component, and Fig. 10 is the assembly diagram of the actual object.

          

Figure 3 Figure 4

 

Figure 6 (click on the image to enlarge)

    For cell broadcasting, the current mono broadcasting can no longer meet the needs of the majority of music lovers, and more listeners want to receive high-quality FM stereo broadcasting. Figure 6 is based on Figure 1 adds a level of FM stereo coding circuit composed of BA1404 , so that the circuit transmission sound quality has been further improved, it can transmit FM stereo signals. BA1404 is a FM stereo modulation transmitter integrated circuit. Here we have abandoned its high-frequency oscillation part, and directly used the 19KHZ pilot signal output by its 13- pin and the audio signal output by 14- pin to modulate the FM oscillation circuit of MEC002A . . The components connected to pins 1 and 18 form an FM pre-emphasis network. With the FM receiver's FM de-emphasis network, the desired frequency response can be effectively obtained and the received sound quality can be optimized. In addition to providing power instructions, the LED also provides a stable operating voltage for the BA1404 . The two pads are also reserved for the connection pads of the microphone volume control potentiometer and power switch. The user can modify it according to the schematic diagram, and add the power switch and microphone volume control potentiometer.

Picture 7

Figure 7 is the assembly diagram of the FM stereo three-kilometer board, Figure 8 is the solder layer circuit board diagram of the circuit, Figure 9 is the component assembly diagram, and Figure 10 is the physical assembly diagram.

   

Figure 8 Figure 9

   Precautions for use: (1) Audio source and antenna: Due to the slightly higher transmission power of the unit, the field strength it generates is sufficient to affect the normal operation of the surrounding electrical appliances with poor shielding. It is recommended to use the above two transmissions during use The board is shielded with a metal box and leads the antenna to the outdoor or a place far away from the sound source. During the actual test, this circuit board uses a whip antenna to directly lock on the circuit board for transmission. When the MP3 player is used as the sound source, the transmitter is close to the transmitter. The MP3 player is not disturbed, but when the Walkman or VCD player is used as the sound source, it is close. The transmitter board will not work properly within three meters, and other audio sources have not been tested; however, the above phenomenon will not occur when using an outdoor antenna for transmission. In addition, the use of a computer as a sound source should also be far away from the transmitter, especially for the stereo transmitter board, it will produce noise. The outdoor antenna is best to use the GP antenna introduced by the Minshi website or the " half wave dipole" antenna made by the horn antenna. The 50- ohm coaxial cable is used to transmit the RF signal. Using a 1.5- meter whip antenna will greatly increase the transmission distance compared to the 1- meter whip antenna provided with the original board . (2) Power supply: Due to the large operating current of the launch board, the power supply requirements are high, and the output current of the power supply is required to be not less than 800 mA at 18V . Therefore, a power supply with an output power that is too small will prevent the launch board from working properly. For example, if two 9V stacked batteries are connected in series, the battery will be exhausted in a very short time. When the DC adapter is used for power supply, the output current is also required to be greater than 800 mA. The operating voltage must be selected from 13.5 to 18V. The test power supply we use is made of TL431 precision voltage reference IC. (3) Regarding the solution of temperature drift: The three-kilometer board will generate a small range of frequency drift with the increase of operating temperature in the first few minutes of startup, and finally stabilized at an operating frequency. The more effective solutions are: A . Replace the CV2 and CV4 adjustable capacitors in the circuit with fixed ceramic capacitors (with black-point high-frequency capacitors) with a capacity in the range of 20-68P. (Note: The change in the value of CV2 has an effect on the transmission frequency) Fix the heat sink to the metal chassis, increase the heat dissipation area, so that the heat can be dissipated as soon as possible, and a small fan can be added to the heat sink if possible. There are two suggestions that can effectively overcome the frequency drift caused by the temperature of the three-kilometer board.

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