Production and debugging of wireless FM microphone

The circuit diagram of the wireless FM microphone is shown in the figure below, where the LC parameters are important factors that determine whether the circuit oscillates and the transmission frequency, especially L and C3 in the resonant circuit, which determine the FM transmission frequency and are the key to the success of the production.

After theoretical derivation, precise measurement and installation verification, the author uses the following formula to select the resonant circuit LC value accurately and reliably. The formula is: N = 2.565 × 107d / φ2f2C, where N, d, φ, f, and C represent the number of turns of the resonant coil L, the diameter of the enameled copper coin used, the inner diameter of the coil (i.e. the diameter of the temporary frame used to wind the coil), the transmission frequency, and the resonant circuit capacitance value. The units used in the formula are all practical units, namely: turns, millimeters, megahertz, and picofarads. For example, if the resonant capacitor C is selected as 15pF and the transmission frequency is 96MHz, if enameled wire with d = 0.6mm is used, the coil can be wound on an ordinary ballpoint pen core with a diameter of φ = 3.5mm. According to the above formula, 9.1 turns are required, and 9 turns are actually wound. Note: (1) The number of turns calculated by this formula refers to single-layer dense winding, that is, the turns are closely arranged without gaps, and they are out of the mold after winding. The diameter of the enameled copper wire (silver-plated wire is better) should be as large as possible, which can reduce the loss resistance of the coil and improve the quality factor Q, so as to increase the gain of the high-frequency oscillation circuit. (2) When using this formula, adjust the values ​​of various parameters appropriately so that the transmission frequency is between 88MHz-108MHz and try not to be close to the edge frequency. And make the number of turns an integer. In addition, C4 determines the bandwidth, generally 3-10P is selected, and 5-8P is appropriate. C2 is related to the modulation depth, and generally 100-300P is selected (in some circuits, this capacitor is installed between the base and the ground. At this time, a capacitor with a capacity of 500-1500P can be selected. C5 is the antenna signal coupling capacitor, and the capacity can be selected in the range of 10-20P.

Transistor BG works in high frequency state, and it is necessary to select high frequency transistors with cut-off frequency greater than 100MHZ, and the collector junction capacitance should be small. After actual installation verification, it is easier to debug successfully than using 3DG204 , 3DG56 , 3DG80 , 9018 and other ultra-high frequency tubes, rather than 3DG6, 3DG201 and other general high frequency tubes, and the cut-off frequency of 3DG6 and 3DG201 is greater than 100MHZ, which is enough to meet the requirements, and the stability is also good. The p value of the transistor should not be too high, preferably between 60-100, so as not to cause unstable oscillation frequency.

Installation Tips

The pins of high-frequency components must be short. On the one hand, it reduces interference and is conducive to the normal operation of the circuit. On the other hand, it makes the LC parameters accurate and stable, so that the transmission frequency falls accurately between 88-108MHZ. If the pins of L or C3 are too long, it is equivalent to increasing the turn effect of L, thereby increasing the inductance. For a coil with only a few turns and a small diameter, it is even equivalent to adding one or two turns, thereby reducing the resonant frequency a lot. This cannot be ignored. The layout of components should be separated from high and low frequencies, and the connection lines should not cross each other. There should be a gap of more than 1mm between adjacent printed circuits on the circuit board. Solder one by one, and the solder joints should be reliable and smooth. The circuit board diagram of the schematic diagram is shown in the figure below.

Circuit debugging and troubleshooting

1. Make the circuit oscillate normally

A convenient and feasible way to determine whether a circuit is oscillating is to connect a milliammeter or a multimeter set to the 10mA current range in series in the power supply circuit to monitor the current of the entire machine. When a screwdriver is used to touch the metal rod to the collector of the BG, the current should increase and then return to its original value after leaving the collector, which proves that the high-frequency oscillation is working normally. When touched, the current value changes greatly, indicating that the oscillation is strong. If it does not change, it means that the circuit is not oscillating. The reasons why the circuit does not oscillate are: (1) The DC operating point is not suitable: the bias resistor R2 can be adjusted to make the current of the entire machine between 2-3mA (3-7mA for a 3V power supply circuit); (2) The high-frequency performance of the BG is poor or it has been damaged during installation and needs to be replaced for verification; (3) C2 and C4 are improperly selected and deviate too much from the normal value; (4) There is a short circuit or open circuit fault inside C2, C3, and C4. For a short circuit, it is easy to detect it using general methods. For an open circuit fault, it is difficult to determine without a capacitance meter, but the possibility of such a fault is very small. (5) If there are short circuits or false solder joints on the circuit board or solder joints, check carefully and re-solder. In addition, do not use non-insulating or corrosive flux such as zinc chloride solution and solder paste, as they will deteriorate the insulation performance of the circuit board and cause serious leakage between components. If they have been used, use pure alcohol to clean them and wait until they are cool before debugging.

2. The circuit starts to oscillate.

But the signal cannot be received. If the above method proves that the circuit oscillates normally, but no receiving point can be found in the entire radio FM band, this is because the value selection of L and C3 in the resonant circuit is biased, causing the oscillation frequency to exceed the frequency range of the FM band. Most of the unsuccessful installations by beginners are due to this problem. It can be solved by replacing C3 or properly stretching or compressing L. This situation is mostly caused by the nominal value error of C3. If the value of C3 is measured with a capacitance meter, the above formula is used to calculate and wind L, the above phenomenon will not occur.

3. Unstable work

The main reasons for unstable operation are: (1) poor BG performance or too low β value; (2) C4 value is too large or too small; (3) poor contact at the welding point or inside the component; (4) when changing the orientation or holding the machine in different positions, the unstable operating frequency is caused by the single-tube machine being easily induced by the human body, resulting in frequency deviation. You can turn the radio frequency knob to find the correct frequency, or add a shield to solve the problem.

4. Low sound, but high noise.

(1) Weak high-frequency oscillation: The transmitting power is too small or the receiver is too far away. This can be improved by properly adjusting R2, replacing a tube with a slightly higher p value, rewinding L with a wire with a larger diameter, or properly lengthening the transmitting antenna.

 (2) The microphone performance is not good, or the bias resistor R1 has an inappropriate resistance value (the phenomenon is often that the sound is small but the noise is not necessarily large). R1 ​​can be adjusted so that the voltage at point a is about 1/2 of the power supply voltage, so that the sound is the largest without distortion.

(3) The receiver is not tuned correctly and receives a parasitic oscillation frequency. If no noise-free receiving point can be found in the entire band, the above-mentioned "circuit oscillation, but no station can be found" fault can be handled.