Production and debugging of wireless FM microphone

The circuit schematic diagram of the wireless FM microphone is shown in the figure below. The LC parameter is an important factor that determines whether the circuit oscillates and the transmission frequency, especially L and C3 in the resonant circuit. It determines the FM transmission frequency and is the key to the success of the production. key.

The author has conducted theoretical derivation, precise measurement and installation verification. Use the following formula to select an accurate and reliable resonant circuit LC value. The formula is: N=2.565×107d/φ2f2C, where N, d, φ, f, and C respectively represent the number of turns of the resonant coil L, the diameter of the enameled copper coin used, the inner diameter of the coil (that is, the diameter of the temporary skeleton used to wind the coil), Transmission frequency, resonant circuit capacitance value. The units used in the formula are all practical units: turns, millimeters, megahertz, and picofarads. For example, if the resonant capacitor C is 15pF, the emission frequency is 96MHz. If an enameled wire with d=0.6mm is used, a coil can be wound on an ordinary ballpoint pen core with a diameter of φ=3.5mm. Calculated by the above formula, 9.1 turns need to be wound. , it is enough to actually wind 9 turns. Note: (1) The number of turns calculated according to this formula refers to a single layer of dense winding, that is, the turns are closely arranged without leaving any gaps, and they emerge after being wound. 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, improve the quality factor Q, and increase the gain of the high-frequency oscillation circuit. (2) When using this formula, adjust the parameter values ​​appropriately so that the transmitting frequency is between 88MHz-108MHz and as close to the edge frequency as possible. And make the number of turns an integer. In addition, C4 determines the frequency bandwidth, generally 3-10P is selected, and 5-8P is appropriate. C2 is related to the modulation depth, and 100-300P is generally selected (in some circuits, the capacitor is placed between the base and the ground. This is optional. Capacitors with a capacity of 500-1500P, C5 is the antenna signal coupling capacitor, select a capacity of 10-20P.

The transistor BG works in a high-frequency state, so a high-frequency transistor with a cutoff frequency greater than 100MHZ must be selected, and the collector junction capacitance must be small. It has been verified by actual installation that it is easier to successfully debug using ultra-high frequency tubes such as 3DG204, 3DG56, 3DG80, 9018, etc. than using general high-frequency tubes such as 3DG6, 3DG201, and the cut-off frequencies of 3DG6 and 3DG201 are greater than 100MHZ, which is sufficient to meet the requirements and is stable. Sex is also better. The p value of the transistor should not be chosen too high, preferably between 60-100, to avoid unstable oscillation frequency.

Installation points

The pins of high-frequency components must be short. On the one hand, it is to reduce interference and facilitate the normal operation of the circuit. On the other hand, it is to make the LC parameters accurate and stable, so that the transmission frequency accurately falls 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 resonance. There are many frequencies, which cannot be ignored. The component layout should be separated for high and low frequencies, and the connections should not cross each other. There should be a gap of more than 1mm between adjacent printed circuits on the circuit board. One place should be soldered and the other should be soldered. The solder joints should be reliable and smooth. The circuit board diagram of the schematic diagram is shown below.

Circuit debugging and troubleshooting

1. Make the circuit oscillate normally

A convenient and feasible way to determine whether the circuit is oscillating is to connect a milliammeter or a multimeter set at the 10mA current range in the power circuit to monitor the current of the whole machine. When holding a screwdriver and touching the metal rod to the BG collector, the current should be Increase the change and restore the value after leaving, which proves that the high-frequency oscillation is working normally. When touched, the current value changes greatly, indicating strong oscillation. If it does not change, the circuit does not oscillate. The reasons why the circuit does not vibrate are: (1) The DC operating point is inappropriate: the bias resistor R2 can be adjusted to make the current of the whole machine between 2-3mA (3-7mA for a circuit with a 3v power supply); (2) BG The high-frequency performance is poor or damaged during installation and needs to be replaced to confirm; (3) C2 and C4 are improperly selected and deviate too much from the normal values; (4) There are short circuit and open circuit faults inside C2, C3, and C4. Short circuits are easy to detect with general methods, while open circuit faults are difficult to judge without a capacitance meter, but the possibility of such faults is very small. (5) If there are short circuits or false solder joints on the circuit board or solder joints, they need to be carefully inspected and re-soldered. In addition, do not use non-insulating or corrosive fluxes such as zinc chloride solution and solder paste. They will deteriorate the insulation performance of the circuit board and cause serious leakage between components. If it has been used, you can use pure alcohol to scrub it and wait until it cools down. Try debugging again.

2. The circuit starts to oscillate.

But the signal cannot be received. If the above method has been used to prove that the circuit starts to oscillate normally, but no receiving point can be found in the entire frequency modulation band of the radio, this is due to the deviation in the value selection of L and C3 in the resonant circuit, causing the oscillation frequency to exceed the frequency modulation band. Frequency range, most of the unsuccessful installation by beginners is due to this problem. It can be solved by changing to C3 or stretching or compressing L appropriately. This situation is mostly caused by the error in the nominal value of C3. If you use a capacitance meter to measure the value of C3, use the above formula to calculate and wind L, the above phenomenon will not occur.

3. Unstable work

The main reasons for stable operation include: (1) Poor BG performance or too low β value; (2) C4 value is too large or too small; (3) Welding joints or poor internal contact of components; (4) Changing orientation or holding different positions Sometimes, the operating frequency is unstable because the single-tube unit is susceptible to human body induction, causing the frequency to shift. You can turn the radio frequency knob to find the correct frequency, or add a screen cover to solve the problem.

4. The sound is small. Loud noise.

(1) Weak high-frequency oscillation: The transmitting power is too small or the distance from the receiver is too far. You can adjust R2 appropriately and replace the tube with a slightly higher p value, rewind L with a larger diameter wire, and lengthen the transmitting antenna appropriately. methods have been improved;

(2) The microphone performance is not good, or the resistance value of the bias resistor R1 is inappropriate (the phenomenon is often that the sound is small but the noise is not necessarily loud). You can adjust R1 so that the voltage at point a is about 1/2 of the power supply voltage to make the sound the loudest and It is better not to be distorted;

(3) The receiver is not tuned accurately and what is received is the spurious oscillation frequency. If you cannot find a noise-free receiving point in the entire band, you can deal with the above-mentioned "circuit oscillation, but no station found" fault.