Design and production of wireless FM microphones

To achieve frequency stabilization, ceramic oscillators are used.

FM wireless microphone uses sound to change the oscillation frequency to achieve the purpose of transmitting the sound. The signal transmitted in the wireless state can be received using an FM tuner.

Most FM wireless microphones use LC oscillation circuits. However, the LC oscillation circuit is easily affected by changes in power supply voltage or temperature changes, causing the frequency to fluctuate.

Generally, although the power supply voltage can be stabilized relatively easily, there are still temperature changes. As a result, the frequency of LC oscillation will still change.

The FM wireless microphone produced by this method must be realigned with the reception stability of the receiver every time it is used. That is to say, it must be tuned.

In order to avoid the need to readjust the reception every time, a ceramic oscillator with better frequency stability of the oscillation frequency can be used.

Ceramic oscillators perform similarly to crystals. Figure 1 shows the electrical characteristics of a ceramic oscillator. Figure (a) is the equivalent circuit, and Figure (b) is the reactance characteristics.

The oscillation frequency occurs when the reactance is inductive between fs and fp.

Construction of Ceramic Oscillator

The inductance range fs~fp of the ceramic oscillator is dozens of times that of the inductance range fs~fp of the crystal. Therefore, when doing frequency modulation (FM), it is easier to use ceramic oscillators to achieve high modulation, or "loudness", "pickup sensitivity", etc.

Overview of the radio microphone produced

Figure 2 shows the block diagram of the radio microphone produced this time, and Table 1 shows the design specifications of the FM wireless microphone. The receiver can use an FM tuner, so its receiving frequency is between 76MHz and 90MHz.

(A general FM radio can be used for reception, but in order to avoid violating the Radio Wave Act, its range of use is limited to indoor use. It is characterized by small frequency variation.)

Transmission frequency FM transmission band

76MHz~90MHz

Radio wave type FM

Possible transmission distance 20m

Frequency offset ±75kHz

Power supply voltage DC306V~6V

Built-in battery

Frequency drift within ±20KHz

(Since there are few types of ceramic oscillators, 12MHz, which is easier to obtain, is used here. Therefore, a 7-fold frequency circuit is required.)

In order to avoid violating the radio wave law, the transmission distance of the call produced this time is up to 20m, and its frequency offset (the width of frequency change due to frequency modulation) is the same as that of FM broadcasting stations of ±75kHz.

The power supply uses nickel-cadmium batteries or ordinary dry batteries (3 or 4 pcs), and the working principle voltage range is about 3.6V ~ 6V. Taking into account the selectivity of the FM tuner, the frequency variation here is set to be within ±20kHz.

Using oscillator circuit for frequency modulation

Figure 3 shows the structure of an oscillation circuit that can form frequency modulation. The basics of this oscillation circuit are as shown in Figure (a). This is the unadjusted oscillation circuit shown in Chapter 3, Section 3-4.

Here, a ceramic oscillator CSA12.0MX (Murata Manufacturing Co., Ltd.) is used in series with a variable capacitance diode 1SV50. The modulation signal is directly added here to change the electrostatic capacitance of the variable capacitance diode to achieve the purpose of FM modulation.

In order to easily understand the working principle of frequency modulation, the unadjusted oscillation circuit is represented by the equivalent circuit in Figure (b).

(for

To make the circuit easier to oscillate, use transistors with higher fT. Use variable capacitance diodes to directly form an FM modulation circuit. )

Ceramic oscillators work in the field of inductance, so the oscillation circuit can be regarded as a coil (inductor) working principle.

Although the electrostatic capacitance of the variable capacitance diode is connected in series, since the inductance of the ceramic oscillator is large, the entire ceramic oscillator and variable capacitance diode still operate in the form of a coil (inductor).

However, the inductance of this coil will change due to the electrostatic capacity of the variable capacitance diode. Therefore, changing the voltage applied to the variable capacitance diode can also change the oscillation frequency.

However, no matter how high the vibration frequency of the ceramic oscillator is, it will not be higher than 30MHz and cannot directly oscillate to the 76MHz to 90MHz of the FM broadcast band. Here, it oscillates at 12MHz, which is multiplied 7 times to become 84MHz.

There is no resonant circuit connected to the output of the unadjusted oscillation circuit. Therefore, the output waveform of the unadjusted oscillation circuit will not be a beautiful waveform, but a distorted waveform containing high harmonic components. However, since it uses a high frequency that is 7 times higher, a slight distortion of its waveform will not have much impact.

The smaller the appearance of the finished wireless microphone, the better. However, considering the convenience of production, the size was decided to be approximately 20mm × 100mm.

The power supply can use a 3.6V, 50mAh nickel-cadmium battery, which is installed on the substrate and charged using the DC6V~9V output voltage of the AC Adapter. Therefore, charging terminals are provided on the substrate.

The power supply can also use dry batteries, for example, use three single AA or AA dry batteries connected in series. Although the power supply voltage at this time is 4.5V, the circuit values ​​do not need to be changed.

It should be noted that general dry batteries cannot be recharged, so they cannot be connected to the AC Adapter for charging.

Adjust the output level of the wireless microphone

For wireless microphone adjustment, you can use an FM receiver with a level meter.

To adjust the output level, use an adjustment driver to adjust L1. This adjustment screwdriver cannot be made of metal, it can be made of plastic, and its front end can be cut into a shape that can be inserted into the L1 iron core for adjustment.

Adjust as shown in Figure 11. Slowly rotate the L1 iron core so that the FM tuner's tuning meter indicates the maximum.