In order to achieve frequency stabilization, a ceramic oscillator is used.
The FM wireless microphone is used to change the oscillation frequency using sound to achieve the purpose of transmitting the sound. This signal transmitted in the wireless state can be received by an FM tuner or the like.
Most FM wireless microphones are used using an LC oscillation circuit. However, the LC oscillation circuit is susceptible to the fluctuation or temperature change of the power supply voltage, and the frequency changes.
Generally, although the power supply voltage can be relatively easy to stabilize, there is still a temperature change. As a result, the frequency of LC oscillation changes.
The FM wireless microphone made by this method, and must be reorganized with the receiver's receptionary rate at each time. It is also necessary to tune it.
In order to avoid re-adjusting reception each time, a ceramic oscillator having a better frequency stability of the oscillation frequency can be used.
The performance of the ceramic oscillator is similar to that of the crystal. Figure 1 shows the electrical properties of the ceramic oscillator. Figure (a) is an equivalent circuit, and Figure (b) is a resistance characteristic.
The oscillation frequency generates an electric resistance between the inductive FS and FP:
Texture of ceramic oscillator
In the inductive field of ceramic oscillators, the inductive fields of FS ~ FP crystals fs ~ fp dozens of times. Therefore, when performing frequency modulation (FM), the use of ceramic oscillators easily achieve high modulation or "loudness", "pickup sensitivity", and the like.
Summary of the radio microphone produced:
Fig. 2 is a block diagram showing the radio microphone made in this time, and the design specification of FM wireless microphone is shown in Table 1. The receiver is a FM tuner, and therefore, its reception frequency is between 76 MHz to 90 MHz. (You can use a general FM radio to receive, however, in order to avoid violation of the radio wave method, its scope is only used indoors. The characteristics are small.)
Transfer frequency FM transmission frequency band
76MHz ~ 90MHz
Electric wave type FM
Possible transmission distance 20m
Frequency offset ± 75kHz
Power voltage DC306V ~ 6V
Battery
Frequency drift within ± 20kHz
(Due to less types of ceramic oscillators, 12MHz, which is easier, and therefore, it is necessary to use a 7 frequency circuit.)
In order to avoid violation of the waves method, the call transmission distance produced by the time is 20m, and its frequency offset (due to the frequency change in frequency changes) and the FM broadcast station are also ± 75 kHz.
The power supply is a nickel-cadmium battery or a general dry battery (3 or 4), and the working principle voltage range is about 3.6V to 6V. Taking into account the selectivity of the FM tuner, the set frequency changes within ± 20 kHz.
Using oscillation circuit to do frequency changes
Figure 3 shows the configuration of an oscillation circuit that can form a frequency change. This oscillating circuit is basically shown in Fig. (A), which is the non-adjustment oscillation circuit shown in Section 3-4.
In this case, the variable capacitance diode 1Sv50 is used in series using a ceramic oscillator CSA12.0MX (Village Making Center), and the modulation signal is directly added to this, and the electrostatic capacitance of the variable capacitance diode can be changed to achieve the purpose of the FM modulation.
In order to easily understand the working principle of frequency modulation, the equivalent circuit of the oscillation circuit will be expressed as the equivalent circuit of Figure (B).
(In order to make the circuit easy to increase, the higher transistor is used. Use a variable capacitance diode, directly constitute the FM modulation circuit.)
The ceramic oscillator is working principle in the field of inductance, and therefore, the oscillating circuit can be considered as a coil (inductance) working principle.
Although there is an electrostatic capacitance of a variable capacitance diode in series, since the amount of inductance of the ceramic oscillator is very large, the whole of the ceramic oscillator and the variable capacitance diode is in principle the working principle of the coil form (inductor).
However, the inductance of this coil will vary depending on the electrostatic capacity of the variable capacitance diode, and therefore, the voltage applied to the variable capacitance diode can also change the oscillation frequency.
However, even if the vibrating frequency of the ceramic oscillator is high, it will not be higher than 30 MHz and cannot directly oscillate 76 MHz to 90MHz. Here is an oscillation at 12 MHz, and then 7 times is 84 MHz.
The output without adjusting the oscillation circuit does not connect the resonant circuit, and therefore, the output waveform without adjusting the oscillation circuit does not become a beautiful waveform, but is a distortion waveform having a high harmonic component. However, due to the use of 7 times high frequency, the waveform is slightly distortion, and there will be no significant effect.
The finished product of the wireless microphone is so good, but considers the convenience of production, the decision is about 20mm × 100 mm.
The power supply can be used with 3.6V, 50 mAh nickel-cadmium battery, mounted on the substrate, and charging the output voltage of the DC6V ~ 9V of the AC Adaptor. Therefore, a charging terminal is provided on the substrate.
A dry battery can also be used, for example, using three single 4 or single 5 dry batteries. Although the power supply voltage at this time is 4.5V, the value of the circuit does not need to be changed.
It should be noted that the general dry battery cannot be charged, so it cannot be connected to the AC Adaptor for charging.
Adjust the output level of the wireless microphone
For the adjustment of the wireless microphone, you can use the FM receiver of the Level Meter.
The adjustment of the output level is to use the adjusted starter (Driver) to adjust L1. This adjustment should not be a metal material, and the plastic material can be used to cut its front end to insert the L1 core.
Adjustment is shown in Figure 11, slowly rotating the L1 core, making the Tuning Meter of the FM tuner indicate the maximum.
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