Radio microphone. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Audio equipment

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The advantages of radio microphones over traditional corded microphones are well known. However, the high cost of branded products for many potential consumers makes them inaccessible. There is only one way out of this situation: make a microphone yourself. True, the high technical characteristics inherent in industrial devices are unlikely to be achieved. Nevertheless, in some cases, even a simple home-made radio microphone can fully satisfy the needs of its owner.

On the pages of the magazine "Radio" and in other amateur radio literature, many articles were published with descriptions of various radio microphones. Unfortunately, some of them, in my opinion, have such disadvantages as low stability of the carrier frequency, high current consumption, and unsatisfactory design.

Particularly big trouble is caused by the departure of the carrier frequency during prolonged operation of the microphone due to the discharge of the power source, which is usually used as a 7D-0.115 rechargeable battery.

The use of quartz frequency stabilization [1] makes it possible to obtain only a narrow-band FM, which is undesirable due to a decrease in the quality of sound transmission. In addition, the stability of the carrier frequency, as the author of the mentioned article himself points out, depends on the position of the operating point of the varicap, which is determined by the stability of the supply voltage or, at best, the voltage generated by the simplest parametric stabilizer R2VD1. In addition, such a stabilizer consumes a current of about 7 mA, which is comparable to the current consumed by all other elements of the radio microphone, and ultimately leads to increased power consumption of the power supply.

Certain operational inconveniences are presented by the antennas used in some microphones in the form of a hanging wire. It is difficult to recognize the loop antenna as successful [2].

These shortcomings forced the author of these lines to abandon the use of similar circuit solutions when developing their version of the radio microphone and choose parametric stabilization of the frequency of the master oscillator with the adoption of effective measures to improve its long-term stability.

Schematic diagram of the radio microphone is shown in fig. 1. It consists of a voltage stabilizer, a battery discharge control device, an amplifier modulator, and a three-stage transmitter. The voltage stabilizer is made on the basis of what is described in [3]. It is assembled on a microassembly VT1, transistors VT2, VT3 and is designed to power the microphone master oscillator. The advantage of such a stabilizer is a rather large stabilization factor (about 2000) with a very low current consumption (less than 1 mA), which allows for good frequency stability throughout the entire microphone session. The source of exemplary voltage is the transistor VT3 in a diode connection. It is selected during tuning until a voltage of about 7 V is obtained at pin 1 of the VT6 microassembly. The same voltage is used as a reference for the DA1 comparator, on which the power supply discharge control device is made. Resistor R3 sets the ignition threshold of the HL1 LED when the supply voltage drops to the maximum allowable value (~ 7 V).

The modulator amplifier is made on a DA2 chip and a VT4 transistor. As DA2, the K513UE1 chip is used. working in various electret microphones, including the MKE-9 microphone used in the described device, the required sensitivity is set by resistor R9. The signal amplified by this cascade is fed through the L1 choke to the VD1 varicap, which provides frequency modulation of the master oscillator signal on the VT5 transistor. The oscillator frequency was chosen twice as low as the operating frequency of the microphone. Cascades on transistors VT6 and VT7 perform the functions of a frequency doubler and a power amplifier, respectively. This construction of the RF path significantly reduces the influence of the operator's hand on the frequency of the master oscillator through the antenna located inside the microphone housing.

(click to enlarge)

The design of the microphone can be any, it is only important that the requirements for the installation of high-frequency circuits are observed. The author's version of the design is shown in Fig. 2. Body parts are made of ebonite. The decorative grille was used from the MD-85 microphone. The spiral antenna is wound with PEL 0.5 wire on a cylindrical frame with a diameter of 28 mm. Winding pitch - 7, length - 68 mm. Coils L2, L3, L5 are wound on frames with a diameter of 5 mm with trimmers made of 50VCh ferrite with a diameter of 4 mm and contain 8 (L2, L5) and 6 (L3) turns of PEL 0,3 wire. Chokes DM-1 are used as coils L1, 4-0,1. Trimmer resistors R3, R9 - SPZ-19A, constant -MLT 0,125. Oxide capacitors -K50-20, tuning capacitors - KT4-25, the rest - KM-3, KM-4.

The radio microphone is powered by a 7D-0.115 battery with a voltage of 9 V.

Adjust the radio microphone according to the generally accepted method. Resistor R8 is selected until a voltage equal to half the output voltage of the stabilizer is obtained at the positive terminal of capacitor C6, R10 - according to the minimum distortion of the low-frequency signal. With the help of capacitor C15, the required frequency deviation is selected. Capacitor C22 allows you to adjust the excitation level of the output stage, i.e., actually set the output power of the transmitter.

According to the scheme described above, two radio microphones with operating frequencies of 66,5 and 67,5 MHz were assembled, which were tested in a concert hall. Reception was carried out on a two-channel receiver, assembled according to a standard scheme. The VHF-1-05S block was used as the HF path, and the K174XA6 microcircuit was used as the IF. With a current consumption of 20 mA, the microphone is able to work continuously for three hours. The carrier frequency drift when the supply voltage was reduced to 7 V did not exceed 35 kHz.

In order for the current through the VT3 transistor to remain stable during temperature fluctuations, the VT1 VT2 transistors must be included in one assembly, for example, KR198NT5 (6, 7, 8) with any letter index. In this case, for one of the assembly transistors, the base and emitter must be connected to each other and connected to the VT2 base, and its emitter to the VT2 collector.

Literature

1. Abramov A. Radio microphone with quartz frequency stabilization of the transmitter. - Radio. 1995. No. 9. p. 27.
2. Ruzmatov V. Radio microphone with loop antenna. - Radio. 1995, No. 7. p. 17.
3. Perlov V. Transistors and diodes as zener diodes. - Radio. 1976, No. 10. p. 46.

Author: A.Bovkun, Kharkov, Ukraine

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