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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Radio microphone for lecturers. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Audio equipment Recently, descriptions of radio microphones often appear in amateur radio literature. This article will focus on a radio microphone that can work in the same room with speakers. This is achieved through the use of a compressor chip with a built-in noise suppressor. Radio microphones (RMs) are typically used for sound amplification and are used in the same room as high power loudspeakers. Obviously, if the RM can receive a sound signal from a source located at least a few meters away from it, then an acoustic tie will inevitably arise. Therefore, the RM is equipped with sensitivity controls and threshold noise suppressors. Usually the regulators are set in such a way that the RM does not amplify the voice of a person who is 1,5 ... 2 m away. To ensure decent sound quality, the RM must operate in the VHF band with wideband FM. It is obligatory to have an AGC (compressor) and a noise suppressor in the RM modulator. Since the task of the PM is to transmit the signal to the input of the power amplifier without wires for only a few meters, it is not necessary to have an output power of more than a few milliwatts, especially since this increases the battery life. If the task is to create an RM for vocalists, then it is very difficult to obtain high sound quality, since expensive microphones with good characteristics are needed here, and the noise suppressor requires very careful tuning. For radio systems for lecturers, the requirements are less stringent, since in these cases intelligibility is important and it is enough that the voice is simply recognizable. Therefore, inexpensive electret microphones can be used. Usually, the lavalier microphone and transmitter are placed in a pocket under outerwear so that the lecturer's hands remain free and the RM is invisible to others. If there is a compressor in the modulator, the lecturer's voice will be heard well, even if he turns away, but the voice of his interlocutor will not be heard if the squelch threshold is set correctly. The radio microphone, the circuit of which is shown in fig. 1, this is a refinement of the device published in [1]. For PM modulators, the SSM2166 chip, which is a microphone preamplifier with an adjustable compressor and noise suppressor, turned out to be very convenient. A simple tone control on the elements R17-R19, C21, C22 allows you to get the desired sound timbre [2]. Due to the passive regulator, it was necessary to raise the compressor threshold by reducing the resistance of the resistor R8 to 7,5 kOhm. Of the inexpensive electret microphones, very good results are shown by microphones sold on the radio market under the name "fungus". Convenient and microphone attachment with pins.
The voltage regulator assembled on the DA2 chip (KR1170EN6) of the "Low drop" series, that is, with a small input-output voltage drop, has been greatly simplified. A small current consumption (about 22 mA) allows you to use a battery with a capacity of 0,15 Ah (15F8K-U) for power. For recharging, the device is provided with a current stabilizer on a VT5 transistor and a charging process indicator - an HL2 LED. On the side of the housing there is an X2 connector for connecting a network adapter that provides a rectified voltage of +12 V. The scheme of the transmitter itself has been changed. The master oscillator at a frequency of 87,9 MHz is assembled on a VT2 field-effect transistor with an insulated gate. It is necessary to set the drain current within 12 ... 14 mA. It is possible to electronically adjust the frequency (approximately 2 MHz) using the tuning resistor R13. Switch SA1 closes the feedback circuit to ensure generation in the low-frequency path. This is only needed to set up the transmitter and receiver. The L5 coil is frameless, wound with a wire 1 mm in diameter on a mandrel 6 mm in diameter and contains 4 turns with a tap from the first one. The output coil (L3) is also wound on the same mandrel and contains 10 turns of wire with a diameter of 0,6 ... 0,8 mm. Inductors L1, L2, L4 - any type of inductance from 56 to 100 μH. The radio microphone is housed in a plastic case of the KM-26 type. On fig. 2 shows a drawing of a printed circuit board. There are few connecting wires on the side of the component placement, and it is quite acceptable to use a single-sided jumper board. Connector X1 (mono JACK 3,5) for a microphone and a two-color LED HL1 indicating power on and battery discharge are displayed on the top cover of the case.
The board has a special cutout to secure the connector to the bottom of the case where the board is placed. Then it is enough to unscrew one lag screw to remove the upper part of the case and gain access to all the controls. After all, the initial setting of sensitivity, noise reduction threshold and tone is usually carried out once. A clip for a belt or pocket is fixed on the top cover of the case. There is also a hole for the HL2 LED. Since the battery can last for a very long time without being replaced, the battery replacement window cover in the case is glued. The SA2 power switches and the SA1 positive feedback circuit are brought to the left cover of the PM housing. It is advisable to shorten the switch handles, especially for SA1, in order to exclude the possibility of accidental switching on. Moreover, the SA1 switch is only needed when tuning analog receivers for the characteristic low-frequency "buzz" and it is perfectly acceptable to limit the ability to turn it on only with a screwdriver or similar object. On the same left cover there is an X2 connector for turning on the network adapter. You can choose any connector type, as long as it fits the adapter and is placed on the board. When setting up the device with tuned capacitors C11 and C12, they achieve the highest readings of the field indicator when the antenna (microphone cable) is on. Tests of the RM layout confirmed that it is not only not inferior, but sometimes it turns out to be more convenient in operation than some inexpensive industrial devices of this purpose. And a few words about the radio receiver of the system. The most convenient were pocket scanning receivers with a digital scale. You just need to set a constant tuning frequency for such a receiver and switch it to AC power. The device of such radio receivers is considered in [3]. If you do not turn off the "Scan" button, then when you turn off the RM, the receiver will immediately tune in to the frequency of the next broadcast radio station and the lecture will end with some kind of fun music or advertising. To introduce a constant setting, the easiest way is to cut the track leading to the varicap and apply constant voltage there from the multi-turn potentiometer engine through a resistor with a resistance of several hundred kilo-ohms. The small-sized potentiometer RP1-48 is easily placed together with a simple voltage regulator in the battery compartment of even small receivers. It is advisable to find a receiver with a dynamic head and with an external antenna, since it will not be possible to connect a receiver with telephone wires as an antenna to the amplifier input. You can also connect an external antenna to a receiver that does not have one. How to do this is described in [3], but DC power is supplied to the output transistor through telephones. Therefore, to remove the signal from the telephone jack, you need to include a resistor between the collector of the output transistor and the power supply. The signal is taken through a decoupling capacitor. Literature
Author: E.Kuznetsov, Moscow
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