RADIO MICROPHONES

SEVERAL SCHEMES

In general, radio microphones are a constructive combination of an ordinary radio transmitter, the microphone itself, a transmitting antenna, auxiliary devices (optional) and a power source. Radio microphones can be used for holding cultural events, monitoring what is happening in the children's room, especially important with infants, as well as for obtaining information that is contrary to the law, i.e. This radio bugs, listening devices, A this is a criminal offense.

A generalized block diagram of the radio microphone is shown in Fig. 1. The radio transmitter consists of a microphone amplifier - ULF with AGC (its presence is optional), a modulator, a master oscillator (SG), a power amplifier (PA), a matching device (SU) and a power supply (PS). The transmitting antenna WA2 is connected to the output of the matching device, and a microphone (VM1) is connected to the ULF input.

Figure 1 - block diagram of a radio microphone

The presence in the block diagram of the receiving antenna WA1 of the receiver (you can also use the transmitting antenna WA2 as it) and control devices (CDs), switched on by a control signal (command) of radio microphones, which, due to their complexity and high cost, are not very common. The sensitivity of the receiver may be low, since the switching command must be powerful to avoid false alarms. In a particular case, the control device can be triggered by a person’s voice. However, such radio microphones are used in cases where the value of the information obtained is no less than the cost of obtaining it.
When implementing a block diagram, it is necessary to select transistors with a minimum saturation voltage 1)us, which makes it possible to effectively use low-voltage power supplies, increase the efficiency of the device and the output power of the transmitter.
For efficient operation of the transmitter, transistors with cutoff frequency frp should be selected in accordance with the operating frequency f of the selected range. For example, with fp<700 МГц должны быть выбраны транзисторы с frp>3 GHz type KT3101A, KT3132, etc.
In addition, the listed types of transistors are conditionally unpackaged, have small dimensions and good technical characteristics, which allows minimizing the dimensions of the radio microphone.
The given block diagram can also be implemented on integrated circuits. For example, make a microphone amplifier using K548UN2, which has a very high gain with a supply voltage of only 1.2 V (designed specifically for hearing aids). The transmitter can be implemented on KF174PS4. This will make it possible to obtain a miniature radio microphone with high technical characteristics.
The frequency of the master oscillator must be stabilized by a quartz resonator. This will increase the stability of the radio microphone. If it is intended to operate for a long time under significant temperature changes, it is easier to tune in to the signals of a quartz oscillator. The absence of quartz can cause the operating frequency to drift and make it difficult to tune the receiver to it. In simple RMs, quartz stabilization is not used, since this complicates the circuit and increases the dimensions of the device.
Radio microphones, as a rule, use master FM (FM) oscillators, and AM oscillators are used rarely and mainly in the HF range (in walkie-talkie radio stations), when it is necessary to quickly and with minimal hardware costs collect information. However, AM devices have low noise immunity and short range. The use of FM generators makes it possible to significantly increase the noise immunity of the RM and obtain approximately a doubling of the operating range.
Let's consider some circuits of radio microphones, since companies involved in their production, as a rule, do not provide circuit diagrams.
The PM circuit usually consists of two parts, one of which functions as an RF oscillator, and the other functions as a microphone amplifier. The oscillations of the HF generator are emitted by the transmitting antenna WA2 and are picked up by a radio receiver tuned to its frequency. The HF part of the radio microphone is usually made with 1-2 transistors, the microphone amplifier - with 1-3, depending on the required gain, that is, on the required maximum distance to the sound source, which ensures normal speech intelligibility.
Microphone amplifier circuits are well established in modern hearing aids, where miniaturization and performance have reached their limits. Therefore, for RM, many technical solutions “microphone - amplifier” can be borrowed from hearing aid technology.
The circuit of the simplest radio microphone with only two transistors is shown in Fig. 2.
With the parameters of the elements indicated in the diagram, its range is several meters, the modulation is amplitude, the operating range is 25 m (11.9 MHz).

Figure 2 - schematic diagram of a radio microphone.

The MP-4 microtransmitter circuit, advertised by the private Kyiv company RKF, is shown in Fig. 3.
With the element ratings indicated in the diagram, the device operates in the frequency range 68...74 MHz and with an antenna length of 1.2 m provides a range of up to 200...300 m.

Figure 3 - Diagram of the MP-4 microtransmitter.

One of the simplest radio microphone circuits with just one transistor is shown in Fig. 4.
The radio microphone is a hybrid of a regular telephone and a microtransmitter operating in the VHF range 66...74 MHz. Its peculiarity is that it does not require autonomous power supply, since for this purpose the voltage drop across resistor R5, which occurs when the telephone is picked up and a call is made, is used. The range of the transmitter depends on the length of the antenna and is several meters. The device is connected in series with the telephone on any part of the line from the telephone set to the telephone exchange.

Figure 4 - Diagram of a radio microphone powered from a telephone exchange line

A radio microphone operating in the FM range 88... 108 MHz is shown in Fig. 5. To increase the output power of the RF generator, it is made of two transistors. The device uses a sensitive electret microphone MKE-3.

Figure 5 - Diagram of a radio microphone operating in the FM range 88... 108 MHz

A simpler PM circuit for the same range is shown in Fig. 6. Its feature is the presence of a smooth adjustment of the operating frequency within the range using a miniature variable capacitor included in the generator circuit. The range is tens of meters.

Figure 6 - Diagram of a simplified radio microphone operating in the FM range 88... 108 MHz

For obvious reasons, the devices under consideration have high requirements for minimizing the size of the board and the entire product. Of primary importance for their implementation is the principle of the electrical solution of the circuit itself. For the schemes under consideration, due to the absence of a master oscillator, quartz resonator, AFC, and AGC, many parameters of the radio microphone can be critical. For example, increased sensitivity of the circuit when sounds are close and loud enough can lead to overmodulation of the signal, which sharply worsens speech intelligibility.
The considered RMs operate in the broadcasting bands KB, VHF, FM. Therefore, the signals they transmit are received by conventional radio receivers that have these ranges. It is also necessary to work out the installation of the printed circuit board, since due to the design features of VHF equipment, the stability of the device’s operation depends on this.
Many of the above disadvantages are absent when the above adjustments are introduced, which makes it possible to obtain very good technical characteristics, but increases the size and weight of the radio microphone, and this necessitates the need to disguise it as large household items. An example is the product of the American company "LEA Inc.", where the radio microphone is disguised as a baseball cap, belt, etc. It should also be noted that the task of minimizing the size of radio microphones has led to the use of modern technologies, for example, the production technology of hybrid microcircuits.
Let's look at other components of the radio microphone. The main requirement for microphones used in radio microphones is small dimensions. In practice, you can use telephone capsules DEMSH-1 A, TG-2K, TG-7, TON-2, dynamic speaker heads with a power of 0.05...0.5 W and even sound piezoelectric transducers (“tweeters”) of the ZP1, ZPZ type , ZP5, which allows you to significantly reduce the dimensions of the devices. However, the best results are obtained when using special miniature microphones of the MKE-3 type, as well as M-3 type microphones from hearing aids and MM-5 electrodynamic miniature microphones with dimensions of 9.6x9.6x4 mm. They are designed to work as part of various electronic equipment for industrial and household purposes and for organizing communications in studios during radio and television broadcasts in the nominal frequency range of 500...5000 Hz.
As mentioned above, the transmitting antenna is an integral part of the radio microphone and is most often structurally made in the form of a piece of insulated wire with a length of 10... 30 to 120 cm, or an elastic pin of the same dimensions. Such an antenna parameter as the effective height reflects the relationship between the dimensions of the antenna and its efficiency. It is known from antenna theory that a quarter-wave emitter radiates efficiently, but in practice it is necessary to make an antenna with a length of L «A/4 so that it can be easily camouflaged. Therefore, it turns out that with the same transmitter parameters, an antenna with a larger length has a greater effective height, and therefore a greater range of the radio microphone.
To minimize the long antenna, it is made in the form of a spiral, which is several times shorter than a straight wire. In order to increase the effective height of the antenna, so-called extension coils (a piece of wire wound in the form of a spiral coil) can be connected to the devices. It should be noted that at a higher operating frequency of the PM, a smaller antenna is required. It can be disguised as household items (belts, belts, frames, rods, including telescopic rods, nets, etc.).
The power source for radio microphones, if they are installed in household appliances powered by an alternating current network, is usually the network itself. Otherwise, rechargeable batteries and batteries with a voltage of 1.5...12 V are used. They are also subject to weight and size restrictions. Such power supplies must have low internal resistance and high capacitance. The best characteristics are provided by lithium power supplies of the ML type and silver-zinc type SC, which have a flat discharge characteristic. The difference between the initial and final voltages of the source during its normal operation is minimal, which ensures the stability of the electrical characteristics of the RM over time. The high voltage of the power supply allows the use of transistors with a higher saturation voltage in the PM, which allows obtaining greater power of the radio transmitter, and therefore greater range. In practice, you can use watch cells (from wristwatches and microcalculators) and 1.5 V batteries of the type STs, MTs, RC, CR 316.332, TsNK-0.45, D-0.05; D-0.1, D-0.25, Krona batteries, flat batteries of the type used in American Polaroid and Kodak instant cameras (the “bug” can work for several months).
The design of the RM can be very diverse, including custom-made. Most often, custom-made devices are made in a one-time design. In this case, they cannot be repaired or altered, since they are filled with epoxy resin. For restored radio microphones, the best sealant is Termesil paste, since it does not violate the electrical parameters of the radio elements and the device, and is elastic, which allows it to be removed if necessary. It is also possible to use sealant of the “Vixint” type for filling, which is also suitable for sealing and is the same transparent. This increases the maintainability of the device, since it is possible to open a specific element by determining its location visually.
The industry mass-produces radio microphones of the "walkie-talkie" type: intercoms in the form of a children's toy - the "Khvylya" set (JSC "Neva" plant, Khmelnitsky) and for household use - the "PORTA" set (PO "LORTA", Lviv ). Their electrical circuits are built on the principles described above, however, there is no need to obtain permission to purchase and operate them, since the power of the transmitters does not exceed the 10 mW permitted by law.

Editor's note: the described radio microphones can have dual uses: those that do not contradict the laws, for example, listening to a child's room where an infant is, and those that contradict them - unauthorized recording of information. In the latter case, the actions constitute a criminal offense. Only special units of the Ministry of Internal Affairs and the FSB with the sanction of the prosecutor have the right to obtain information using hidden radio microphones.

Roman Parshin,
[email protected]

The page was prepared based on materials from the magazine CIRCUIT ENGINEERING

As you know, simple radio microphones (RMs) have disadvantages such as: low frequency stability, changes in parameters when the power supply changes, frequency drift when touching the antenna, etc. Those who are quite happy with such RMs may not read further :-) So that you understand what we mean Let's go further, I want to quote from the book by V. Kiyanitsa “The best designs of radio microphones” “.... let's consider the principle of stabilizing the operating frequency of the RM using a quartz resonator (popularly quartz). This is the second, radical method of stabilizing the frequency of the RM (the first is parametric). And There are certain difficulties here, the main one of which is the weak susceptibility of quartz to frequency modulation. Namely, frequency (and not amplitude, as is often the case) modulation is necessary for the high-quality operation of the frequency detector of a VHF receiver. Yes, it’s understandable: that’s what quartz is for keep the frequency for which it is made, and not “whack” it in accordance with the vibrations of anyone’s vocal cords. Therefore, those who have already tried radio microphones with quartz could not help but pay attention to the booming sound that arises due to insufficient modulation of the signal. Some circuit tricks make it possible to obtain modulation sufficient for speech signals, however, the full audio frequency band necessary for high-quality operation of the PM is achieved, as a rule, using... frequency multiplication. …..The main difficulty encountered when setting up a frequency multiplier is the ability to adjust its output circuit to the required harmonic (it is difficult to distinguish the second, third, and higher ones due to the fact that the amplitude of the harmonics decreases with increasing frequency). Even experienced radio amateurs sometimes spend a lot of time on such work. That’s why creating a good radio microphone is considered a rather difficult and expensive task.”

Transmitter characteristics:

• frequency range 27…28 MHz
• output power 0.5W
• AF range 300…3000Hz
• emission bandwidth 11 kHz
• frequency deviation at maximum modulation 2.5 kHz
• supply voltage 9 V
• current consumption 100mA

The signal from the microphone is sent to the direct input of op-amp DA1. A voltage divider R2R3 is connected to the input, which creates half the supply voltage on it, allowing the op-amp to operate with single-pole power supply. Circuit R7 C5 C6 is connected between the inverting input and output; it sets the coefficient. gain and frequency characteristic.

The output voltage of the amplifier is detected by diodes VD1 VD2 (KD522A) into a negative voltage that is applied to the gate VT1. This transistor bypasses the inverting input with capacitor C6 and changes the coefficient. OS. Through R11 R12, the voltage from the op-amp output is supplied to a varicap matrix which modulates the frequency of the quartz resonator. The master oscillator is made on VT2, circuit L2C13 in its circuit is configured to the middle of the range. It produces RF voltage which, through L3, is supplied to the output stage at VT3.

The DC operating point is created by the R17 R18 chain. The amplified voltage through the low-pass filter (L4 C16 C17) is supplied to the extension coil L5, and through C19 to the antenna.

All coils except L1 are wound on frames with a diameter of 7 mm with a 100HF core with a diameter of 2.8 mm.
L2 - 6
L3 - 3
L4 - 8
L5 - 20
turns of PEV wire 0.2mm
L1 - DPM choke -0.06 - 16 µH

Literature

• 500 schemes for radio amateurs\

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It is clear that radio microphones have an undeniable advantage over their wired counterparts. However, their high cost in factory-made versions restrains the growth of their use. The solution is obvious - make the device yourself. And although it is unlikely that it will be possible to achieve high performance in a homemade device, this will be an acceptable way out of this situation. Radio microphones based on amateur radio designs have a number of disadvantages. These are low stability of the carrier frequency, high current consumption, poor design, loss of the carrier frequency during prolonged operation of the microphone due to the discharge of the power supply, narrowband FM due to the use of quartz frequency stabilization, which entails a decrease in the quality of the audio path, increased energy consumption due to certain circuit design solutions.

Radio microphone with quartz transmitter frequency stabilization

A printed circuit board with the parts of a radio microphone located on it, except for the microphone itself, elements and the power switch.

Radio microphone with loop antenna

Simple radio microphones

In fact, the necessary frequency deviation of the radio microphone transmitter can be achieved without the use of varicaps, and the circuit itself can be implemented with just one transistor.

Radio microphone

The range of the radio microphone can reach 25 meters from the receiver and about 3-4 meters from the microphone. Setting up a radio microphone comes down to tuning it to a section of the VHF band that is free from radio stations.

Radio microphone

The best solution for a radio microphone is quartz stabilization of the carrier frequency of the transmitter master oscillator shown below. The device operates in the widespread range of 66-74 MHz and consists of a two-stage audio amplifier, a master oscillator and a frequency modulator on a varicap.