Wireless microphone LIEN. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Microphones, radio microphones

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The radio microphone LIEN (translated from French - communication) is designed for one-way communication in the VHF band, as well as for sounding discos and other events.

The radio microphone (PM) LIEN operates at a frequency of 70 MHz (VHF1 band) and is a micropower transmitter with frequency modulation. The PM circuit (Fig. 1) is highly economical and, operating from a 9-volt Korund battery, consumes a current of 6 ... 15 mA. Since the maximum allowable discharge current of Corundum is 20 mA, an LED power-on indicator HL1 is introduced into the PM circuit. With a small current consumed by it (3 mA), it does not overload the battery, but significantly increases the usability of the PM.

Fig.1. Schematic diagram of a radio microphone

The microphone amplifier, which is part of the MKE-3 electret microphone, is powered by an unstabilized voltage through an L-shaped RC link (R1-C3) and provides an AF voltage of up to 30 mV at the output. This signal is fed through the coupling capacitor C2 to the input of the amplifier on the transistor VT1. To improve the temperature stability of the cascade, the bias voltage to the base VT1 is supplied from the collector through R2, and R5 is introduced into the emitter circuit. Capacitor C5 is a blocking capacitor and cuts off the RF components penetrating the ultrasonic frequency circuit from the generator to VT2.

The cascade on the transistor VT2 is a capacitive three-point. Resistive divider R7-R8 determines the bias voltage (U_cm) based on VT2, which operates in cutoff mode (class C). Therefore U_cm based on VT2 can be selected within +0,8 ... +1,2 V. Parallel to the trimmer resistor R8, two silicon diodes are connected that stabilize U_cm and minimize the frequency drift of the generator when the battery is discharged.

Transistor VT2 is covered by positive feedback using C8. The collector VT2 includes a parallel oscillatory circuit L1-C7. When used as a trimmer capacitor C7, its slot is connected to the cold end, that is, +9 V, to reduce parasitic capacitance. The exact value of the inductance L1 is set by a brass or ferrite core (the introduction of a brass core into L1 gives a smaller increase in coil inductance compared to ferrite).

The frequency modulator is assembled on the elements R6, VD3, C5. When the AF voltage is applied from the output of the UZCH through the resistor R6, the VD3 varicap changes its capacitance. From the anode VD3 through C5, the modulating voltage is applied to the tap (4th turn from the top) of the coil L1. This is done to reduce the modulation depth. In a simplified (non-retractable) version of L1, the right (according to the diagram) output C5 can be connected to the lower output L1. You can also reduce the modulation depth by reducing the capacitance C5 or using a varicap as VD3 with a lower capacitance overlap coefficient. In practice, when overmodulation occurs (deviation is more than 150 ... 250 kHz), the capacitance C5 should first of all be reduced.

The RF signal, modulated by the AF voltage, is fed through the coupling coil L2 to the WA1 antenna, made of a PEL 0,96 single-core copper wire. WA1 - type Short whip (short pin) has a length of 184 ... 206 mm, which is selected experimentally when setting up. An important factor for ensuring the stable operation of the RM is the mechanical strength (immobility) of the components of the oscillatory circuit, and especially the antenna.

Before turning on the radio microphone, carefully check the installation. Then it is recommended to check the resistance between the power contacts. The resistance of the measured circuit should not be zero and should change when the polarity of the tester connection changes.

Further, a DC milliammeter with the shortest possible length of connecting conductors is included in the PM power supply circuit. The current consumed by the radio microphone should not exceed 20...25 mA. Otherwise, check the installation again and eliminate possible short circuits. With Iп = 3...18 mA, you can start setting up the PM for direct current:

• set the voltage on the microphone +1,2 ... +3 V by selecting R1;
• set the voltage to 0,5Up on the collector VT1;
• set U=+0,8...1,2 V based on VT2.

Now you can start setting up the generator:

• put a VHF receiver tuned to the desired range (70 MHz) at a distance of at least 2 m from the radio microphone;
• turn on the power supply of the RM and achieve the appearance of generation by rotating the slot of the tuning capacitor C8 with a dielectric screwdriver. The occurrence of generation can be controlled by ear by the characteristic frequency capture (disappearance of the hiss of the receiver). To avoid tuning the receiver to the harmonic, do not place the receiver closer to the PM;
• tune the oscillatory circuit in the VT2 collector circuit with a brass or ferrite core to a resonance frequency (70 MHz) according to the maximum capture width of the broadcasting range between two stations (tuning is possible to a different frequency from the edge of the range or on any free section of the broadcasting range, equidistant from two neighboring stations) .

In case of unsatisfactory results, you should change the capacitance C7 and repeat the setting. To reduce the tuning time, it is recommended to replace the capacitor C7 with a tuning capacitance of 6 ... 30 pF. If the tuning results are satisfactory, you can try to further increase the resonance amplitude by changing the number of turns of the L5 coil by 10 ... 1%.

The oscillation amplitude will be maximum when the elements of the oscillatory circuit are in balance, that is, when the reactances L1 and C1 are equal. Coarse tuning of the L1-C7 circuit is carried out by selecting the number of turns L1 and (or) changing the capacitance C7, and smooth tuning is carried out by a tuning core. The presence of resonance can also be controlled by the minimum Ip. To control Ip, in order to avoid a noticeable frequency drift, you should use a milliammeter with a minimum length of connecting conductors.

It is better to repeat the setting several times with a successive change in the parameters C8, L1, C7, focusing on the minimum current consumed when the oscillatory circuit enters resonance and the maximum bandwidth of the VHF receiver. Therefore, it is more convenient to use a receiver with an arrow setting indicator. And as the power emitted by the radio microphone increases, the distance between the receiver and the RM should be increased.

You can specify the depth of deviation (the magnitude of the change in the frequency of the FM signal) by selecting the capacitance of the coupling capacitor C5 (C5 \u1,2d 10 ... 5 pF). With an increase in CXNUMX, the depth of deviation increases. The capacitance of this capacitor should be such that even in the loudness peaks when the receiver is operated from the RM, there are no crackles, distortions, and even more so, excitation and disruption of radio reception. This type of excitation should not be confused with the characteristic whistle that appears when the RM is close to the receiver tuned to its wave. In this case, to remove the excitation (acoustic feedback), it is enough to reduce the volume of the receiver.

Next, the Lien radio microphone is connected to a battery pack (for example, two 3336L batteries), its frequency is adjusted and the range is checked. After tuning, the core of the inductor L1 is filled with paraffin, and the rotors of the trimmer capacitors are stopped with nitro paint.

The tuned Lien radio microphone was tested in operation with the Ishim-003 broadcasting receiver and had a range of up to 500 m (with line of sight).

You can speed up the process of adjusting a roughly tuned RM using a wavemeter (Fig. 2). The wavemeter consists of a parallel oscillatory circuit C1-C2-L1, a diode detector VD1 and a low-pass filter C3. The parameters of the wavemeter circuit are similar to the parameters of the parallel circuit of the radio microphone. A tester (multimeter) is connected to sockets XS1, XS2 of the wavemeter in the mode of a DC voltmeter (measurement range - 12 V).

Fig.2. Wavemeter

Measurement of the strength of the alternating magnetic field in the antenna PM produced as follows. RM included. The WA1 antenna of the radio microphone (evenly, along its entire length) is wrapped around two or three turns of a flexible stranded wire in insulation and this wire is pulled from the PM antenna in the direction of the arrow (Fig. 2), while simultaneously measuring the voltmeter readings. The maximum readings of the wavemeter are achieved by adjusting the RM contour and the length of its antenna. You can start a similar procedure when using a quarter-wave pin as an antenna. The wavelength L for a given resonance frequency can be calculated using the formula:

L = C/f

where L is the wavelength, m; C is the speed of light (300000 km/s); f is the frequency in megahertz.

The wavelength L for a frequency of 70 MHz is 4,2857 m, and the quarter-wave pin (L / 4) has a length 4 times less - about 107 cm.

In the RM circuit, resistors of the OMLT, VS and similar small-sized resistors with a dissipation power of 0,125 W can be used. Trimmer resistor R8 - type SPZ-22. Capacitors C3, C10 - K50-6, K50-16, K50-35 or similar oxide; C1, C2, C4 ... C7, C9 - type KM4, KM5, K10-7 or any other ceramic (non-inductive). Trimmer capacitor C8 - type KT4-23. It is permissible to replace the VD3 D902 varicap with almost any silicon or germanium diode with a capacitance Cd of more than 1 ... 3 pF. You can find a replacement for VD3 using the table.

Transistor VT1 can be replaced by transistors KT315B, G, and VT2 - KT368B. Diodes VD1, VD2 - any silicon with a direct voltage drop of at least 0,7 V. The value of the resistor R6 can be any in the range from 10 to 100 kOhm.

The inductor L1 is wound on a frame with a diameter of 6,3 mm with a PEV wire ø0,5 ... 0,55 mm with a winding pitch of 1,5 mm. L1 contains 5 turns and has a tap from the 4th (top of the diagram) turn. A coil made of silver-plated copper wire has a high quality factor and is easier to enter the generation mode. You can silver the wire in a spent photo fixer (sodium hyposulfite). But the best results are obtained by using ready-made coils from VHF receivers with a resonance frequency of about 70 MHz, for example, from the VHF-2-01E unit from the Ilga-301 radio.

Structurally, the RM is made on a board of fiberglass foiled on both sides with a thickness of 1,5 ... 2,5 mm. One side of the board is a screen, and the other side, cut into 8x4 mm cells, is being assembled. Board size - 110x27 mm.

Author: A. Oznobikhin, A. Lebedev

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