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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Radio microphone. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Civil radio communications Words are superfluous about the popularity of radio microphones, more and more often any performance from the stage, rally, meeting, public event is not complete without them. Since medium and high-end industrial devices are expensive and inaccessible, there is a wide field of activity for radio amateurs here. Below we offer a description of an amateur radio microphone, which is well designed and has improved parameters compared to other homemade ones. This radio microphone is designed for sounding events, listening to a children's room, etc. The device operates in the VHF band at a frequency of 87,9 MHz, specially reserved for radio microphones, and its signals are received by a conventional broadcasting receiver with a VHF-2 band. The range of the radio microphone within the line of sight - more than 200 m Unlike the similar designs described earlier [1], this radio microphone is more complicated, but it has a number of advantages. It has a mic amplifier AGC that picks up weak sounds and eliminates strong non-linear distortion when loud sounds are sent directly to the microphone. The described radio microphone has a relatively high frequency stability and good use of the supply battery, in particular, its performance is maintained when the supply voltage is reduced from 10 to 5 V. Scheme and principle of operation. The diagram of the radio microphone is shown in fig. 1. The transmitter is assembled on a VT4 transistor in a single-stage circuit. Such a solution for a miniature device, such as a radio microphone, is justified, since the use of a separate master oscillator and an output stage in the transmitter leads to a decrease in its efficiency and an increase in dimensions.
As you know, the frequency of an LC generator operating in the 100 MHz region depends significantly on the supply voltage. For example, the author investigated the widespread capacitive "three-thin" with the inclusion of a transistor according to a common base circuit. According to this scheme, the transmitter of the radio microphone described in [1] is included. The generator frequency drift was more than 1 MHz when the supply voltage changed from 5 to 10 V. The introduction of a voltage stabilizer into the radio microphone would lead to an increase in losses. Therefore, in the device under consideration, the transmitter is powered directly from the source. Unlike those described earlier, the transmitter contains two circuits - the L1C9C10C12C13VD2 circuit that sets the generator frequency, and the L3C15C16 output circuit associated with the antenna. This improves the stability of the generated frequency. The master circuit is connected to the transistor VT4 according to the Clapp circuit recommended for building transmitter master oscillators [2]. The effect of changing the parameters of the transistor VT4 when changing the supply voltage to the driving circuit is minimized by choosing a small coefficient of inclusion of the transistor in the circuit (determined by the capacitance of capacitors C10, C12, C13). To increase the temperature stability of the frequency, capacitors C9, C10, C12, C13 with a small TKE are used, and the coefficient of inclusion in the driving circuit of the varicap VD2 is small due to the small capacitance of the capacitor C9. The output P-loop allows you to match the antenna with the output of the transistor VT4 and improves the filtering of higher harmonics. Note that the conventional circuit attenuates harmonics in proportion to (n2-1), and the P-circuit - n(n2-1), where n is the harmonic number [3]. The output circuit is tuned to the frequency of the second harmonic of the driving circuit. This reduces the influence of the output circuit on the driving circuit through the capacitance of the collector-base junction of the transistor VT4, thereby improving the frequency stability of the transmitter. Due to all these measures, the frequency drift of the transmitter when the supply voltage changes from 5 to 10 V is small and the receiver does not need to be tuned during operation. The sound signal from the BM1 electret microphone is fed to the input of a microphone amplifier assembled on an operational amplifier (op-amp) DA2. The microphone receives power through the resistor R1 and the decoupling circuit R5C2. To reduce the power consumption at the DA2 site, a micropower OS K140UD12 was used. Resistor R10 sets the current consumption of the op-amp to about 0,2 mA. High power is not required from the microphone amplifier, because it is loaded on the varicap, and the power to drive the varicap, which is a reverse-biased diode, is extremely small. Resistor R7 and the resistance of the drain-source section of the field-effect transistor VT1 form a negative feedback circuit that determines the gain of the microphone amplifier. The channel of the field effect transistor VT1 serves as an adjustable resistance in the AGC system. When the gate-source voltage is close to zero, the channel resistance is about 1 kOhm and the gain of the microphone amplifier is close to 100. When the voltage increases to 0,5..-.1 V, the channel resistance increases to 100 kOhm, and the gain of the microphone amplifier decreases to 1. This provides an almost unchanged signal level at the output of the microphone amplifier when the signal level at its input changes over a wide range. Capacitor C4 creates a drop in the frequency response of the microphone amplifier in the high-frequency region to reduce the modulation depth at these frequencies and prevent the spread of the transmitter signal spectrum. Capacitor C3 blocks the DC feedback circuit of amplifier DA2. Through the resistor R4, the bias voltage required for a unipolar supply is supplied to the non-inverting input of the op-amp DA2. Transistor VT3 performs the function of the AGC system detector and controls the field effect transistor VT1. The threshold for operating the AGC system is set by a trimming resistor R12. When the signal from the output of the microphone amplifier and the triggering bias voltage from part of the resistor R12 in total equal to the opening voltage of the emitter-base junction of the transistor VT3, the latter opens, applying voltage to the gate of the field effect transistor VT1. The channel resistance of the field effect transistor VT1 increases, and the gain of the microphone amplifier decreases. Thanks to AGC, the amplitude of the signal at the amplifier output is maintained at a practically constant level. This level can be adjusted by changing the bias voltage of the transistor VT12 with resistor R3. The R9C5 circuit sets the response time constant, and the R8C5 circuit sets the AGC recovery time constant. To compensate for temperature changes in the opening voltage of the emitter-base junction of transistor VT3, voltage is applied to resistor R12 from diode VD1. Transistor VT3, the AGC threshold formation circuit R11R12VD1 and resistor R4, through which the bias is applied to the non-inverting input of the op-amp, are powered by the voltage regulator DA1. The same voltage is applied through resistor R14 as a bias voltage to the VD2 varicap. Since the capacitance of a varicap significantly depends on the bias voltage applied to it, stringent requirements are imposed on its stability. Therefore, the DA1 stabilizer is the KR142EN19 microcircuit, which is a parallel-type voltage stabilizer [4]. By choosing resistors R2 and R3, a stabilization voltage of about 3,5 V is set at pin 3 of the DA1 chip. The ballast resistance is a current generator on a field-effect transistor VT2, which increases the efficiency of the stabilizer. Details. It is permissible to use fixed resistors MLT, S2-23, S2-33 with a tolerance of no more than ± 10% in the device, any small-sized trimming resistor R12, ceramic capacitors - K10-17, K10-73, KD, KT. Capacitors C9, C10, C12, C13, C16 must be of the M47 group according to TKE. Capacitors C1, C4, C11 - groups M750 or M1500 according to TKE. Capacitors C6, C7, C8, C14 - H90 group according to TKE. Trimmer capacitor C15 - KT4-23. Capacitor C2 - K50-35 or K50-68. It is advisable to take capacitors C3, C5 with a low leakage current, for example, K53-18 V. Instead of the transistor KP10ZE (VT1), it is permissible to use KP10ZI or KP10ZZH. Instead of the VT3 transistor, any low-power silicon one with a current transfer coefficient of at least 100 is suitable. We will replace the KT368BM (VT4) transistor with KT368B, KT368A (M), the KV121A (VD2) varicap with KV121B. The K140UD12 (DA2) op amp has good internal frequency correction, is stable when operating with unity gain, and its replacement with other types of op amp is undesirable (in particular, the micropower op amp KR1407UD2 was excited). Import analogue of the DA1 chip - TL431. Microphone VM1 - electret (NMC or domestic MKE-332). The inductor L1 is wound on a frame with a diameter of 6 mm with a trimmer from the FPF circuit of the image of the radio channel module of USST TVs. The number of turns is 8. The winding is made turn to turn with a wire with a diameter of 0,25 mm. Inductor L2 is wound on a resistor 02-33-0,5 W with a resistance of about 1 MΩ or more. It contains 60 turns of wire with a diameter of 0,06 mm. The winding is divided into three sections of 20 turns. The winding is carried out in bulk, and gaps of at least 0,5 mm wide are left between the sections. A standard RF choke with an inductance of 5 uH will also work. The inductor L3 is wound on a frame with a diameter of 5 and a length of 20 mm with a brass or copper trimmer. The author used a frame with a trimmer from the contour coil of the PTK-11 drum switch from a tube TV. The winding contains 7 turns of wire with a diameter of 0,8 mm, wound round to round. The turns of all coils should be fixed with glue or varnish to prevent them from slipping. Installation of the device can be hinged or printed. When making a microphone, a number of requirements must be met. Capacitor C6 and resistor R10 are connected as close as possible to the terminals DA2. The elements of the transmitter must have the shortest connections between themselves, the capacitor C11 is located as close as possible to the transmitter. Inductive elements L1, L2, L3 must have a mutually perpendicular orientation in space. The capacitor rotor 015 is connected to the common wire of the device.
The design of the antenna is shown in fig. 2. For its manufacture, a copper winding wire with a diameter of 0,8 mm is needed, the coil contains 17 turns wound in one layer turn to turn. After winding, the turns are fixed with glue. Establishment. First, the coil trimmer L1 should be completely screwed into the coil, the rotor of the capacitor C15 should be set to the middle position, and the coil trimmer L3 should be screwed inward to the middle of its winding. By applying a supply voltage of 7,5 V, a voltmeter with a resistance of at least 10 kOhm / V measures the voltage at the points indicated in the diagram. The measured values must not differ from those indicated by more than ±0,3 V. Then, with a resistor R12, the voltage between its engine and the emitter of the transistor VT3 is set within 0,25 ... 0,3 V. The broadcasting receiver is turned on in the VHF-2 range and tuned to the operating frequency. The receiver and the adjusted radio microphone are placed next to each other. The volume of the receiver is set to correspond to a loud conversation. With a screwdriver made of dielectric material, smoothly rotate the L1 coil trimmer until a loud sound appears in the receiver's loudspeaker, which will indicate that the radio microphone transmitter is tuned to the receiver frequency. Turn off the receiver. The setting of the output circuit of the transmitter is carried out using a wavemeter. Due to the fact that the output circuit is initially detuned, the signal emitted by the transmitter antenna may be weak to be detected by the wavemeter. Therefore, the author connected the wavemeter circuit through a 1,5 pF capacitor to the connection point of the inductor L3 and the radio microphone antenna, connecting the common wires of both devices with a short conductor. Adjust the wavemeter to the maximum readings on the operating frequency of the radio microphone. With a detuned output circuit, a signal with the frequency of the master circuit may be present at the antenna output, so the wavemeter must be tuned exactly to the frequency of 87,9 MHz. With a dielectric screwdriver, the rotor of the capacitor C15 and the trimmer of the coil L3 are smoothly rotated alternately, achieving the maximum readings of the wavemeter. When, during the tuning process, the wavemeter indicator arrow starts to go off scale, it is necessary to disconnect it from the radio microphone and carry out further tuning to the maximum signal emitted by the antenna, also achieving the maximum wavemeter readings. After that, a sound source is placed next to the radio microphone, for example, a tape recorder, the volume of which is set at the level of a whisper. Taking the receiver to another room, turn it on and tune to the frequency of the radio microphone. If the signal heard by the receiver is quiet and unintelligible, the resistor R12 reduces the bias voltage of the transistor VT3, achieving an intelligible sound of the receiver. Set the volume of the tape recorder to scream level. If the signal heard by the receiver is strongly distorted, the resistor R12 increases the bias voltage of the transistor VT3, again achieving an intelligible sound of the receiver. This completes the adjustment - the radio microphone is ready for work. Literature
Author: A.Naumov, Saransk
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