VHF FM radio station 2. Scheme, description

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VHF FM radio station. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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Radio settings

Receiver operating frequency range - 144 146 MHz
Transmitter frequency - 145,2 MHz ± 250 kHz
The real sensitivity of the receiver is 3 μV / m
The output power of the transmitter on a load with a characteristic impedance of 75 Ohm - 3 W
Supply voltage - + 12V (10-14V)
Current consumption when receiving not pain - 50 mA
Consumption current during transmission, no more than - 800 mA

The radio station is designed to operate in the amateur band 144-146 MHz. It is built according to a simple scheme with minimal use of scarce parts. The receiving path is built according to a simple superheterodyne circuit with one frequency conversion and an intermediate frequency of 6,5 MHz. There is no quartz stabilization of the receiver, the tuning frequency depends on the heterodyne circuit, consisting of a coil and a varicap, controlled by a variable tuning resistor. This allows you to cover a fairly wide range in a simple way, but significantly reduces the stability of the tuning. The transmitter is stabilized by a 48,4 MHz quartz resonator (from defense equipment). The use of quartz stabilization in the transmitter provides the necessary stability, but leads to a narrowing of the tuning range. The circuit diagram is shown in Figure 1.

Fig.1 (click to enlarge)

Switch SB1 is shown in the "receive" position. This switch is a toggle switch with a middle neutral position, it switches only the antenna and the power supply. In the extreme positions - "reception" and "transmission", in the middle position the radio is turned off.

In the "receive" position, the signal from the antenna jack through C3 is fed to the input circuit L1C4, connected completely to the first gate of the MOSFET VT1, operating in the UFC. The gain of this stage depends on the bias voltage at its second gate, which can be adjusted by the trimmer resistor R1. The L3C6 circuit is the load of this amplifier and is partially connected to its output.

From tap L3, the amplified signal is fed to one of the inputs of the balanced mixer of the A1 microcircuit, which performs the functions of a frequency converter and a local oscillator. The frequency of the local oscillator, and hence the tuning of the receiver, is determined by the setting of the L4 C9 C11 C12 C13 C10 VD1 circuit. The restructuring is carried out using the VD1 varicap. The intermediate frequency is 6,5 MHz, so the local oscillator frequency varies between 137,5-139,5 MHz.

Intermediate frequency oscillations are isolated in the L5C14 circuit and fed through the capacitor C 16 to the A2 microassembly. The use of such a common microassembly as UPCHZ-1M or UPCHZ-2 from USST TVs makes building a radio station more affordable. The microassembly contains a 6,5 MHz piezoceramic filter, an eight-stage IF, an FM detector with a resonator in a phase-shifting circuit, and an adjustable pre-amplifier. The active part of the assembly is made on K174UR4.

From the output of the microassembly, the low-frequency signal is fed to the UMZCH, assembled on three transistors VT2-VT4 in a two-stage circuit. Volume control is performed by a variable resistor R9, which controls the adjustable ultrasonic microassembly. A speaker-microphone BA25 is connected to the UMZCH output through a separating capacitor C1.

A characteristic common malfunction of UPCHZ microassemblies is the impossibility of adjusting the volume using an adjustable amplifier. Therefore, if a microassembly from among the waste of a TV workshop is used, it makes sense not to connect output 7 A2, but to control the volume using a variable resistor included instead of a trimmer R8. However, this will make it difficult for the volume control to be in tune.

In transmission mode (SB1 in the opposite position shown in the diagram), the supply voltage and the antenna are connected to the transmitter. The signal from the speaker-microphone VA1 is fed to a low-frequency limiting amplifier on transistors VT8 and VT9, which amplifies the signal to the required level. Through R20, the low-frequency voltage enters the reverse bias circuit of the VD5 varicap and creates frequency modulation.

The master oscillator is made on the transistor VT7. Its base circuit includes a quartz resonator at a frequency three times lower than the frequency of the transmitted signal, in this case at 48,4 MHz, but it is possible to use resonators for other frequencies in the range from 48 to 48,6 MHz (144-145,8 MHz ), if there are several resonators from this range, you can install an RF connector or an RF switch and change the resonators, thus rebuilding the transmitter. A frequency shift circuit is connected in series with the resonator, consisting of an L15 coil and a VD5 varicap. With the help of this circuit, FM and a slight restructuring of the transmitter occur (using resistor R22).

The transmitter power amplifier is two-stage, on transistors VT5 and VT6. The interstage and output circuits L12C34, L9C30 and LL7C27 are tuned to the third harmonic of the quartz resonator. Chokes L6 and L5 are connected between bases and emitters VT8 and VT11. The output stage is designed to connect an antenna with an equivalent resistance of 75 ohms, for communication with an antenna located at some distance from the radio station, a PK-75 cable is used. If necessary, you can easily reconfigure the transmitter output to a load of 50 ohms. Capacitors C28, C3O and C35 are used to prevent failure of the power amplifier transistors in case of accidental short circuits between the trimmer capacitors with an air dielectric.

Resistors MLT 0,125, loop capacitors KD or KG with a minimum TKE are used in the receiving path and modulator. Oxide - K50-16, K50-35 or imported. Trimmer capacitors - ceramic type KPK-MN. The rest - any suitable. Transistor KP350 can be replaced with KP306, microassembly UPCZ-1M can be replaced by UPCZ-2, but you will need to slightly change the layout of the receiver printed circuit board. Instead of the K174PS1 chip, the K174PS4 is suitable. Zener diode - any 6-8V. Varicap type KV109, KV102, KV104.

HF coils of the receiving path do not have frames. Their outer diameter is 6 mm, they are wound with a silver-plated wire with a diameter of 0,7 mm. The length of the winding L1 is 9 mm, the number of turns is 5, the tap is from the first. Length L3 - 7 mm, 4 turns, taps from the 1st and 2nd. Turn count from the side of the wire connected to the power wire. The L4 coil is wound with the same wire, but on a ceramic frame with a diameter of 5 mm, the winding length is 10 mm, the number of turns is 4. After winding and tuning, its turns are fixed with epoxy glue.

The IF coil L5 is wound on the frame from the UPCHZ circuit of the 3-USCT TV (SMRK-1-6, SMRK-1-4). The frame, screen and core are used. It contains 30 turns of PEV 0,12 with a tap from the 15th turn.

The transmitter uses only Air Dielectric Trimmer Capacitors. Ceramic is not recommended. Frameless coils (except L15 and chokes L8, L11, L14) are wound with silver-plated wire with a diameter of 0,7 mm. L6 has an inner diameter of 10 mm, winding length 80 mm, number of turns 3,5. L7 - inner diameter b mm, winding length 5 mm, number of turns 1,5. L9 - inner diameter 10 mm, winding length 12 mm, 3 turns. L10 - inner diameter 6 mm, winding length 6 mm, 3 turns. L12 - inner diameter 8 mm, winding length 7 mm, 3 turns. L13 - inner diameter 6 mm, winding length 20 mm, 8 turns. L8, L11, L14 - identical chokes wound on MLT 0,5 resistors with a resistance of more than 100 kOhm, each contain 30 turns of PEV 0,2. Coil L15 is wound on a frame like L5, it contains 10 turns of PEV 0,2, it does not have a screen.

Structurally, the radio station is made in a metal case with dimensions of 180X180X52 (a finished case was taken from a faulty car radio). The body is divided by a shielding tin partition into two compartments. One compartment measuring 170X70X50 is also divided by a tin partition into three more equal compartments. This compartment is located at the rear edge of the case. In this place, a massive steel plate with dimensions of 170X50X5 mm is installed, it acts as a radiator for the transmitter transistors and is tightly pressed against the rear wall of the case. Holes are drilled in it in which threads are cut for installing VT5-VT7 transistors, the same holes (but without threads) are made in the back wall of the case. It turns out that the radiator plate is screwed to the case, and the role of the mounting bolts is performed by the cases of these transistors. The compartments are arranged in such a way that each compartment contains one transmitter stage: a master oscillator with a quartz resonator, a preamplifier and a final amplifier with an output circuit. Installation of all parts of the transmitter is carried out in a volumetric way on the mounting petals. Feed-through capacitors C26, C31 and C33 are installed in the holes of the partitions between the stages. Also, holes for interstage connections are drilled in the partitions.

In the second compartment there is a receiving path built on a printed circuit board made of one-sided foil fiberglass, and a transmitter microphone amplifier board.

PCB 1
PCB 2

Speaker-microphone and volume control are placed in a separate housing. The tuning resistor, equipped with a simple circular scale, and the SB1 toggle switch are displayed on the front panel (the panel opposite to the one on which the radiator plate is located) If you need to move the "reception-transmission" switch, you need to replace SB1 with an electromagnetic relay, and install an additional toggle switch to turn off the power completely.

Establishing the receiver, with serviceable parts, comes down to setting the UZCH DC mode on transistors VT2-VT4. By selecting the value of R11, the voltage at the emitters VT3 and VT4 is set equal to half the supply voltage. Then you need to tune the L5C14 circuit to a frequency of 6,5 MHz and select the R10 rating in such a way that you get the widest range of volume control. As a signal for tuning, you can use the PCH signal received from the radio channel of the ZUSTST TV, or use the GSS, supplying a 6,5 MHz signal, FM modulated, with a level of 1 mV. Then you need to adjust the high-frequency circuits.

The transmitter is set up using an RF generator, starting with a power amplifier on VT5. At the same time, the quartz resonator is turned off, and through a capacitor with a capacity of 10-30 pF to the bases of transistors (first VT5, then VT6 and then VT7) a signal from a 145,2 MHz generator. At the same time, a load equivalent is connected at the output of the transmitter - a 2 W resistor with a resistance of 75 Ohm (or 51 Ohm), and the setting is controlled by the level and frequency of the RF voltage on it.

The modulator is tuned with the L15 coil, choosing the position of the core at which the quality of the sound received by the control receiver is the best.

Author: A. Ivanov.; Publication: N. Bolshakov, rf.atnn.ru

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