Encyclopedia of radio electronics and electrical engineering / radio reception

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VHF FM broadcasting is now widespread and covers ever larger geographic and frequency bands. The popularity of VHF radios is growing every day. The proposed radio receiver (Fig. 1) is designed to receive radio stations with FM modulation of the "VHF" and "FM" bands. The receiver is powered by three batteries (700MA / 4), the basis is the K174XA34 chip, which converts the input modulated FM signal into sound.

Rice. 1. Schematic diagram of the radio receiver (click to enlarge)

The radio frequency signal comes from the antenna to a broadband circuit consisting of C1, C2, L1 and VT1 junction capacitance. The circuit has a weak resonance at a frequency of 100 MHz. The use of such a switching circuit made it possible to dispense with the restructuring of the input circuits when operating in the VHF and FM bands. The first UHF stage is assembled on VT1, which compensates for losses in the input circuits. Then the signal is further amplified by VT2, the load of which is the matching transformer T1. The use of T1 made it possible to abandon the additional circuit. From winding II T1, the RF signal is fed to pins 12 and 13 of DA1. Further transformation takes place inside DA1.

The K174XA34 design has a number of original solutions. These include the construction of the IF path on active RC filters, in which the intermediate frequency is reduced to 70 kHz. The IC uses a correlation system for noiseless tuning and frequency synchronization, which reduces the frequency deviation from 75 to 15 kHz and thus ensures low non-linear distortion at low IF. With the ratings selected on the diagram, the harmonic coefficient of the low-frequency signal with an input signal with a deviation of 75 kHz is no more than 2,3%, and with a deviation of 22 kHz - 0,7%.

The K174XA34 input signal range is from 1,5 to 110 MHz, so this IC can be used as a receiver booster with an input frequency of 1,5 ... 110 MHz by stabilizing the local oscillator frequency with quartz. The use of a quartz or piezo filter at the input of such an IF makes it possible to improve the quality of the RF signal processing.

This IC can be used in portable radio stations, and the local oscillator frequency f_г should be stabilized and the input frequency will be f_г ± 70 kHz.

To pin 5 of DA1, using SA2, local oscillator coils L2 or L3 are connected, which set the "FM" and "VHF" ranges, respectively. The frequency tuning of the receiver is carried out by the VD4 varicap, to which the tuning voltage is applied from the variable resistor R8. The tuning voltage on R8 is stabilized by the HL1 LED and a stabilizer consisting of VT3, VD1 ... VD3, R6, R9. This circuit reduces the dependence of the tuning voltage on changes in the supply voltage.

The low-frequency signal from pin 14 of DA1 is fed through the R12-C18-R11 circuit to the UZCH input to DA2. The UZCH IC is switched on in a bridge circuit, R10 sets its operating mode. The output power of the UZCH when powered by 4 V at a load of 4 ohms is 1 watt. The UZCH microcircuit does not require additional cooling and remains operational when the supply voltage drops to 1,6 V. The AF input signal at pins 1 and 3 should not exceed 50 mV.

Receiver tuning consists in tuning L2 and L3 so that when R8 is tuned, the entire range ("FM" and "VHF") is covered. By adjusting L1, you can achieve equalization of sensitivity in these two ranges. The value of R11 must be selected according to the maximum undistorted AF signal in the loudspeaker. With proper installation and serviceable parts, this completes the setup.

The receiver is mounted on a printed circuit board made of double-sided fiberglass with dimensions of 70x40 mm (Fig. 2). On one side are printed circuit tracks, and the other side is the "+" supply. Those elements that go to "+" are soldered directly to the foil. In the diagram, these points are marked with a cross, the remaining holes for the leads are countersunk to prevent a short circuit with the foil. This arrangement of the circuit reduces the possibility of self-excitation.

Rice. 2. The printed circuit board of the radio receiver

Details

Transistors VT1, VT2 - KT3127A; VT3 - KT3102A...E, KT342A...G. All resistors are MLT-0,125, they are installed vertically on the board. Capacitors C1, C2, C4, C7, C8, C9 - KM-5A; C3, C5, C6, C11 ... C16, C18 - from the IBM-286 motherboard, but other small-sized ones with a capacity of 0,1 ... 0,22 μF can be supplied; С17 small-sized, imported. LED HL1 - AL307BM. VD4 -КВ109А...В. R8-SPZ-4AM, R12 SPZ-4VM. R12 applied with power switch. Coils L1...L3 are wound on frames d5 mm with an inner core from SB-12A. The core is bitten off to a length of 5 mm. L1 contains 4 turns, L2 - 2,5 turns, L3 - 3,5 turns of PEV-2 wire d0,33 mm. T1 is wound on a 50VCh K7x4x2 ring. Winding I contains 12 turns, winding II - 4 turns of PEV-2 wire d0,27 mm. T1 must be glued to the board through an insulating gasket. Dynamic head VA1 - any broadband with a winding resistance of 4 ... 16 ohms. The SA2 range switch is taken from a car radio. The quality of its sound depends on the body of the receiver. For example, it is very difficult to get high-quality sound in a small case. The receiver board can be inserted, for example, into a three-program indoor loudspeaker.

A feature of the receiver is the instability of the tuning voltage with a strong change in ambient temperature. With such a low supply voltage, the use of zener diodes is undesirable. Therefore, the receiver does not use fixed settings for the selected radio stations.

The receiver remains operational at a supply voltage of 2,7 to 6 V. A higher voltage will cause DA1 and DA2 to fail. If only one range is used, then there is no need for SA2, as well as UHF on VT1. When the receiver is operating in one range, you can also exclude the cascade on the transistor VT2, and connect the collector VT1 to the I winding of the transformer T1. This increases the sensitivity of the receiver.

You can somewhat improve the temperature stability by using instead of HL1 infrared LEDs with a total voltage of 2 ... 2,5 kOhm.

If you install the receiver in a device with a network power supply, you can enter fixed settings, and stabilize the power with a KS162 type zener diode. In this case, it is necessary to limit the voltage on R8 to 2 V.

Although the receiver does not have an AFC, it works quite stably when powered from the mains. As an antenna, it is best to use a telescopic antenna, 300 ... 500 mm long. Current consumption in rest mode - about 300 mA. If the receiver will be powered by a network power supply, the stabilizer can be made on the KR142EN5A chip.

Author: A. Shcherbinin, Barnaul; Publication: cxem.net

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