Stereo in a simple radio receiver. Scheme, description

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Stereo in a simple radio receiver. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / radio reception

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Radio amateurs today try their hand not only in the design of radio receivers, but also with no less success are engaged in the modernization of industrial production equipment. Readers are invited to one of the curious amateur developments.

Any, even the simplest and cheapest pocket VHF radio with a range of 88 ... 108 MHz, which is now quite a lot on the Russian market (especially Chinese-made), supplemented by a simple stereo decoder, will allow you not only to follow the latest news while in the country or on a fishing trip , but also, using stereo telephones, to listen to music programs of stereo broadcasting with high quality.

Of all the variety of inexpensive imported pocket receivers currently on sale, perhaps the best in terms of price-quality ratio is the TECSUN R-212T model made in China.

Small dimensions, good sensitivity (2...3 μV in the VHF range), good selectivity, efficiency, continuous range from 64 to 108 MHz make this radio very suitable for equipping it with a stereo decoder.

In the manufacture of the stereo decoder, the task was to minimally change the design of the receiver itself. Therefore, after modernization, outwardly it differed only in the presence of an additional jack for stereo phones.

The choice of imported chips for the stereo decoder is due to the lack of domestic analogues. Products of the company "PHILIPS" were used: TDA7040T - a stereo decoder of frequency-modulated signals with a PLL and TDA7050T - a stereo amplifier with output to stereo headphones. They were purchased by the author from the catalog of the Promelectronica store (620107, Yekaterinburg, Kolmogorova st., 70, the cost of the set at the time of purchase was 9 rubles 50 kopecks).

Here are some specifications of microcircuits, taken from various sources, as well as determined experimentally.

TDA7040T is a PLL stereo decoder, the microcircuit is made in a surface-mounted package, its dimensions (without leads) are 5x4x1,2 mm:
Supply voltage - 1,8 ... 7 V (at a voltage of less than 1,8 V, the microcircuit provides an output signal in the "Mono" mode
Consumption current - 5...7 mA.
Input complex stereo signal voltage - 100 mV
Channel separation - not worse than 26 dB (with a supply voltage of 3 V at a frequency of 1000 Hz)
Harmonic coefficient - 0,2%
Signal to noise ratio - 65 dB.
TDA7050T is a stereo amplifier, the dimensions of the case are the same as the previous microcircuit:
Supply voltage - 1,6 ... 6 V
Consumption current - 5...7 mA
Minimum load resistance - 32 0m
Rated output power - 35 mW per channel (at Upit = 3 V and load resistance 32 0m)

The microcircuit has protection against short circuits in the load connection circuits.

In the receiver being finalized, the design basis is the integrated circuit AM / FM of the CXA1191M (SONY) path, which determines its good characteristics. The microcircuit is operational when the supply voltage drops to 1,6 V. The current consumption when working on headphones and at an average volume does not exceed 10 mA (in the "Mono" mode). The complex stereo signal (CSS) to the stereo decoder is taken from pin 23 (AM/FM demodulator output) of the CXA1191M chip.

Note that in a mono receiver design, the spectrum of the demodulated low frequency signal is always limited. In my receiver, this element is a capacitor between the output of the demodulator (frequency detector) and the common wire (C15), this reduces the noise level in the "Mono" mode. In a stereo version of the radio, to obtain good channel separation, it is necessary to reduce the capacitance of this capacitor to 0,015 microfarads.

The proposed design of the stereo decoder has the following specifications:

Channel separation - no worse than 26 dB.
Supply voltage - 3 V.
Current consumption in stereo mode - 10 mA.
The maximum undistorted output voltage at the load is 32 0m - 1 V (in each channel).

Schematic diagram of the stereo decoder is shown in fig. 1. A complex stereo signal is fed through the corrective circuit R1C1R2 to the input of the stereo decoder (pin 8 of the DA1 chip). In stereo mode, the correction circuit determines the quality of the channel separation and the timbre of the sound. In the reference oscillator adjustment circuit (pin 3), the tuning resistor R5 sets the VCO operation mode according to the best channel separation.

Stereo in a simple radio receiver

After decoding, the signals of the left and right channels from pins 5 and 6 of the DA1 chip through the dividers R11R13 and R12R14 are fed to the inverting inputs of the stereo audio amplifier chip (pins 2 and 3 of the DA2 chip).

The device for indicating the stereo mode of operation is made on transistors VT1 and VT2, the HL1 LED is included in the collector circuit of the latter. In the absence of KSS at the input of the decoder, at pin 7 of the DA1 microcircuit, the control voltage has a level of about 640 mV, which keeps the VT1 transistor in the open state. The transistor VT2 is accordingly closed, and the HL1 LED is off. When the KSS appears, the control voltage level decreases to 200 mV, the transistor VT1 closes, and VT2 opens, the HL1 LED lights up, indicating the "Stereo" mode.

Installation of a stereo decoder and changes in the basic design of the receiver.

The stereo decoder board (Fig. 2) is made of double-sided foil fiberglass 1,5 mm thick. You can use one-sided fiberglass, but then to attach the XS1 stereo phone jack, you will need to glue the "patch" of foil on the installation side of the parts.

Stereo in a simple radio receiver

The mounted stereo decoder board is installed in a small gap between the receiver's sound head and the back cover of the housing (Fig. 3).

Stereo in a simple radio receiver

To limit its movement in a horizontal plane, a plastic stop must be glued in place in the cover of the receiver housing. The height of the board on the side of the tuning resistors R2 and R5 should not exceed 7 mm, and on the side of the transistors VT1 and VT2 - no more than 9 mm. To withstand these dimensions, it is necessary to drown the tides of the leads of trimmer resistors and capacitors into the board, having previously countersunk the holes with a 3-4 mm drill.

When installing the board with a piece of adhesive tape, it is necessary to isolate the metal case of the range switch and the section of the stereo decoder board that comes into contact with it. The wires connecting the receiver board to the decoder board should be no longer than 50...60 mm, this will ensure the convenience of assembling and disassembling the receiver. Under the power compartment cover, drill two holes with a diameter of 3 mm opposite the tuning slots of the tuning resistors R2 and R5.

The printed track of the range switching circuit is used as the signal transmission circuit (SCC) (Fig. 4). To do this, break the track from pin 15 of the CXA1191M chip (band switching signal input) and connect it to pin 23 of the same chip (FM demodulator output). Then you need to remove the foil between the terminals of the range switch and connect them to the receiver board and to the decoder board according to the figure.

Stereo in a simple radio receiver

For the display device to work, it is necessary to remove the wire connecting the cathode of the HL1 LED with pin 19 of the CXA1191M chip (setting indication output). The LED cathode is connected to the resistor R10 of the stereo decoder (install on the receiver board). The conductor from the anode HL1 is broken, and the anode of the LED is connected with a flexible wire to the power plus (Fig. 4). Remove the resistor R5 (220 0m) of the receiver. The remaining empty sections of the printed conductors are used to install the capacitor C10 of the stereo decoder. Resistors R11 and R12 with capacitors C7 and C9 are connected from the side of the elements.

Used radio elements: transistors VT1, VT2 - KT3102A, transistors of this series with letter indices from B to E are also suitable; capacitor C10 - 47-100 uF (6,3 V, small in an insulating case, the remaining capacitors - K10-17B; resistors R2, R5 - SP3-19a, the rest - MLT-0,125 W.

To simplify the modernization of the receiver, it was decided to abandon the AM band and use the AM / FM band switch to switch the supply voltage and the KSS signal to the decoder board. As a result, in the upper position of the switch, the receiver operates in mono mode, the stereo decoder board is de-energized and the KSS signal is not supplied to it. In the lower position of the switch, the receiver operates in the "Stereo" mode with access to the additional socket XS1 of stereo phones.

In stereo mode, it is still possible to listen to transmissions simultaneously through the speaker head using the volume control, or through the second head phones using the receiver's standard connector.

The station tuning LED serves as a stereo mode indicator. In this case, the stereo mode indication is also a fine-tuning indication and allows you to reduce the current consumption by eliminating one LED. The brightness of the LED also allows you to control the voltage of the receiver. If it is less than 1,85 V, then the LED goes out.

The volume control in the stereo decoder is not provided due to the lack of a small-sized dual variable resistor and the difficulty of installing it in this receiver. The optimal volume level is set by selecting resistors R11 and R12. The volume also depends on the type of stereo phones used. An external volume control will make the operation of the radio much more convenient and comfortable. The author uses inexpensive stereo phones with volume control.

To install the output jack XS1 of stereo phones in the body of the receiver, a hole with a diameter of 5 mm was drilled to fix it.

Since the author additionally used a stereo decoder only for the CCIR system (with a pilot tone), then in the design of the receiver with an extended VHF range (64 ... 108 MHz), programs in stereo mode will only be played in the 88 ... the radio station is intended to operate in stereo transmission mode. A good replacement for the TDA108T chip can be K7050UN174, which allows you to adjust the volume in two channels with a conventional variable resistor (see "Radio", 23, No. 1997, p. 2, 36, 37, 53; No. 54, p. 3, 32).

Author: V.Zdorovtsev, Georgievsk, Stavropol Territory

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