Encyclopedia of radio electronics and electrical engineering / radio reception

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Lightweight miniature self-powered walking radios have recently become very popular, probably also because they are relatively cheap. Is it possible to give this miniature "walking companion" the sound quality that satisfies listening to programs in the comfort of home? It turns out that it is possible, and this article is devoted to this.

Miniature pocket receivers, which are now commercially available in large numbers, are invariably popular among fans of listening to radio broadcasts. They are very convenient in the country, in the forest and on vacation because of their small size and weight, as well as good sensitivity in all ranges. Many of these receivers are operated at home, in apartments. And if outside the city the volume of sound, autonomy of power for the most part suits many, then when used in an apartment, there is a desire to get more power and better sound quality from the receiver (especially when receiving in the VHF bands), and it is desirable to have power from the network.

These problems are quite easy to solve if radio amateurs have a stationary audio frequency power amplifier or other sound-reproducing equipment with loudspeakers (hereinafter referred to as UZCH) and incorporating a power supply with an output voltage of +5 ... 30 V. Having made minor modifications to the UZCH and by connecting the receiver with a cable, you can listen to your favorite programs with good sound quality. When the cable is disconnected, the receiver can be used as usual.

Let's start with the simplest option, in which minimal modifications are performed. A receiver with a supply voltage of 3 V is taken as the basis - the bulk of the receivers currently in use have just such a power supply option. As examples, the author considers options for improvements with "TECSUN R1012" receivers with a CXA 1191M (SONY) chip installed in it and a "HUASHI HS-311" receiver with a KA22425D (SAMSUNG) chip, which is an analogue of the first. The proposed modifications are easily implemented with other receivers with a supply voltage of 6 or 9 V.

The improvements consist in installing an additional connector on the UZCH case in any convenient place and manufacturing a power stabilizer with a voltage of +3,5 V. Its circuit is shown in fig. 1. It is made on an integrated circuit DA1, the output voltage is set by a tuned resistor R2.

The stabilizer is assembled on a printed circuit board, the drawing of which is shown in Fig. 2.

This board is built into the UZCH. The input of the stabilizer is supplied with a constant voltage within +5 ... 30 V. The output of the stabilizer is connected to the socket of the additional connector XS1 (pin 3), the UZCH input is also connected here (pin 1). The XP1 plug (Fig. 1) of the connecting cable is connected to the XS3 connector, which is connected with the XP2 plug to the receiver jack intended for an external power source and the XP2 plug to the headphone jack. When connecting the XPXNUMX and ХРЗ plugs in the receiver, the built-in power supply (to save battery power) and the built-in sound head are automatically turned off respectively.

When the receiver is working together with the UHF, the volume control of the receiver should be set to a position below the average level. Further volume and tone adjustments are carried out by the UZCH controls, and tuning to the station, as usual, by the receiver tuning knob. If, when working through the UHF, strong distortions are heard, then the signal level should be slightly reduced with the receiver's volume control.

This is the easiest, but not entirely successful way to connect. The fact is that most pocket receivers have their own ultrasonic frequency converter with large non-linear distortions. In addition, its frequency response is limited by the frequency spectrum within 100...10000 Hz. That is why, with this option, the volume control in the receiver must be set to a low volume position to minimize the effect of the distortions present in the receiver.

The best option is to connect the UHF to the output of the receiver's demodulator (in the receiver's operating mode in the VHF band). In the microcircuits mentioned above, this is pin 23. For this option, the receiver should cut the printed track coming from the 3H output (pin 27) of the microcircuit to the socket for connecting headphones, and connect the socket to pin 23 of the microcircuit with a wire. Now, when connecting the receiver to the UZCH, the volume control of the receiver should be set to the minimum volume position so that the sound head built into the receiver does not work, and the signal to the UZCH will go directly from the demodulator. With this inclusion, the entire resulting "audio complex" sounds much better. True, the modified receiver will not be able to listen to programs on headphones, but this, in most cases, is not such a big loss.

In the scheme of Fig. 1 microcircuit stabilizer DA1 can be replaced with a domestic type KR142EN12A, any domestic or imported resistors and capacitors. ONTS-VG-1/5-V can be used as the XP16 plug, ONTS VG-1/5-R can be used as the XS16 socket. The type of XP2, XP3 plugs is determined depending on the design of the sockets installed in the receiver.

A further improvement in the sound of receivers in combination with a stereo amplifier can be achieved by installing a stereo decoder in the UHF, which will allow you to get stereo sound reproduction. To do this, you need to assemble the structure according to the scheme of Fig. 4. It is based on a dual-system stereo decoder DA1 (KR174XA51). The signal to it comes from the output of the demodulator of the receiver through the XS1 socket, but at the same time, small changes will need to be made in the receiver, which are discussed below.

An emitter follower is made on the transistor VT1, which excludes the mutual influence of the receiver output and the decoder input circuit. From the emitter of the transistor, a complex stereo signal is fed through the low-pass filter R4C4 to the input of the DA1 microcircuit (pin 16). This chip works in a typical inclusion [1]. The device automatically detects the signal encoding system and switches to the appropriate operating mode. The HL1 LED indicates the presence of a stereo signal when receiving stereo broadcasts. From the outputs of the stereo decoder, the 3H signals of the left and right channels are fed to the corresponding power amplifiers. The stereo decoder is assembled on a printed circuit board, the drawing of which is shown in fig. 5.

Changes in receivers to implement stereo playback. As is known, during monophonic playback, the spectrum of the signal from the demodulator output is limited in frequency from above by a value of 10 ... 15 kHz. But to work in stereo, the signal spectrum must be much wider - up to 70 kHz.

On fig. 6 shows a fragment of the circuit diagram in the receivers mentioned above, which must be modified. The designation of the elements corresponds to the version given on the printed circuit boards (without brackets - for the "TECSUN R1012" receiver, in brackets - for "HUASHI HS-311"). All changes are clear from the figure, the newly introduced elements are indicated by thicker lines, and there is an apostrophe in the positional designation. Capacitor C1 'can not be installed, it is needed to reduce the noise level and was not used in conjunction with the KA22425D microcircuit. Refinement of the receiver "TECSUN R1012" is described in [2].

The transistors used in the stereo encoder can be any of the KT315, KT3102 series, the HL1 LED - any domestic or imported with parameters close to the specified one.

Establishing a stereo decoder consists in setting half the supply voltage at the emitter of transistor VT1 by selecting resistor R2. The trimmer resistor R1 limits the maximum level of the CSS at the input of the stereo decoder at the level of 200...250 mV. The trimmer resistor R11 achieves the operation of the decoder generator when receiving stereo transmissions both in the OIRT system (polar modulation) and in the CCIR system (with pilot tone). In this case, the HL1 LED should light up.

In addition to the receivers described above, stereo receivers can also be connected to the UZCH, then a stereo decoder will not have to be made. For example, the MANBO S-202 miniature stereo receiver uses the CXA1538M chip, which incorporates a CCIR stereo decoder. Complete with an amplifier for stereo phones on the TDA7050 chip installed in the receiver, this receiver provides a fairly high-quality stereo sound at the output. Therefore, to connect to the UZCH receiver, use stereo as the XP3 plug, and use pin 1 for the right channel (left - pin 1) in the XS4 jack and XP1 plug. The only drawback is that this receiver does not have a socket for connecting an external power supply, but it is not difficult to install it. In addition, with this receiver, the antenna is the connecting wire of the headphones. Therefore, it may be necessary to install an additional antenna socket.

It should be noted that in order to work with UHF, the receiver's volume control must also be set to a low volume in order to avoid overloading the amplifier and the appearance of strong distortions.

In conclusion, I would like to note that in this way almost any receiver, both imported and domestic, can be connected to the UZCH, and not even a new one, and even with a faulty own path of the audio frequency amplifier. It is only necessary to set the required voltage at the output of the manufactured stabilizer, as well as to refine the output circuit of the frequency modulator.

Literature

1. Alenin S. Microcircuits of the K174 series. Dual-system stereo decoder KR174XA51. - Radio, 1999, No. 5, p. 43-45; No. 6, p. 47.
2. Zdorovtsev V. Stereo - in a simple VHF receiver. - Radio, 1999, No. 1, p. 6, 7.

Author: I.Potachin, Fokino, Bryansk region

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