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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING VHF prefix to the direct amplification receiver. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception The capabilities of a direct amplification receiver are limited by its sensitivity; therefore, radio stations in the MW and LW bands are usually received on it. However, a relatively simple prefix, which is described in the article, is able to supplement such a receiver with another popular range - VHF. Developed by the author almost ten years ago and described in the collection "To Help the Radio Amateur", issue 100 (DOSAAF publishing house, 1988), the direct amplification receiver has gained a second "life": now it can receive radio stations operating in two new bands - VHF-1 (65,8 ... 74 MHz) and VHF-2 (88 ... 108 MHz). This became possible thanks to the use of a simple set-top box (Fig. 1), made on a single microcircuit and connected to the antenna input of the radio receiver.
The prefix is a kind of converter that converts (together with the input circuit of the receiver) the signals of radio stations of the indicated VHF bands with frequency modulation (FM) into frequency signals of the MW band with amplitude modulation (AM). Therefore, almost any direct amplification receiver designed to receive MW radio stations is suitable for receiving VHF radio stations. The prefix with the receiver works like this. The signal from an external antenna, which is connected to the XS1 jack, enters the microcircuit through the C1L1 filter - it passes only the signals of VHF radio stations, significantly attenuating the signals of lower frequencies. Depending on the position of the movable contact of the SA1 switch, the local oscillator of the set-top box operates either in the VHF-1 or VHF-2 range (positions "1" or "2", respectively). The local oscillator signal is mixed with the received signal, resulting in the formation of signals of difference and sum frequencies that are in the CB range (0,5 ... 1,6 MHz). From the output of the set-top box, they go directly to the magnetic antenna of the radio receiver. True, while this is an FM signal with a bandwidth of more than 50 kHz, and the receiver is designed to receive AM radio stations with a signal bandwidth of about 10 kHz. Therefore, if you tune the receiver exactly to the frequency of the converted signal, there will be very noticeable sound distortion. To avoid this, they try to "locate" the FM signal on the slope of the amplitude-frequency characteristic (AFC) of the magnetic antenna. This explains Fig. 2, where curve 1 is a typical frequency response of a magnetic antenna, and 1.1 is signal distortion 34 in the case of an FM signal with frequency Fo applied to a magnetic antenna with deviation (frequency change) AF.
It is possible to sharply reduce distortion by reducing the quality factor of the input circuit of the receiver, as illustrated by curve 2 and the 3H 2.1 signal for such a case. For this purpose, a resistor R1 and a capacitor C8 are installed in the console, which allow shunting the input circuit. When working with a set-top box, the receiver is tuned to a section of 1,2 ... 1,6 MHz of the CB range, and with a variable capacitor C7, the set-top boxes select such a local oscillator frequency so that the sound in the dynamic head of the receiver is practically not distorted.
There are several versions of the set-top box and its connection to the receiver. If the dimensions of the receiver allow, the set-top box is placed inside its case and connected in accordance with the diagram shown in Fig. 3a. A SA2 switch is installed on the receiver case, which turns on the set-top box, and the positive power wire is connected through the SA2.1 section to the SA1 switch of the receiver. The output of the set-top box is connected through the SA2.2 section directly to the oscillatory circuit (magnetic antenna) of the LC receiver. To connect an external antenna, which is a piece of wire 0.2 ... 1 m long, you can use the receiver's antenna socket, if any, or install an additional XS1 socket on the case. All connecting conductors must be of the minimum length. If the receiver case does not allow placing the attachment inside it, then it can be made in the form of a removable module and connected to the receiver in accordance with the diagram shown in Fig. 3,6. To do this, the prefix is \u1b\u2bequipped with plugs XP4, XP1, located on its board (Fig. 2), and not the receiver case, sockets XSXNUMX, XSXNUMX are installed. Otherwise, all connections will be the same. The antenna in this case is connected directly to the set-top box.
All parts of the attachment are mounted on a printed circuit board made of double-sided foil fiberglass with a thickness of 1 ... 1.5 mm, a sketch of which is shown in fig. 5.
In this case, all printed conductors are placed on one side of the board, and the second is left metallized and connected to a common wire around the perimeter of the board in several places. There are relatively few parts used in the console. A variable capacitor - 1KPVM or similar with an air dielectric, a minimum capacitance of 2 ... 5 pF and a maximum of 24 ... 30 pF, the remaining capacitors are KM, KD, KLS. Resistor R1 - MLT-0,125. Switch SA1 - PD-9.2 or similar small-sized slide switch. As plugs XP1, XP2 and jacks XS2, XS3 use connectors (jack and plug) for connecting small phones. The inductors are wound with wire PEV-2 0,5 on a mandrel with a diameter of 4 mm and contain: L1 - 18 ... 20 turns, L2 - 13, L3 - 23..25 turns. Coils L2, L3 are installed directly between the terminals of the switch and the variable capacitor, and the capacitor C8 is installed between the middle contact of the XP2 connector and the printed circuit board using the surface mounting method. The middle contact of the XP1 connector is connected to the positive conductor of the board with a piece of thin wire. From the mounting side, the board can be closed with a plastic or metal cover. Establishing a set-top box comes down to setting the boundaries of the local oscillator frequency tuning range. Since the intermediate frequency (IF) value in the system is low, the tuning range of the local oscillator practically corresponds to the range of received frequencies. Therefore, to control the frequency, you can use an ordinary VHF receiver that has both bands. Its antenna is placed as close as possible to the working set-top box and, by tuning the local oscillator frequency with a variable capacitor, it is fixed by the VHF receiver by the disappearance of noise in its dynamic head. If the frequency tuning range of the local oscillator of the set-top box is shifted in one direction or another, it will be necessary to change the parameters of the coils L3 (in the VHF-1 range) or L2 (in the VHF-2 range). When shifting the range towards higher frequencies, you need to add a few turns, and towards lower frequencies - unwind or slightly stretch the turns of the coil. At the end of the tuning, it is necessary to cover the turns of the coils with a layer of epoxy glue - this will increase their rigidity and will eliminate the undesirable so-called microphone effect. Then the set-top box is connected to a direct amplification receiver according to one of the above schemes and tuned to VHF radio stations (as mentioned above, the receiver must be switched on to the MW band with a tuning frequency of 1,2 ... 1,6 MHz, free from broadcast radio stations) . If the reception is accompanied by distortions, they are eliminated by a smoother adjustment of both the receiver and the set-top box, as well as by selecting a lower resistance resistor R1. In general, it is better to replace R1 with a tuning resistor for the time of tuning, and after determining the resulting resistance, install a constant resistor instead. It should be noted that with a high saturation of VHF radio stations, which is typical, for example, for Moscow, the reception of some of them may be accompanied by interference from other radio stations. Eliminate this interference by tuning the receiver to a different frequency range. The prefix operates on a voltage of 6 ... 9 V and consumes a small current - 1 ... 2 mA. At a lower voltage, the local oscillator of the set-top box will work unstably. If desired, the set-top box can be configured to receive the sound of television broadcasts. If the positive power wire is common in the direct amplification receiver, then the wiring of the power supply conductors of the set-top box is also changed accordingly. In the case when the input circuit of the receiver is connected to the positive power wire, it is permissible to exclude the capacitor C8 from the prefix. Author: I. Nechaev, Kursk; Publication: N. Bolshakov, rf.atnn.ru
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