Automatic switch of TV inputs. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Automatic switch of TV inputs. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Телевидение

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Many users of various video equipment are faced with the problem of feeding television signals to the TV from several sources (two or three antennas, a VCR, etc.). To solve it, the author of the article suggests using an automatic diode switch.

The development of central and local television broadcasting, the growing use of video technology lead to the need to automatically switch television signal sources when choosing the appropriate programs. The device for switching three inputs of such sources by a mechanical relay with electronic logic control was used to expand the capabilities of the SVP4-10 program selection unit and is considered in the author's article "12 programs instead of 6" ("Radio", 1997, No. 4).

Of course, switching inputs with a mechanical relay is not the best option due to parasitic capacitances and contact inductances. This is not important for different frequencies of received programs. But if the same program is received by different antennas, for example, by direct and reflected beam, relay switching becomes unacceptable due to the summation of the main and spurious signals. For the same reason, the adder of signals from various antenna inputs will also be unsuitable.

The problem is quite simply solved by using a switching diode switch that opens the working input and blocks the non-working ones. Such an algorithm, combined with a low capacitance of closed diodes (0,5...1,5 pF) and a low differential resistance of open diodes (0,5...5 Ohm), makes it possible to better suppress interfering signals. When choosing them, one should be guided by the fact that the best parameters are diodes KD420A, KD407A and a little worse - KD409A, KD413A, KD514A. However, you can try other high-frequency or pulse diodes with parameters close to the indicated values.

Automatic switch of TV inputs. Schematic diagram of the automatic switch

Schematic diagram of the proposed input switch is shown in the figure. Antenna socket "C" (local television) is connected to the TV when programs are selected in the SVP4-10 unit via command inputs 1 and 2, socket "B" (high-frequency signal) - when a program is selected via command input 3. When selecting programs, not connected to inputs 1-3, the TV signal is received through the antenna socket "M" (Central TV). The control input 4 provides binding of the switched sources to the UHF range. Permission for switching in the first two cases is the coincidence of levels 0 on the command and control inputs, and in the third - their mismatch.

Chip DD1 generates control voltage, and chips DD2, DD3 switch diodes VD3-VD14. At zero control voltage "C", "M" or "B", an open diode (VD6, VD10 or VD14) closes the signal to a common wire, and closed diodes (VD3 and VD4, VD7 and VD8 or VD11 and VD12) prevent it from passing to TV input. With a single control voltage, the state of the diodes changes and the signal passes through the switch almost without loss.

To reduce parasitic capacitances and inductances, diodes and their associated capacitors are soldered directly on the petals of the antenna sockets with a minimum lead length. Blocking capacitors C3, C5, C7 are best used without leads or after tapping the existing leads. The parallel connection of the inverters of the DD2 and DD3 microcircuits is used to increase the current through the diodes, in our case equal to 10 mA.

Placement and connection of part A of the device are the same as described in the above article. Parts B (printed wiring) and C (surface mounting) are located near the antenna jacks of the TV and are connected to part A by a five-wire harness.

The device provides a very efficient switching of television signal sources. Since the interfering signal is significantly suppressed, there is absolutely no image overlap on the TV screen and the specific distortions caused by it.

In conclusion, it should be noted that the considered switch is easy to adapt to other reception conditions and other program selection devices on the TV. It will only be necessary to change the input logic of operation and the number of executive switching channels.

From the editor. For the normal functioning of the switch, it is necessary that the DC input impedance of the SK-D does not exceed 75 Ohm.

Publication: N. Bolshakov, rf.atnn.ru

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