انواع ناخب الأقنية (Tuner) .. كتاب التلفزيون الملون والعادي

Chapter V

Channel voter

E-1 Types of Tuner and its Function:
The channel voter is the first stage in the device that the visual and audio signal enters - and it is an independent unit in the receiving device, located at the front or side of the device - and the voter can choose the desired station in one of the following ways:

A - Rotary voter method.
b - Compressor or contact method.
C - An electronic method on the device and by means of a remote control. The channel selector works to pass a package of frequencies of one channel and prevents the passage of the signals of other channels. It amplifies the signal of this channel and mixes it with the signal of the local oscillator, and as a result you get the medium frequency (IF) signal as It has other functions:
1- It works as a connecting phase between the antenna line and the intermediate frequency circuit (IF) in a way that achieves correct impedance compatibility with the intermediate frequency phase.

​​​​2 - Amplifies the RF radio frequency signal so that the signal-to-noise and interference ratio is doubled.

3- It isolates the local oscillator signal from the antenna and prevents its signal from spreading to neighboring antennas

4- Converts the RF signal into medium frequencies using the super heterodyne method. Figure (5-(4) shows the VHF voter box diagram. The voters can be divided into:

A - Mechanical voter and it has two types

1 - slat type (tower)

2- Disc type.

B - an electronic voter. According to the frequency range in which the VHF voter operates, i.e. the very high frequencies

B - UHF voter, i.e. ultra-high frequencies
according to the elements it contains.

A - An old valve elector that is no longer used except rarely) Figure (1-5).

​​​​​​B - Transistor or semiconductor selectors such as transistors and integrated circuits

As for the voter that may contain valves and transistors together, it is called a hybrid voter (hybrid).
E - 2 Impedance Compatibility Circuit (BALUN): It is a transformer with a core of magnetic iron, on which there are four coils - these are coils of different numbers and diameter by converting the impedance (300 ohms) in parallel into (75 ohms) or (300) ohms unbalanced - Impedance (75 ohms) is often used in VHF voter circuits. The iron core maintains the stability of the impedance conversion ratio within the entire range of received frequencies (Fig. 5-2).
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