الناخب الالكتروني لمجال uhf .. كتاب التلفزيون الملون والعادي

- 5 - 10 The Electronic Tuner for UHF:

The box diagram shows Figure (5 - 36) stages of a UHF tuner tuned using capacitive diodes. We have shown the circuits for each of the RF amplifier, the local oscillator, and the mixer. There are several notes worth noting, which is that the tuning voltage is DC for the capacitive diode It ranges from 1 to 27.5 volts in all UHF channels, and this voltage is used to tune the antenna, RF radio frequency, mixer, and oscillator circuits - and it is noted that the voltage line +18 volts to 06 05 and Q7 can be connected and disconnected - during the work of the VHF field, the UHF field does not work - During the operation of the UHF elector, the medium-frequency output of the elector feeds the VHF elector mixer, so it acts as an amplifier - and during the operation of the UHF elector, the oscillator and the mixer circuits are off. As for the AGC voltage, it has a voltage ranging from .. to 12 (V) and feeds the second gate G2 of the amplifier RF in the UHF field of the transistor (Qs) and Fig. (5) - (37) shows the diagram of an electronic voter for the UHF field.
Figure (5) - (38) shows the elements of the UHF elector and it is noted that there are no variable capacitors for tuning - four capacitive diodes are used for tuning - they are distributed in the antenna - the RF mixer amplifier - the oscillator - and it is possible to tune in the VHF and UHF fields - Tuning to any of them is done with a switch with three positions - VHF mode for low numbers, VHF mode for high numbers, and VHF mode. And after selecting the position of the switch on the appropriate field, we turn the fine-tuning knob, and the channel number appears on a light digital screen - so the fine-tuning key changes. The VHF voter does the IF amplifier when the voter is on

VHF is in the working state, and it enhances and strengthens the signal entering the mid-frequency amplifier for visuals. Thus, the device will have the same signal strength for the VHF and UHF channels. Qvo antenna input

E - 11 - Digital Tuning Technology:

Digital tuning includes the use of the PLL circuit, the Locked Loops circuit, artificial frequencies, the working principle of these circuits (AFT), and the remote control process, which we will discuss in a later chapter.
E-12 Wave traps and filters in the voter:

The filters pass the required frequencies, including the high-pass filter that prevents the passage of frequencies less than (47) megahertz, which are less than the medium frequency (IF). There is a resonant circuit (33) megahertz (MHz) and a resonant circuit (389) megahertz.

VHF and UHF are allowed. There is also an LPF low-pass filter that prevents UHF frequencies from entering a voter by passing only VHF frequencies. Figure (5-40) - (5-41) and 5- (42) There is also a high-pass HPF filter that prevents VHF frequencies from entering a voter, and we must remember the following:

The frequency of the VHF field is limited to (47) - 68 megahertz (MHz) in band 1 of channels 2 - 3 - (4) and between 174 - 223 megahertz (MHz) in band III of channels (5) to (11) and the frequency of the UHF field is limited to between 470-900 MHz, and the channels from (21-69) operate within it. The middle frequency: (3315-4015 MHz) = that is, the channel width: 7 megahertz (MHz).