مبدأ عمل دارة AGC القمة (Peak) .. كتاب التلفزيون الملون والعادي

8-3 Working principle of the Peak AGC circuit:

Figure (8) - (4) This circuit works with a double triode or diodes of semi-conductors working as an AGC voltage rectifier - the binary is connected to the IF signal through the capacitor and the value of R (1) mega ohm. It is the load of the rectification circuit, as the rectifier is not connected unless its board (anode) is positive with respect to its cathode, and then the electrons stream passes from the cathode to the anode. While the negative charge is stored in C, and due to the low impedance that the diode presents when it is in the conductive state, the capacitor C charges to the peak of the applied voltage, which is the value of the synchronization pulse voltage

negative sign period

During the negative part of the incoming signal, the diode's elevator becomes negative with respect to its cathode. The binary is in a state of non-connection, and from the diagram we notice that the entire R-C circuit - and the input coil is connected in series - and when the voltage is present through it, the current passes. Which makes the upper point of: negative with respect to the Earth. Due to the large time constant of the RC circuit, the charge stored in the capacitor discharges slowly through R1 and the slow discharge makes only a small percentage of the charge C discharge during the non-connection period of the valve.

Positive sign period:

When the incoming signal becomes positive again, the diode or diode does not conduct directly because the voltage of the incoming signal must first overcome the remaining voltage in C which may discharge only a small part of its voltage - and the diode connection occurs at the great peak of the positive cycle, which is: the peak of the synchronous pulses - Thus, the voltage across the capacitor is exactly the same. On the sync pulses, which is required, the negative voltage present across R is filtered by R2 and C to eliminate the 15625 Hz ripples of the horizontal sync pulses. As well as the low-frequency component of the image and the DC rectifier voltage goes to feed the RF amplifier and the IF amplifiers of the image, which is a voltage that charges the AGC control.

Disadvantages of the top AGC circuit:

1- A quarter of the RF amplification weakens even when a weak signal is received.

2 - Weak sensitivity of the circuit - it is not quickly affected by the change in the voltage of the input signal when changing the tuning of the channel, especially when the signal is disturbed, due to the passage of a plane, which leads to an increase in the light of the image and then its darkening due to the reflection of the signal wave on the metal of the plane.
3- It is affected by noise signals - because it conducts them in a manner similar to the synchronous pulses. It weakens the amplification of the stages, which makes the noise-to-signal ratio large.

4- The circuit is not affected by a wide range of input signal changes.

8-4 Working Principle of the Gated (Keyed) AGO Circuit:

This method is characterized by preventing the image from being affected by extraneous and fast signals. As caused by the passage of aircraft.

In this method, Figure (8) - (5) negative horizontal synchronization pulses are applied to the collector of the transistor (T151). The negative composite visual signal from the visual-signal amplifier connects it to its base. At this point, the transistor becomes ON. Figure (8-6). Before that, the transistor was in a closed state, because its emitter voltage is less than its base voltage on the one hand, and its collector voltage is positive with respect to its emitter on the other hand. This is what makes the transistor a barrier, meaning that it does not pass any current. We notice in the circuit that the radiator voltage
(+5 volts) is secured by the voltage divider (153) (151) - and the collector voltage is (13) volts. The arrival of the pulses at the base and collector opens the transistor, and the potential difference between its collector and its emitter decreases according to the voltage of the signal entering its base.

Therefore, when the voltage of the signal connected to the visual signal amplifier increases, the negative voltage (VBE) increases and leads to more transistor connections, so its resistance decreases and the latency difference between its collector and emitter decreases, and the capacitor is charged at (156) through the internal resistance of the transistor with a voltage less than the first. At the end of the negative pulse coming to the transistor collector, the transistor closes, and the capacitor (156) begins to discharge its charge through the filter circuit 154 and 156 (R). The level of the signal arriving at the detector returns to its original value. Conversely, when the signal voltage in the detector decreases - in turn, the negative voltage VBE connected to the base of the AGC transistor decreases, so its resistance increases, and the charging potential of the capacitor 156 C increases - and it is discharged through the filter circuit, so the positive voltage connected to the bases of the amplification transistors increases, so their amplification decreases and the signal level decreases. can be connected
AGC to the RF amplifier via a delay circuit - to give room for the RF amplifier to amplify the weak signal as much as possible.

The AGC signal is tuned when tuning the instrument to a very strong station and then to a very weak station. The manufacturer’s instructions cover the necessary recommendations for this: so that the strong signal does not overload the image, or the weak signal causes significant distortion, and Figure (7)- (8) shows a complete box diagram of the work of the AGC circuit, in which a delay circuit is reserved in the way of its voltage affecting the amplification of frequencies. high in voter