Signal Level Control (AGC) Instructor: Dr. George Collins Presented By: Rahul Chitturi

Contents Automatic Gain Control (AGC) Basic AGC Circuit Types of AGC Reverse AGC Forward AGC Various AGC Systems Peak AGC System Keyed AGC System Delayed AGC System Noise Cancelation Diode Noise Gate Circuit Separate Noise Gate Amplifier Video Amplifiers Advantages of AGC Conclusion

Automatic Gain Control (AGC) AGC circuit varies the gain of a receiver according to the strength of signal picked up by the antenna. AGC circuit produces AGC bias , which is a DC voltage proportional to the input signal strength. DC voltage is obtained by rectifying the video signal as available after the video detector. AGC bias is used to control the gain of RF and IF stages in the receiver.

Basic AGC circuit 1. Unidirectional voltage is developed across RL. 2. R1, c1 acts as AGC filter. 3. AGC bias voltages is fed to the RF and IF amplifier. 4. Decoupling n/w – used to isolate the amp stages from common AGC line.

Types of AGC Reverse AGC – Shifting operating point towards cut off. Forward AGC - Shifting operating point towards saturation.

Reverse AGC Reverse AGC requires smaller changes in voltage for full gain control. Voltage divider formed by R1 and R2 provides suitable fixed forward bias R3,C1 Decoupling nw

Forward AGC 1. For any increase in signal strength, VBE must increase to shift the operating point towards saturation. 2. R5 is used in series with the collector circuit. 3. When AGC voltage varies to increase Ic, Vce decreases.

Various AGC systems Peak AGC system / Non keyed AGC system Delayed AGC system

Peak AGC system This system is based on sampling the synch tip level. During synch pulses of the modulated signal, D2 conducts & C1 charges. Voltage across R1is then filtered using R2,C2 & produces AGC voltage. During periods other than synch pulse, diode is under RB condition. But the C1 discharges through secondary winding of the transformer.

Keyed AGC system / Gated AGC In this, AGC rectifier is allowed to conduct only during horizontal synch pulses This is done with the help of fly back pulses derived from the output of horizontal deflection circuit of the receiver.

Delayed AGC AGC voltage is proportional to signal strength. Even weak RF signals will produce some control voltage. This when applied to RF amplifier will tend to reduce its gain. Hence, RF amplifier is not fed any AGC voltage till the signal strength attains a predetermined level.

Method 1 – Diode noise gate circuit Noise Cancellation The circuit which is used to cancel noise in AGC and synch separator circuits is called Noise cancellation circuit. Method 1 – Diode noise gate circuit 1. D2 is used as a switch which opens in the presence of noise. 2. R3 is used to provide forward bias to the diode so that it pass noise free signals to video amplifier. 3. For strong noise signal D2 is RB, thereby stopping noise pulses from reaching the amplifier.

Method 2 – Separate noise gate amplifier The output of video amplifier is the inverted composite video signal. Noise gate is a grounded base amplifier. It amplifies the noise pulse without inversion of its polarity. The output of video amplifier & Noise gate is added. Thus noise pulse is cancelled on addition.

Video Amplifiers RC Coupled Amplifier It is the one which is used to amplify the video signal. An inverting amplifier is used generally, hence inverted composite video signal is obtained as the output and fed to the cathode of the picture tube. RC Coupled Amplifier

High frequency compensation To compensate the fall in gain at high frequencies, peaking coils in different configurations are used. Methods of using peaking coils Shunt Peaking Series Peaking Combination Peaking

Advantages of AGC Maintaining constant contrast and intensity of the picture. Constant contrast when receiver is switched from one station to another. Reducing Amplitude and cross modulation distortion. Increasing gain of weak signals. Reducing flutter in the picture due to passing aero planes and other fading effects. Maintaining constant sound signal. Easy separation of synch pulses.

Conclusion In this presentation we have discussed about the Automatic Gain Control, the basic circuits used for the AGC and its different types. We also discussed about the different systems in which these AGC systems are used and the method in which they are used.

References http://en.wikipedia.org/wiki/Automatic_gain_control http://www.eecg.utoronto.ca/~kphang/papers/2001/martin_AGC.pdf http://v2.embedded.com/design/214600265 http://www.qsl.net/va3iul/Files/Automatic_Gain_Control.pdf http://www.tpub.com/neets/book8/31c.htm

Thank you