About Implementing a AGC

I am trying to Implement AGC,After demodulator block i am using AGC Block.When i have given input to my AGC it is multiplying every sample with the same gain but agc should be choose the which sample has to be multiplied with what amount of gain.so what logic is usefull for me to wirte so that my AGC can choose the gain according to the sample...

[Dimitris Kalogiros]

1) Are you working with matlb or simulink ?

2) do you prefer a feedback or a feedforward structure?

Keep in mind that AGCs aims to keep the average power of the incoming signal almost constant

[Hari Ijjada]

i am working with matlab..i prefer a feedback structure..if you want to decrease the noise level of AGC how can i decrease the level ?

Answer 1

[Dimitris Kalogiros]

Dear Hari

Here you are a simple version of a "feedback" structured AGC

clearvars; clc;

%% time instances
dt=0.001
t=0:dt:20;

%% input signal
A=10; f=0.25;
xin=A*sin(2*pi*f.*t);

%% AGC parameters

% loop filter gain
L=round(10*f*(1/dt));

% Target power
powerTarget=0.2;

%% AGC process
yout=zeros(1,length(xin));
gainAGC=ones(1,length(xin));
for n=1:length(t)
    % amplify current input sample
    yout(n)=gainAGC(n)*xin(n);
    
    % adjust agc gain, with feedback branch
    gainAGC(n+1)=gainAGC(n)*(1 - (1/L)*(yout(n)^2-powerTarget) );
    
end

%% Plot input and output

figure; 
subplot(2,1,1);
plot(t, xin, '-b'); hold on;
plot(t, yout, '-r'); zoom on; grid on;
xlabel('t in sec'); ylabel('amplitude'); 
legend('xin', 'yout'); title('AGC input and output');

subplot(2,1,2);
plot(t, gainAGC(1:end-1)); zoom on; grid on; 
title('gain of AGC');

I have choosen a very simple version of gain adaptation.

Keep in mind that parameter L controls the speed of AGC lock. Large values of L leads to a fast but noisy convergence.

If you run this code, with the parameters I've choosen, you will get the following results:

[Hari Ijjada]

But i want to give voice signal as input....whem i am giving voice sigal how to calculate the value of L ?..

if possible can you tell me how to apply AGC to voice input..

[Dimitris Kalogiros]

It depents on your sampling rate. For example, suppose you have voice signal , sampled at Fs. You can set L=100*Fs or L=1000*Fs (Fs is expressed in Hz)

By the way, you can experiment on the value of L.