Automatic gain control circuit

An automatic gain control (AGC) circuit comprising:

• an amplitude synchronous detector for synchronously detecting an amplitude-modulated signal applied to the input terminal thereof and for producing a demodulated output signal from the output terminal thereof;
• an AGC voltage generating circuit having an input terminal coupled to the output terminal of said amplitude synchronous detector, an output terminal coupled to the input terminal of the amplitude synchronous detector, and a control terminal, for maintaining the output signal level of the amplitude synchronous detector at a predetermined value in response to changes of input signal level to the amplitude synchronous detector, and AGC voltage generating circuit including a first low-pass filter coupled to the output terminal of the amplitude synchronous detector, an AGC detector coupled to the output terminal of said first low-pass filter and a second low-pass filter coupled to the output terminal of said AGC detector;
• and
• a phase-synchronized-state detecting circuit having an input terminal coupled to the output terminal of the amplitude synchronous detector, and an output terminal coupled to the control terminal of said AGC voltage generating circuit for detecting an asynchronous state of the amplitude synchronous detector and for producing a control signal to the AGC generating circuit in response to the detection of said asynchronous state, said phase-synchronized-state detecting circuit including a best detector coupled to the output terminal of said amplitude synchronous detector for detecting a beat signal which is produced from the amplitude synchronous detector when the detector is in the asynchronous state and for producing said control signal in response to the detection of the beat signal thereby preventing a malfunction of the AGC voltage generating circuit.

An automatic gain control (AGC) circuit comprising:

• an amplitude synchronous detector for synchronously detecting an amplitude-modulated signal applied to the input terminal thereof and for producing a demodulated output signal from the output terminal thereof;
• an AGC voltage generating circuit having an input terminal coupled to the output terminal of said amplitude synchronous detector, an output terminal coupled to the input terminal of the amplitude synchronous detector, and a control terminal, for maintaining the output signal level of the amplitude synchronous detector at a predetermined value in response to changes of input signal level to the amplitude synchronous detector;
• a phase-synchronized-state detecting circuit having an input terminal coupled to the output terminal of the amplitude synchronous detector and first and second output terminals, said first output terminal being coupled to the control terminal of said AGC voltage generating circuit, said phase-synchronized-state detecting circuit detecting an asynchronous state of the amplitude-synchronous detector and producing a control signal to the AGC voltage generating circuit through said first output terminal in response to the detection of said asynchronous state, and for detecting a noise in the output signal of the amplitude synchronous detector and producing a noise detection signal through said second output terminal in response to the detection of noise;
• and
• a noise eliminating circuit coupled to the output terminal of the amplitude synchronous detector and to the second output terminal of the phase-synchronized-state detecting circuit for mixing the output signal of the amplitude synchronous detector with said noise detection signal from the phase-synchronized-state detecting circuit so as to substantially reduce the noise,
• whereby the output level of the amplitude synchronous detector is maintained at a predetermined level irrespective of the occurrence of the asynchronous state of the amplitude synchronous detector and the presence of noise in the output signal of the amplitude synchronous detector.