Automatic gain control circuit

Details Automatic gain control circuit Automatic gain control circuit

1986-02-06

1987-05-19

Mullins, James B.

Amplifiers

With control of power supply or bias voltage

With control of input electrode or gain control electrode bias

330279, 358174, H03G 320

Patent

active

046671633

DESCRIPTION:

BRIEF SUMMARY
This invention relates to automatic gain control circuits and particularly, though not exclusively, to automatic gain control circuits for use in the demodulation of FM signals.
In the demodulation of FM signals in a SECAM television sytem using sampling techniques, e.g. switched capacitor filter techniques, non-linearity of the circuit results in the generation of frequency components higher than the Nyquist value which can cause beat notes in the system. For example, the frequency band in a SECAM system is 3.9-4.75 MHz and so the 3rd. harmonic (such as would be generated by a limiter as normally used in FM systems) of this frequency band is 11.7-14.25 MHz. Sampling at a frequency of approximately 17.73 MHz (i.e. four times the PAL sub carrier frequency, as is convenient in a TV receiver) generates a frequency band of approximately 3.5 -6.0 MHz. Further non-linearity produces beating between this generated frequency band and the first-mentioned SECAM frequency band.
In order for the system to remain linear, an automatic gain control circuit must be used to remove amplitude variations in the FM signal. However, known automatic gain control circuits cannot readily be made to operate sufficiently fast to cope with amplitude variations resulting from the passage of such FM signals through tuned circuits, etc.
It is an object of the present invention to provide an automatic gain control circuit wherein the above disadvantage may be overcome, or at least alleviated.
In accordance with the invention an automatic gain control circuit comprises: connected to the input node, a control input and an output; and
automatic gain control means having an input coupled to the output of the controllable gain amplifier means and an output connected to the control input of the controllable gain amplifier means, characterised in that the automatic gain control circuit further comprises divider means having a first signal input coupled to the output of the controllable gain amplifier means, a divide signal input coupled to the input of the automatic gain control means and an output connected to the output node of the circuit, whereby the signal produced at the output of the divider means is substantially independent of the signal level produced at the output of the controllable gain amplifier means.
Preferably the circuit futher comprises multiplier means having first and second inputs each connected to the output of the controllable gain amplifier means and an output connected to the input of the automatic gain control means, and the divider means has a second signal input connected to the output of the controllable gain amplifier means and includes means for multiplying the signals at its first and second signal inputs.
Preferably phase shifting filter means is provided in the signal path to the first signal input of the divider means so that the circuit is frequency discriminating and low-pass filter means is provided in the signal path to the divide signal input of the divider means, the delays produced by the two filter means being so matched as to minimise differential delay effects at the output of the divider means.
One frequency discriminating automatic gain control circuit in accordance with the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:
FIG. 1 shows a block-schematic diagram of the circuit; and
FIG. 2 shows a more detailed circuit diagram of parts of the circuit of FIG. 1.
Referring firstly to FIG. 1, a frequency discriminating automatic gain control circuit for use in an FM decoder in a SECAM TV receiver includes an input node 2 connected to the signal input 4 of a variable gain amplifier 6. The output 8 of the amplifier 6 is connected to each of two inputs 10, 12 of a multiplier 14. The output 16 of the multiplier 14 is connected to the input 18 of a known automatic gain control (AGC) gain generator 20. The output 22 of the AGC gain generator 20 is connected to the control input 24 of the amplifier 6 to control the gain thereof.
The output 8 of th

REFERENCES:
patent: 4563653 (1986-01-01), Perahia

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