Combined feedback and feedforward automatic gain control

Claim 2

An automatic gain control (AGC) system comprising: a main signal path; an AGC circuit; and means for coupling the input end of said AGC circuit to a point along said signal path; CHARACTERIZED IN THAT: the AGC signal generated by said AGC circuit is simultaneously fed forward to first variable attenuator means located in said main signal path for controlling the magnitude of the signal propagating therealong, and fed back to a second variable attenuator means located at the input end of said AGC circuit for controlling the magnitude of the signal in the AGC circuit.

Claim 3

The AGC system according to claim 1 wherein said first and second attenuator means have identical attenuation -vs- AGC signal characteristics.

Claim 4

The AGC system according to claim 1 wherein said first and second attenuator means have linear attenuation in dB -vs- AGC signal characteristics.

Claim 5

The AGC system according to claim 1 wherein said AGC circuit includes, in cascade: said second attenuator means; an amplifier; an amplitude detector; and a differential amplifier having one input port coupled to the output port of said detector, and having its second input port coupled to a direct current reference voltage; and wherein the output signal from said differential amplifier is the AGC signal.

Claim 6

The AGC system according to claim 1 wherein each of said first and second variable attenuator means comprises: a differential amplifier having two input ports; a transistor having base, emitter and collector electrodes; and a diode connected between said collector electrode and ground; means for coupling said emitter electrode to one of the two input ports of said differential amplifier; the other of said input ports being the port to which the AGC signal is coupled; means for coupling the output port of said differential amplifier to the transistor base electrode; a resistor connected between ground and the common junction of said emitter electrode and said one input port; means for coupling an input signal to said base electrode; and means for extracting an output signal at the common junction of said collector electrode and said diode.

Claim 7

The AGC system according to claim 1 wherein a plurality of different frequency signals and a pilot signal are simultaneously transmitted along said main signal path; and wherein said AGC circuit includes a narrow passband filter for extracting said pilot signal from among said plurality of signals.

Claim 8

The AGC system according to claim 1 wherein each of said first and second variable attenuator means comprises a diode whose alternating current conductance varies in response to said AGC signal; and wherein said diode serves as a load to the signal whose magnitude is to be controlled.

Claim 9

The AGC system according to claim 1 wherein each of said variable attenuator means comprises; a 3 dB quadrature couplers having two pairs of conjugate ports 1-2 and 3-4, where ports 1 and 2 are the input and output ports, respectively, of said attenuator; a pair of diodes connected, respectively, to coupler ports 3 and 4 for alternating current signals, and connected in series with respect to direct current signals; and means for coupling said AGC signal to said series-connected diodes.

Claim 10

The AGC system according to claim 8 wherein said means for coupling said AGC signal to the series-connected diodes in each of said variable attenuator means includes an attenuator control circuit whose output current Id varies as the hyperbolic tangent of said AGC signal.

Claim 11

The AGC system according to claim 9 wherein each attenuator control circuit includes: a differential amplifier comprising a pair of transistors; and a transconductor whose output current Id is proportional to the voltage Delta v produced by said differential amplifier and applied to said transconductor; said voltage Delta v being the differential voltage developed between the collector electrodes of said transistors in response to the AGC signal applied to the base electrode of one of said transistors.

Claim 12

The AGC system according to claim 10 wherein the emitter electrodes of said pair of transistors are connected to a common current source whose current varies linearly with temperature; said current source comprising: a diode having one electrode connected to ground, and the other electrode connected to a source of d.c. current through a first resistor; a differential amplifier having a pair of input ports; and a transistor whose collector electrode is connected to the emitter electrodes of the pair of transistors in said attenuator control circuit, and whose emitter electrode is connected to ground through a second resistor; means for connecting one input port of said differential amplifier to the common junction of said other diode electrode and said first resistor; means for connecting the other input port of said differential amplifier to the common junction of said emitter electrode and said second resistor; and means for connecting the output port of said differential amplifier to the base electrode of said transistor.

Claim 13

The AGC circuit according to claim 1 wherein said first variable attenuator means, located in said main signal path, comprises a cascade of n identical attenuators, each of which has a linear attenuation in dB -vs- AGC signal characteristic; wherein said second variable attenuator means has a linear attenuation in dB -vs- AGC signal characteristic; and wherein the magnitude of the AGC signal coupled to the respective attenuators comprising said first attenuator means is 1/nth the magnitude of the AGC signal coupled to said second attenuator means.

Claim 14

An automatic gain control (AGC) system comprising: a main signal path; an AGC network; and means for coupling the input end of said AGC network to a point along said main signal path; CHARACTERIZED IN THAT: said AGC network comprises a cascade of m AGC circuits, each of one of which generates an AGC signal component e1, e2 . . . em which is fed back to a variable attenuator means located at the input end of each of said circuits for controlling the magnitude of the signal in the respective AGC circuits; and in that said signal components are added together in time coincidence to produce a total AGC signal y which is fed forward to other variable attenuator means located in said main signal path for controlling the magnitude of the signal propagating therealong.

Claim 15

An automatic gain control (AGC) system comprising: a main signal path; an AGC network; and means for coupling the input end of said AGC network to a point along said main signal path; CHARACTERIZED IN THAT: said AGC network comprises a cascade of m AGC circuits, each of one of which generates an AGC signal component e1, e2 . . . em which is fed back to a variable attenuator means located at the input end of each of said circuits for controlling the magnitude of the signal in the respective AGC circuits; and in that said signal components are added together in time coincidence to produce a total AGC signal y which is fed forward to other variable attenuator means located in said main signal path for controlling the magnitude of the signal propagating therealong.

Claim 16

The AGC system according to claim 13 wherein each of said AGC circuits includes, in cascade, said variable attenuator means, an amplifier, an amplitude detector, and a differential amplifier having one input port coupled to the output port of said detector, and having a second input port coupled to a direct current reference voltage; and wherein said circuits are connected in cascade by coupling the amplifier output port of one circuit is coupled to the attenuator means input port of the next adjacent stage.

Claim 17

The AGC system according to claim 13 wherein said other attenuator means comprises a cascade of n attenuators; and wherein said total AGC Signal v is divided into n signal components v1, v2 . . . vn, each of which is applied to a different one of said attenuators.