Signal-to-noise ratio detector for automatic gain controlled receivers

Claim 2

In a radio receiver responsive to an input of intermittent reception having periods of signal separated by periods of nosignal, circuitry comprising: means for amplifying the received input, means for detecting the voltage level of the amplified received input, discharge circuit means for delaying the change of the detected voltage level to form a modified detected level for a transient period following termination of a signal period, an automatic gain control circuit responsive to the delayed detected output to adjust the amplification of the received input in accordance with the detected voltage level, means for storing the voltage lEvel detected during a signal period, said means for storing being connected to the discharge circuit means, comparator means for forming a difference voltage by comparing during the succeeding period of no-signal the stored level with the modified detected level, and means for monitoring the difference voltage and determining the peak difference voltage following each of said terminations, whereby the peak difference voltage is indicative of the signal-to-noise ratio of the receiver.

Claim 3

Circuitry as claimed in claim 1 wherein said automatic gain control circuit has a fixed gain recovery time constant for establishing a new level of amplification in response to a change of the detected voltage level at the termination of a signal period, and said discharge circuit means has a discharge time constant, said discharge time constant being substantially shorter than the gain recovery time constant.

Claim 4

Circuitry as claimed in claim 1 wherein said automatic gain control circuit has a fixed gain recovery time constant for establishing a new level of amplification in response to a change of the detected voltage level at the termination of a signal period, said discharge circuit means is an RC circuit, the capacitor of said RC circuit being charged to a voltage level during a signal period by the input, discharged during a no-signal period at a rate determined by the discharge time constant of the RC circuit and simultaneously charged during a no-signal period at a rate determined by the gain recovery time constant, the discharge time constant being substantially shorter than the gain recovery time constant, the net effect being to first discharge and then recharge the capacitor during the no-signal period.

Claim 5

Circuitry as claimed in claim 1 wherein said means for monitoring the difference voltage and determining the peak difference voltage includes a comparator, a clock started by the comparator output, a counter stepped by the output of the clock, and feedback sensing network means for monitoring the state of the counter and developing a voltage representative of the counter state, the representative voltage and said difference voltage being applied to said comparator and said comparator producing an output when the difference between the representative voltage and said difference voltage exceeds a selected threshold.

Claim 6

A selection diversity receiver system having a single output terminal comprising, a plurality of radio receivers, each of said receivers having circuitry as claimed in claim 1, means for comparing the peak difference voltage produced by each of said receivers, and switching means responsive to the comparing means for exclusively applying to the output terminal the input reception of the receiver having the highest peak difference voltage.

Claim 7

Circuitry responsive to an input of intermittent reception having periods of signal separated by periods of no-signal comprising: means for amplifying the received input, detecting means for detecting the voltage level of the amplified received input, delay means for effecting a change in the detected voltage level in a transition period beginning upon termination of a signal period to create a first transient component of the voltage level, which first component changes exponentially with time in one sense, means for effecting a change in the detected voltage level in the transition period to create a second transient component of the voltage level in a sense opposite to the first component, which second component changes exponentially with time independently of the first component, the voltage level in the transient period being a combination of the first and second transient components, and the voltage level in the transient period having a peak value when the magnitude of the positive rate of change of one of the transient components equals the magnitude of the negative rate of change of the other of the transient components, the occurrence of the peak value varying in time in accordAnce with the signal-to-noise ratio of the reception, and means for producing an output representative of the difference between the peak voltage during a transient period and the detected voltage level established during a preceding signal period.

Claim 8

Circuitry responsive to an input of intermittent reception having periods of signal separated by periods of no-signal comprising: means for amplifying the received input, detecting means for detecting the voltage level of the amplified received input, delay means for effecting a change in the detected voltage level in a transition period beginning upon termination of a signal period to create a first transient component of the voltage level, which first component changes exponentially with time in one sense, means for effecting a change in the detected voltage level in the transition period to create a second transient component of the voltage level in a sense opposite to the first component, which second component changes exponentially with time independently of the first component, the voltage level in the transient period being a combination of the first and second transient components, and the voltage level in the transient period having a peak value when the magnitude of the positive rate of change of one of the transient components equals the magnitude of the negative rate of change of the other of the transient components, the occurrence of the peak value varying in time in accordAnce with the signal-to-noise ratio of the reception, and means for producing an output representative of the difference between the peak voltage during a transient period and the detected voltage level established during a preceding signal period.

Claim 9

Circuitry as claimed in claim 6 wherein said means for effecting a change in the detected voltage level to create the first transient component is an AGC circuit arranged to monitor the detected voltage level and control the amplification of the received input, said circuit having a gain recovery time constant which is the time constant of the first transient component.

Claim 10

Circuitry as claimed in claim 7 wherein said means for effecting a change in the detected voltage level to create the second transient component is an RC circuit, the capacitor of said RC circuit being charged during a signal period and discharged during the succeeding transition period with a discharge time constant substantially shorter than the gain recovery time constant.

Claim 11

Circuitry as claimed in claim 6 wherein said means for effecting a change in the detected voltage level to create the first transient component is an AGC circuit arranged to monitor the detected voltage level and control amplification of the received input, and said means for effecting a change in the detected voltage level to create the second transient component is an RC circuit, the capacitor of said RC circuit being charged during a signal period and discharged during the transition period, said first transient component having a time constant equal to the gain recovery time constant of the AGC circuit and the second transient component having the time constant of the RC circuit which is substantially shorter than the time constant of the gain recovery time constant, so that the peak voltage occurs at increasingly later times for increasing signal-to-noise ratios, whereby gain recovery prior to the peak value increases with increasing signal-to-noise ratios.

Claim 12

In a radio receiver circuitry responsive to an input of intermittent reception having periods of signal separated by periods of no-signal comprising: means for amplifying the received input by a controlled gain, means for detecting the voltage level of the amplified received input, a transient delay circuit including a capacitor which is charged by the detected voltage level during a signal period and discharges at the termination of the signal period, an AGC circuit for adjusting the gain of the amplification means in accordance with the output of the transient delay circuit, storage means monitoring the output of the transient delay circuit for storing the detected level during a signal period, comparator means for comparing the output of the transient delay circuit with the stored level to form a difference voltage, and means for detecting the peak difference voltage following each signal period to produce an output indicative of the signal-to-noise ratio of the receiver.

Claim 13

In a radio receiver circuitry responsive to an input of intermittent reception having periods of signal separated by periods of no-signal comprising: means for amplifying the received input by a controlled gain, means for detecting the voltage level of the amplified received input, a transient delay circuit including a capacitor which is charged by the detected voltage level during a signal period and discharges at the termination of the signal period, an AGC circuit for adjusting the gain of the amplification means in accordance with the output of the transient delay circuit, storage means monitoring the output of the transient delay circuit for storing the detected level during a signal period, comparator means for comparing the output of the transient delay circuit with the stored level to form a difference voltage, and means for detecting the peak difference voltage following each signal period to produce an output indicative of the signal-to-noise ratio of the receiver.