Signal-to-noise ratio determination circuit

Claim 2

A signal-to-noise ratio (SNR) determination circuit to determine the SNR of an input signal having a signal component (S) of a known frequency and known phase, as indicated by an additional constant amplitude, coherent reference input signal, and having unknown noise components (N), said input signal having a bandwidth BWs, comprising: input means to receive said input signal; first filter means connected to said input means and having a bandwidth BW1, greater than the signal bandwidth BWs, for decreasing, by a predetermined level, the SNR of the input signal to a level less than -5db. bandpass limiter means having a bandwidth BWL at least as large as BW1, said bandpass limiter means including an input and an output, said output having a constant signal plus noise output power level, said bandpass limiter input being; connected to said first filter means; calibrating means, coupled to said output of said bandpass limiter means and responsive to said signal component at the output of said bandpass limiter means, for coherently detecting said signal component in accordance with said knowledge of both frequency and phase, to derive the SNR of the input to the SNR determination circuit, wherein said calibrating means is adapted to receive said constant amplitude coherent reference signal, and further includes a mixer having first and second inputs, the first mixer input being coupled to said bAndpass limiter means output and said second mixer input being connected to receive said constant amplitude coherent reference signal, said mixer output having an ac and a DC component, and a low frequency bandpass filter coupled to said mixer output for extracting said DC component, said DC component thereby providing a calibrated measurement of the SNR of said input signal.

Claim 3

An SNR determination circuit as recited in claim 1, including a narrow band filter having the center of its pass band at said input frequency S, said narrow band filter being interposed between said bandpass limiter means and said first mixer input.

Claim 4

An SNR determination circuit as recited in claim 2 wherein the center of said narrow band filter is fixed and wherein the input signal to the SNR determination circuit remains constant in frequency.

Claim 5

Intermediate frequency (IF) receiver means including signal-to-noise ratio indicator means having an output representative of the true SNR of a received IF signal, said IF signal having a signal component (S) of a known frequency and phase, indicated by an additional constant amplitude coherent reference input signal, and having unknown noise components (N), comprising: input means having bandwidth BWs to receive said IF signal, and an output, said input means establishing the noise temperature for said receiver means, said output being connected to a terminal; first filter means having a first bandwidth BW1 connected to said terminal to decrease the db level of the SNR at said terminal to a first db level db1, where BW1 greater than BWs; bandpass limiter means having a bandwidth BWL at least equal to said first bandwidth BW1, said bandpass limiter means including an input and an output, said output having a constant signal plus noise output power level, said bandpass limiter means input being coupled to the output of said first filter means; calibrating means coupled to the output of said bandpass limiter means and responsive to the signal component at the output of said bandpass limiter means to derive the SNR of the received radiation therein, said calibrating means being adapted to receive said constant amplitude coherent reference signal, a mixer having first and second inputs, the first mixer input being coupled to said bandpass limiter means output and said second mixer input being connected to receive said constant amplitude coherent reference signal, said mixer output having an ac and dc component, and a low frequency bandpass filter coupled to said mixer output for extracting said DC component thereby providing a calibrated measurement of the SNR of said IF signal; second filter means connected to said terminal, said second filter means having a second bandwidth BW2 less than said first bandwidth BW1, said second filter means being connected to said terminal to convert the db level of the SNR at said terminal to a second db level db2, said first level being related to said second level according to the relationship, SNR db2 - db1 10 log (BW1/BW2) , wherein the output of said second filter means contains the information in the received IF signal and is adapted to demodulation.

Claim 6

Intermediate frequency (IF) receiver means including signal-to-noise ratio indicator means having an output representative of the true SNR of a received IF signal, said IF signal having a signal component (S) of a known frequency and phase, indicated by an additional constant amplitude coherent reference input signal, and having unknown noise components (N), comprising: input means having bandwidth BWs to receive said IF signal, and an output, said input means establishing the noise temperature for said receiver means, said output being connected to a terminal; first filter means having a first bandwidth BW1 connected to said terminal to decrease the db level of the SNR at said terminal to a first db level db1, where BW1 greater than BWs; bandpass limiter means having a bandwidth BWL at least equal to said first bandwidth BW1, said bandpass limiter means including an input and an output, said output having a constant signal plus noise output power level, said bandpass limiter means input being coupled to the output of said first filter means; calibrating means coupled to the output of said bandpass limiter means and responsive to the signal component at the output of said bandpass limiter means to derive the SNR of the received radiation therein, said calibrating means being adapted to receive said constant amplitude coherent reference signal, a mixer having first and second inputs, the first mixer input being coupled to said bandpass limiter means output and said second mixer input being connected to receive said constant amplitude coherent reference signal, said mixer output having an ac and dc component, and a low frequency bandpass filter coupled to said mixer output for extracting said DC component thereby providing a calibrated measurement of the SNR of said IF signal; second filter means connected to said terminal, said second filter means having a second bandwidth BW2 less than said first bandwidth BW1, said second filter means being connected to said terminal to convert the db level of the SNR at said terminal to a second db level db2, said first level being related to said second level according to the relationship, SNR db2 - db1 10 log (BW1/BW2) , wherein the output of said second filter means contains the information in the received IF signal and is adapted to demodulation.

Claim 7

IF receiver means as recited in claim 4, including a narrow band filter having the center of its pass band at said known frequency of said signal S, said narrow band filter being interposed between said bandpass limiter means and said first mixer input.

Claim 8

IF receiver means as recited in claim 5, wherein the center of said narrow band filter pass band is fixed and wherein the IF signal to said input means remains constant in frequency.

Claim 9

In a diversity receiver having a plurality of IF channels, each channel having coherent AGC to maintain IF signal level constant therein and channel combiner means to provide for optimizEd output signal-to-noise ratio (SNR), THE IMPROVEMENT COMPRISING: input means to receive said IF signal associated with each channel having a bandwidth BWs and an output, said input means establishing the noise temperature for said receiver means, said output being connected to a terminal associated with each channel; a SNR determination circuit for each channel, each SNR determination circuit having first filter means having a first bandwidth BW1, connected to said channel associated terminal to convert the db level of the SNR at said terminal to a first db level db1; bandpass limiter means having a constant signal plus noise output level connected to the output of said first filter, the SNR of the input to the bandpass limiter being linearly related to the dbm level of the signal component of the output thereof for a given db range of converted input SNR levels; calibrating means connected to the bandpass limiter means and responsive to the signal component at the output thereof to derive the SNR of said IF channel signal input; second filter means, said second filter means having a second bandwidth BW2 different from said first bandwidth BW1, being connected to said channel associated terminal to convert the db level of the SNR at said terminal to a second db level db2, said first level being related to said second level according to the relationship Delta SNR db2 - db1 10 log 10 (BW1/BW2) wherein the output of the second filter means contains the information in said IF signal and is coupled to said combiner; detection means connected to the output of said second filter means to derive the level of signal component (S) of the input applied to said channel associated terminal; a weighting means, said weighting means connected to the calibrating means and to the detection means to derive a weighting factor for each channel which is proportional to Esi/ENi2, where Es is signal voltage and EN is noise voltage in each channel i at said channel associated terminal, including variable gain means responsive to said weighting factor for adjusting the level of each channel input to said combiner, said variable gain means being interposed between said second filter means and said combiner.

Claim 10

In a diversity receiver having a plurality of IF channels, each channel having coherent AGC to maintain IF signal level constant therein and channel combiner means to provide for optimizEd output signal-to-noise ratio (SNR), THE IMPROVEMENT COMPRISING: input means to receive said IF signal associated with each channel having a bandwidth BWs and an output, said input means establishing the noise temperature for said receiver means, said output being connected to a terminal associated with each channel; a SNR determination circuit for each channel, each SNR determination circuit having first filter means having a first bandwidth BW1, connected to said channel associated terminal to convert the db level of the SNR at said terminal to a first db level db1; bandpass limiter means having a constant signal plus noise output level connected to the output of said first filter, the SNR of the input to the bandpass limiter being linearly related to the dbm level of the signal component of the output thereof for a given db range of converted input SNR levels; calibrating means connected to the bandpass limiter means and responsive to the signal component at the output thereof to derive the SNR of said IF channel signal input; second filter means, said second filter means having a second bandwidth BW2 different from said first bandwidth BW1, being connected to said channel associated terminal to convert the db level of the SNR at said terminal to a second db level db2, said first level being related to said second level according to the relationship Delta SNR db2 - db1 10 log 10 (BW1/BW2) wherein the output of the second filter means contains the information in said IF signal and is coupled to said combiner; detection means connected to the output of said second filter means to derive the level of signal component (S) of the input applied to said channel associated terminal; a weighting means, said weighting means connected to the calibrating means and to the detection means to derive a weighting factor for each channel which is proportional to Esi/ENi2, where Es is signal voltage and EN is noise voltage in each channel i at said channel associated terminal, including variable gain means responsive to said weighting factor for adjusting the level of each channel input to said combiner, said variable gain means being interposed between said second filter means and said combiner.

Claim 11

A diversity receiver as recited in claim 7 further comprising third filter means connected between the bandpass limiter means and the calibrating means to filter out the noise (N) components from said bandpass limiter output.

Claim 12

A diversity receiver as recited in claim 8, wherein said calibrating means is adapted to receive an additional constant amplitude coherent reference signal signal, said calibrating means further including a mixer having first and second inputs, the first mixer input being coupled to said bandpass limiter means output and said second mixer input being connected to receive said constant amplitude coherent reference signal, said mixer output having an ac and dc output, and a low frequency bandpass filter coupled to said mixer output for extraction of said dc output.

Claim 13

A diversity receiver as recited in claim 9 wherein said weighting means includes: a squarer circuit, said squarer circuit being coupled to said low frequency bandpass filter of said calibrating means for squaring the output of said low frequency bandpass filter; a first log amplifier coupled to the output of said squarer circuit; a second log amplifier coupled to the output of said detection means; a subtractor means having first and second inputs and an output, said first subtractor input being coupled to the output of said first log amplifier, said second subtractor input being coupled to the output of said second log amplifier, said subtractor output being said weighting factor.