Signal presence detector and method

A signal presence detector for an input signal having a known spectral distribution but accompanied by strong noise energy which may provide a negative signal-to-noise ratio, comprising:

• a common input for receiving the input signal;
• a first bandpass filter coupled to the common input and responsive to the input signal, the first bandpass filter encompassing one selected region within the spectral distribution;
• a second bandpass filter coupled to the common input and responsive to the input signal, the second bandpass filter encompassing a second selected region within the spectral distribution;
• a detector;
• switching means for applying the outputs of said first and second bandpass filters to said detector in time shared fashion;
• means coupled to said detector and responsive to received energy levels from said bandpass filters which differ by at least a predetermined minimum amount for indicating the presence of the input signal;
• and
• means coupled to at least one of said first and second bandpass filters for balancing the outputs of the filters when the inputs thereto are noise energy alone.

A signal presence detector for a signal having a known spectral distribution and accompanied by substantial noise comprising:

• means responsive to the signal for dividing the signal into two channels;
• a first bandpass filter responsive to a first channel and having a center frequency lying within the spectral distribution;
• a second bandpass filter responsive to the second channel and having a center frequency different from said first bandpass filter and within the spectral distribution;
• a power detector;
• switching means for mutually exclusively applying first one and then the other of the outputs of said first and said second bandpass filters to said power detector;
• and
• means coupled to said power detector and responsive to the existence of different energy levels in the two channels which differ by at least a predetermined minimum amount for indicating the presence of the signal;
• the signal having a spread spectrum characteristic and the center frequency of said first bandpass filter being equal to the center frequency of the spread spectrum.

A signal presence detector for a signal having a known spectral distribution and accompanied by substantial noise comprising:

• means responsive to the signal for dividing the signal into two channels;
• a first bandpass filter responsive to a first channel and having a center frequency lying within the spectral distribution;
• a second bandpass filter responsive to the second channel and having a center frequency different from said first bandpass filter and within the spectral distribution;
• a detector;
• switching means to mutually exclusively apply the outputs of said first and said second bandpass filters to said detector;
• and
• means coupled to said detector and responsive to the existence of different energy levels in the two channels for indicating the presence of the signal;
• said switching means comprising:
• switching control means;
• summing means providing an output to said detector under control of said switching control means;
• a first mixer responsive to said first bandpass filter and said switching control means for providing an input to said summing means;
• and
• a second mixer responsive to said second bandpass filter and said switching control means for providing an input to said summing means.

The method of detecting the presence of a spread spectrum signal having a spread spectrum characteristic having a center frequency comprising the steps of:

• balancing noise energy derived in two different frequency bands within the spread spectrum characteristic of the signal, the frequency bands being chosen to have a determinable relative difference in energy of the signal within the two different frequency bands and so that a first one of the two different frequency bands has a center frequency at the center frequency of the spread spectrum characteristic and a second one of the two different frequency bands has a center frequency at a different region within the spread spectrum characteristic from the center frequency of the first one of the two different frequency bands;
• filtering the signal in the two different frequency bands;
• successively sampling total energy, including noise energy, present in the two different frequency bands upon filtering the signal;
• and
• ascertaining the presence of the spread spectrum signal by the existence of an energy difference in excess of a predetermined amount in the two different frequency bands.