Surface acoustic wave-matched filter and differential detector for demodulating spread spectrum signals

A differential detection system which is provided with a one-input-two-output surface acoustic wave-matched filter to demodulate a data signal, subjected to a four-phase modulation by a pseudo-noise code sequence, from a phase-modulated signal obtained by a spread spectrum modulation of said data signal;

• said filter comprising:
• an input transducer provided on a piezoelectric substrate;
• a first output transducer coded in correspondence to said pseudo-noise code sequence and provided on said piezoelectric substrate;
• and a second output transducer coded in correspondence to said pseudo-noise code sequence and disposed on said piezoelectric substrate at the succeeding stage of said first output transducer by the length of said pseudo-noise code sequence as viewed from said input transducer;
• and wherein:
• correlation signals from said first and second output transducers of said surface acoustic wave-matched filter are input into respective ones of independent two-output dividers;
• the one output signal of said two-output divider connected to said first output transducer is multiplied by a (+π/4) phase-shifted version of the one output signal of said two-output divider connected to said second output transducer, so that the multiplied output signal is applied to a low-pass filter to obtain a differentially detected output signal corresponding to an in-phase component of said four-phase modulated data signal;
• a (+π/4) phase-shifted version of the other output signal of said two-output divider connected to said first output transducer is multiplied by the other output signal of said two-output divider connected to said second output transducer, so that the multiplied output signal is applied to a low-pass filter to obtain a differentially detected output signal corresponding to a quadrant component of said four-phase modulated data signal;
• and
• decision on said data signal is completed by the use of said differentially detected output signals corresponding to said in-phase and quadrature components of said data signal.