Patent US 4,598,293

An intruder detector system employing Doppler radar consists of an encoder for modulating RF pulses with a cyclic pulse code, the code being cycled at a frequency equal to the RF pulse frequency and the pulses being modulated on a VHF carrier for transmission along one of a pair of parallel radiating cables. The signal received from the other cable is demodulated and digitized by a zero crossing detector functioning as a 1-bit analog-to-digital converter. A bank of exclusive-OR gates functioning as 1-bit correlators then simultaneously correlates each successive bit of the digitized signal with the values of the respective bits in each fixed position of the transmitted code sequence.