Patent US 4,149,121

A four phase to two phase correlator is incorporated within the demodulator section of a spread spectrum modem to process a received RF signal modulated by a transmitted PN code and a digital data signal. The correlator includes a first modulator for combining the channel 1 and channel 2 R code signals with the received RF input signal to generate a first channel 1 RF signal and a first channel 2 RF signal. The first channel 1 RF signal is modulated by the transmitted PN code signal, the channel 1 R code signal and the digital data signal. The first channel 2 RF signal is modulated by the transmitted PN code signal, the channel 2 R code signal and the digital data signal. A second modulator combines the channel 1 and channel 2 PN code signals and the channel 1 and channel 2 R code signals with a first unmodulated RF signal to generate a second channel 1 RF signal which is modulated by the channel 1 PN code signal and by the channel 1 R code signal, and a second channel 2 RF signal which is modulated by the channel 2 PN code signal by the channel 2 R code signal. A third modulator combines both channels of the first and second RF signals to generate a third RF signal which is modulated only by the digital data signal.