| 摘要 |
There is disclosed frequency modulation (FM) or phase modulation (PM) communication with a receiver having a limiter, in spread-spectrum operation for security purposes. The spectrum is spread on transmission by superimposing a psuedo-noise code modulation on the intelligence modulation of the carrier. On reception the spectrum is collapsed by auto-correlation of the pseudo-noise code. The pseudo-noise spectrum is produced by a digital coder which causes phase modulation of the carrier at the transmitter and the spectrum is collapsed by an identical coder which remodulates the received signal at the receiver which is operated in precise synchronism with the coder at the transmitter. To maintain synchronism a reference oscillator signal which periodically varies the phase of the received carrier is injected into the coder network at the receiver; this reference oscillator signal is processsed so that it passes through the limiter of the receiver. The modulated carrier from the transmitter is received in parallel paths. In one path the received carrier is modulated directly by the code from the coder at the receiver; in the other path, the received carrier is modulated by the code from the coder, periodically delayed in synchronism with a reference oscillator. The resulting so modulated carriers are correlated and the sum of the independently correlated carriers is impressed on the receiver as a carrier which is modulated by the intelligence and in addition is phase modulated in synchronism with the reference oscillator. The phase modulation is not suppressed by the limiter of the receiver and operates through a phase detector, low pass filter and voltage controlled oscillator, which are elements of a tracking loop, to maintain the coder at the receiver in synchronism with the received coder. The spread-spectrum operation disclosed lends itself to embodiment in adapters for converting conventional clear-signal communication equipment constructed in the past to spread-spectrum operation.
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