| 摘要 |
A secure communications system requires special arrangements for the establishment and maintenance of framing and synchronization. A framing word is transmitted at a fixed frequency, and a key necessary to unscramble the transmission is transmitted in a fixed position in relation to the framing word. Synchronization information (reference clock bursts) is scrambled with the intelligence and may only be recovered once the key is available. When framing is being maintained, the receiver's phase-locked loop opens only during the short reference clock bursts in the incoming signal, and the framing signal generator is able to predict the occurrence of successive framing signals without reference to the incoming signal. When framing is lost, the receiver enters a "soft lock" condition in which its phase-locked loop is opened to receive the entire incoming signal and in which the framing signal generator refers solely to the incoming signal to generate framing signals. |
