RECEIVER FUNDAMENTALS

2-9 Point-to-Point.—Most point-to-point, long-haul circuits between naval communications stations quickly use up the available frequency spectrum that ssb provides. Independent sideband (isb) transmission is normally used to compensate for the deficiency. Isb is used extensively in naval communications to expand our traffic capabilities. You will find there is a similarity between ssb and isb. Isb uses outputs from two sideband generators; it suppresses both carriers and then filters out an upper sideband from one and a lower sideband from the other. We then combine the two remaining sidebands and transmit an envelope with upper and lower sidebands that contain different intelligence. Isb can be used with MULTIPLEXING (a method for simultaneous transmission of two or more signals over a common carrier wave) to transmit a lot of intelligence on one circuit. Independent sideband and multiplexing will be discussed in more detail in chapter 3. Ship-to-Shore.—Many ships handle enough message traffic to justify ship-to-shore teletypewriter circuits. Depending on traffic load, these circuits may contain from one to four (minimum) teletypewriter circuits on one sideband circuit. If the traffic load warrants more than one teletypewriter circuit, we usually use time division multiplex or frequency division multiplex (mux) equipment. This equipment is capable of handling many incoming and outgoing circuits. One circuit normally is used as an ORDERWIRE CIRCUIT for operator-to-operator service messages and for making frequency changes when necessary. The remaining circuits are available for handling official message traffic. Q8. Why are frequency multipliers used? Q9. What are two advantages of ssb transmission? Q10. What is the purpose of an order-wire circuit? Ship-to-Ship.—Ship-to-ship ssb teletypewriter circuits are in wide use today. Their main application is with task force or task group networks or with several ships in company. By using this type of network, ships can send their outgoing messages to another ship that relays traffic ashore. You can see this procedure saves manpower and circuit time, prevents individual ships from overcrowding ship-to-shore circuits, and conserves the frequency spectrum. Depending on the number and types of ships in company, the guard can be shifted to other ships from time to time. A major advantage of these circuits is that electronic cryptographic devices can be used to send classified messages without need for manual encryption. These circuits are used for incoming as well as outgoing traffic, and they can use either hf or uhf communications equipment. RECEIVER FUNDAMENTALS An AM receiver processes amplitude-modulated signals received by its antenna. It delivers an output that is a reproduction of the signal that originally modulated the rf carrier at the transmitter. The signal can then be applied to some reproducing device, such as a loudspeaker, or to a terminal device, such as a teletypewriter. Actual AM receivers vary widely in complexity. Some are very simple; others contain a large number of complex circuits. FUNCTIONS Whatever its degree of sophistication, a receiver must perform certain basic functions to be useful. These functions, in order of their performance, are reception, selection, detection, and reproduction.