The first of these devices was designed in 1853 by Julius Wilhelm Gintl of the Austrian State Telegraph. The telegraph companies were keen to have such a device since the ability to have simultaneous traffic in both directions had the potential to save the cost of thousands of miles of telegraph wire. They were an early form of the hybrid coil. The first duplexers were invented for use on the electric telegraph and were known as duplex rather than duplexer. Source: from Federal Standard 1037C History Note 3: A duplexer must provide sufficient isolation to prevent receiver desensitization. Note 2: A duplexer must provide adequate rejection of transmitter noise occurring at the receive frequency, and must be designed to operate at, or less than, the frequency separation between the transmitter and receiver. Note 1: A duplexer must be designed for operation in the frequency band used by the receiver and transmitter, and must be capable of handling the output power of the transmitter. Most professionally engineered sites ban the use of notch duplexers and insist on bandpass duplexers for this reason. On shared-antenna sites, the bandpass duplexer variety is greatly preferred because this virtually eliminates interference between transmitters and receivers by removing out-of-band transmit emissions and considerably improving the selectivity of receivers. There are two predominant types of duplexer in use - "notch duplexers", which exhibit sharp notches at the "unwanted" frequencies and only pass through a narrow band of wanted frequencies and "bandpass duplexers", which have wide-pass frequency ranges and high out-of-band attenuation. Significant attenuation (isolation) is needed to prevent the transmitter's output from overloading the receiver's input, so such duplexers employ multi-pole filters. For example, the transition between the uplink and downlink bands in the GSM frequency bands may be about one percent (915 MHz to 925 MHz). Modern duplexers often use nearby frequency bands, so the frequency separation between the two ports is also much less. With a duplexer the high- and low-frequency signals are traveling in opposite directions at the shared port of the duplexer. These are effectively a higher-performance version of a diplexer, typically with a narrow split between the two frequencies in question (typically around 2%-5% for a commercial two-way radio system). In radio communications (as opposed to radar), the transmitted and received signals can occupy different frequency bands, and so may be separated by frequency-selective filters.
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