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20.1 Introduction One of the most difficult problems in a bidirectional communications system is coexistence, that is, desensitized by an a very sensitive receiver that is adjacent noisy transmitter. We may ask how much the receiver sensitivity is degraded due to the emitted noise that leaks from the transmitter into the receiver. From a different perspective, the problem is to define the reduction of a receiver's “noise-floor” performance at its threshold input as a consequence of the transmitter noise leakage. In a narrow-band full duplex communications system, in which the transmitting and receiving channels do not overlap in band, the problem is solved by using a front-end receiving∕transmitting duplexer filter, as demonstrated in Fig. 20.1. The requirements of the transmitting portion of the RF duplexer are to attenuate any kind of white noise, or other noise, emitted from the transmitter into the receiving front-end low-noise amplifier (LNA). The transmitter noise composed from a wideband with spurious and discrete intermodulation (IMD). Hence, the requirements from the transmitter-duplexer portion are to attenuate the transmitter out-of-band noise, spurious and harmonic IMD, which can leak into the receiver. The receiver portion of the duplexer filter attenuates the transmitter signal power, transmitter in-band noise, and prevents the transmitter power and noise from going into the receiver's front end, thus preventing the compression of the receiver. Leakage of any transmitter signal into the receiver section may prevent proper signal reception. The similar concept of an RF front-end (RFFE) duplexer is used when community access television (CATV) channels and direct broadcast satellite (DBS) channels are fed from the same photodetector (PD). The PD is connected to the CATV RF chain via a low-pass filter (LPF) with a sharp elliptic response that rejects the DBS band, which occupies the 950–2100 MHz. The DBS section is isolated from the CATV band by an HPF with an ecliptic response that rejects the 50–870 MHz band filter. Design methods are given by Zverev.
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