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Modern communication systems need new filters, diplexers and multiplexers to meet electrical performance, size and manufacturing cost requirements, which are difficult to achieve using classical topologies. Diplexers are widely used in communication systems for reducing mass and volume of the required hardware. They separate different bands of a signal into different ports and enable the use of the same antenna for different frequency bands, resulting in more compact systems. Alternatively, a diplexer combines two different signals with different spectral components into one common port. A typical architecture of a transceiver RF front-end is shown in Figure 1.

The diplexer is one of the key components in the overall system, as shown in the figure within the dotted line. The transmitter and receiver operate in different frequency bands and are duplexed to the antenna by the transmit-receive diplexer. The diplexer consists of a power divider and two channel filters that can have very stringent specifications. The transmit filter must reject out-of-band noise generated in the power amplifier, so it must have a high level of stop band attenuation, particularly in the receive band. It should have a low pass band insertion loss in order to maximize the DC to RF efficiency of the power amplifier. On the other hand, the receive filter should have high attenuation in the transmit band in order to protect the front-end low noise amplifier (LNA) from the large transmit signals, which would otherwise saturate the receiver or produce high levels of distortion. The most important parameter in the diplexer design is the isolation between the receive and transmit channels.