Introduction to Communications CircuitsIn summary, as stated earlier, RF design is a combination of techniques used in low-frequency analog design, and techniques used in traditional microwave design. On chip if distances between components are small, some analog design concepts are used. That is, transmission lines are generally not used and convenient impedance levels are chosen. For example, impedances higher than 50W may be used to save power, or low impedances may be chosen to achieve higher bandwidth in the pres-ence of capacitance. It should be noted, however, that the low frequency technique of aiming for infinite input impedance and zero output impedance is not possible at radio frequencies. Design is often done using small signal models, and internal signals are often described using voltage or current as is common for low frequency analog design. Conversely, at chip boundaries and for components separated by long distances, transmission lines are used and impedances are chosen to optimize power transfer or minimize noise according to standard microwave techniques.1.3 Radio-Frequency Integrated Circuits Used in a Communications TransceiverA typical block diagram of most of the major circuit blocks that make up a typical superheterodyne communications transceiver is shown in Figure 1.1. Many aspects of this transceiver are common to all transceivers.This transceiver has a transmit side (Tx) and a receive side (Rx), which are connected to the antenna through a duplexer that can be realized as a switch or a filter, depending on the communications standard being followed. The input pre-selection filter takes the broad spectrum of signals coming from the antenna and removes the signals that are not in the band of interest. This may be required to prevent overloading of the low-noise amplifier (LNA) by out-of-band signals. The Figure 1.1 Typical transceiver block diagram.