**1. Introduction**

High-frequency circuits are necessary to process the microwave signal between the radiating element and the digital signal processing (DSP) unit. Although, analogue-to-digital converters (ADC) are constantly increasing their frequency and power handling capability, they are still away from being able to detect very low power RF signal or capable to generate high RF power. Microwave circuits, therefore, are inserted to perform high-frequency processing so the ADC, and consequently the DSP unit, can more easily handle the RF signal. Such high-frequency processing functions are: amplification (low-noise, gain or high-power), signal combining or splitting, signal routing, phase and amplitude modulation, signal measurement, finally frequency generation and conversion.

A possible way to cluster these functions is divide them in front-end or backend functionalities. The prior are typically connected to the radiating element, implementing low-noise or high-power amplification and some form of signal routing and phase and amplitude modulation. Instead, microwave back-ends are connected to the ADC and therefore provide all the functionalities so the RF signal can be profitably delivered to the digital section. Such functions are typically more complex functions such as extraction of signal characteristics, frequency generation and conversion.

The two sub-system and the relevant circuits will be discussed in the following.

In this chapter, we will not describe signal filtering, being this an extremely extensive topic, excellently covered by Matthaei et al. work [1] and other chapters in this book.
