6.1: Introduction

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This chapter considers frequency generation and frequency translation modules. In a receiver the basic frequency translation module is a mixer that is used in a receiver to take a signal at a high frequency to a low frequency, the baseband signal, where it can be more easily sampled by an analog-to-digital converter (ADC) and then digitally processed. In a transmitter the signal flow is reversed and a low frequency baseband signal is translated to a high-frequency signal where it can be more easily transmitted. A local oscillator (LO) is input to the mixer to provide the translation mechanism by varying the impedance of an element at the LO frequency. A voltage-controlled oscillator (VCO) allows for agility of the oscillator. A VCO with a phase detector embedded in a phase-locked loop (PLL) provides a well-defined variable frequency oscillation signal. Special RF signal sources are based either on special characteristics of certain semiconductor devices or on vacuum devices in which a beam of electrons interacts with an RF field in a controlled way. These sources produce very high RF powers up to several megawatts or substantial power at very high frequencies, up to several terahertz.