Mixers and Multipliers
This chapter begins with a basic theory describing the operational principles of frequency conversion in receivers and transmitters. The different types of mixers, from the simplest based on a single diode to a balanced and double-balanced type based on both diodes and transistors, are described and analyzed. The special case is a mixer based on a dual-gate transistor that provides better isolation between signal paths and simple implementation. The frequency multipliers that historically were a very important part of the vacuum-tube transmitters can extend the operating frequency range.
7.1 BASIC THEORY
In wireless transmitters, it is generally required to convert a low-frequency baseband information signal to high frequency or frequencies at which the resulting signal can be effectively transmitted via antenna by electromagnetic propagation to the desired destination. This can be done either by a direct modulation of the high-frequency signal source or using a frequency converter traditionally called the mixer. Generally, any device (diode, vacuum tube, or transistor) that exhibits amplitude-nonlinear behavior can serve as a mixer, as nonlinear distortion results in the production of frequencies not present in the input. However, mixer as itself is fundamentally a linear device, which is shifting a signal from one frequency to another, keeping the properties of the initial signal (amplitude and phase), thus generally providing a linear operation. At ...