SYNTHESIZER DESIGN Building a Microwave Frequency Synthesizer — Part 1 : Getting Started

T he frequency synthesizer is a key building block of virtually any microwave test and measurement, communication or monitoring system [1-5]. Synthesizers come in a variety of forms ranging from a tiny PLL chip to a bench-top signal generator. Traditionally we imagine them as a connectorized metal box or " brick " which can be put into a more complex instrument or subsystem. This series of articles presents an overview of microwave frequency synthesizer technologies and design techniques with a specific focus on the synthesizer " brick " modules. It starts with general requirements and specifications followed by a review of the main synthesizer architectures. Direct analog, direct digital, and indirect techniques are compared in terms of performance, circuit complexity, and cost impact. A simple, single-loop PLL example is used to demonstrate the most important aspects of the design process from a general block diagram and component selection to schematic, board layout, assembling, and testing. The design trade-offs are further analyzed and complemented with a detailed review of the most advanced synthesizer solutions including DDS-based and multiloop schemes. A frequency synthesizer can be treated as a black box that translates one (or more) input base (reference) frequency to a number of output frequencies as shown in Figure 1. The box contains individual components such as voltage controlled oscillators, frequency dividers, multipliers, phase detectors, mixers, amplifiers , etc., which being properly connected, perform this translation function. Although all synthesizers exhibit significant differences as a result of specific applications, they share basic characteristics and design objectives as listed below. The synthesizer's characteristics are divided into a few groups depicting its fre-trol range). In addition to these signal-related behaviors, a specification defines how it interfaces to the outside world (control interface, bias, power consumption, size, etc.). Frequency Coverage or Range and Frequency Resolution or Step Size are the fundamental synthesizer specifications set by a particular application. Some applications (e.g., test and measurement) require wide band-width and fine frequency resolution while others need a relatively narrowband (10-20%) coverage with a rough step or just a single fixed frequency. Frequency Stability is determined by the Synthesizer design methods have evolved over time, and this series of articles offers an update on well-established and recently-developed techniques Figure 1 · The fundamental concept of a frequency synthesizer.