An Overview Of Frequency Synthesizers

Introduction

Frequency synthesizers have become indispensable in many of today's advanced measurement and production systems, as well as in stand-alone uses. Typical applications range from ATE and NMR medical imaging to satellite earth station oscillators, from magnetic storage media testing to crystal production, from mode locking of lasers to ECM. Precision timing, radar simulations, and Doppler systems all make use of synthesizers. This document gives a brief overview of frequency synthesizers, discusses the technology behind them, and outlines the synthesizers solutions offered by Programmed Test Sources (PTI).

Synthesizer overview

Frequency synthesizers are basically variable radio frequency generators that are very accurately and quickly settable and possess high stability. Within a specified frequency range, they can be programmed either manually or remotely to practically any output frequency. This output frequency is as accurate and as stable as a built-in frequency standard, usually a crystal oscillator, or as accurate and stable as an external precision standard, which may be connected to the synthesizer in lieu of its own standard. Where very high stabilities are desired, atomic or molecular standards are often used.

Most commercial frequency synthesizers use a decimal read-out or indicator system. The least significant step or digit determines resolution, how closely the synthesizer can be set to any arbitrary frequency. Resolution ranges from megahertz to microhertz, depending on use; some synthesizers offer a choice of resolution to match capability (and price) to users' need. (Although read-out or indication of setting is normally decimal, remote control frequency setting may use other coding.)

The ideal of a pure frequency, a single spectral line, is not attained in practical synthesizers. All produce unwanted frequencies, called spurious outputs, and they also have, like any oscillator, harmonics. While harmonics are at least one octave removed and thus not often troublesome, the suppression of other unwanted frequencies is a major challenge of synthesizer design; units differ widely in this respect, and this is of major impact regarding cost. The same is true of the very close-in noise around the carrier that constitutes unwanted phase modulation. These perturbations are variously called broadband phase noise, spectral density distribution of phase noise, residual FM, and short-term fractional frequency deviation.

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