Wide-Band High-Resolution Determination of Absolute Frequency

In the determination of absolute frequency, conventional approaches have been either frequency or period counting, both of which are subject to resolution-bandwidth tradeoffs and frequency distortion. This paper presents a new technique that avoids such tradeoffs and separates the filtering and sampling functions, giving greater flexibility and enabling one to work down to the limits established by the noise included with the signal. The technique is the use of a commercial frequency synthesizer as the local oscillator in a phase-locked loop. The paper describes this approach, discusses certain modifications that must be made to the synthesizer if it is to work in this configuration, and discusses the design of a type 2, sixth-order feedback system that produces a better approximation to ideal filter response than is obtained by conventional second-order designs.