Patent No. 3641445 Frequency analyzer

 

Patent No. 3641445  Frequency analyzer (Green, Feb 8, 1972)

Abstract

A frequency analyzer is provided wherein real-time, narrow band-pass filters having large dynamic range are synthesized for each frequency. Each filter consists of a local clock driven synchronous detector followed by a simple integrating circuit. Large dynamic range is achieved through the use of a two-state switching device as the synchronous detector. Since the output of the synchronous detector depends on the phase relationship between the input signal and the local clock, two phase-quadrature filters are provided for each frequency. The output from the filter pair is processed in a manner that yields a signal whose amplitude is independent of the input signal-local clock phase relationship.

Notes:

BACKGROUND OF THE INVENTION

This invention relates generally to a frequency discriminator and more particularly to a high resolution, real-time frequency analyzer having large dynamic range.

In order to perform significant frequency analysis of signals such as HF radar signals, it has often been necessary to increase the spectral dispersion of the real-time-signal so that adequate effective resolution could be achieved with analog frequency analyzers of the type now available having relatively limited intrinsic resolution. Such a situation will arise whenever it is necessary to separate closely spaced frequency components, for example, in separating signals involving slightly different doppler shifts.

The desired increase in dispersion can be accomplished by tape recording the real-time signal at one tape transport velocity and playing it back into a frequency analyzer at a greater velocity. While this practice enhances the effective resolution of the analog analyzer, it is somewhat cumbersome and real-time frequency analysis is necessarily precluded.

Alternatively, the spectral dispersion increase may be effected in approximately real-time through the use of a digital memory bank into which a digitalized version of the input signal is entered at the real-time rate followed by a readout operation that is performed more rapidly. Most implementations of this sort are restricted by the capacity of the memory bank that can be economically provided.

A further distinct disadvantage that can be cited regarding this alternate method lies in the complex circuitry that is required to interface the digital memory with an otherwise analog system.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a device capable of performing real-time frequency discrimination without recourse to increased spectral dispersion through the use of complicated time compression techniques.

It is a further object of the present invention to provide a high resolution real-time frequency analyzer consisting of adjacent filter channels.

According to a broad aspect of the invention there is provided a frequency discriminator comprising two phase-quadrature filters coupled to an input signal for converting said input signal into two phase-quadrature components, and means for summing said phase-quadrature components.

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While I have described above the principles of the invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of the invention as set forth in the objects and accompanying claims.