Pulse or digital communications – Spread spectrum – Frequency hopping
Utility Patent
1998-03-19
2001-01-02
Pham, Chi H. (Department: 2731)
Pulse or digital communications
Spread spectrum
Frequency hopping
C375S224000, C708S309000
Utility Patent
active
06169760
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention was made with Government support. The Government has certain rights in this invention.
This invention relates to the measurement of the frequency of a signal, and, more particularly, to a technique which functions very rapidly and with great accuracy.
It is often necessary to determine the frequency of an electrical signal. In the best-known approach to frequency measurement, the number of zero-crossings of a periodic signal occurring in a fixed sampling period of time is counted and converted to a frequency value. The longer the sampling period, the greater the accuracy of the frequency value. This technique works well if the available sampling period is longer than at least several cycles of the periodic signal. In other circumstances, however, the zero-crossing technique has shortcomings.
For example, frequency hopping signal synthesizers are used in a number of applications such as frequency-hopping spread spectrum communications. In this approach, the output frequency of the signal synthesizer whose signal carries information is varied rapidly between preselected or randomly selected frequencies. The signal synthesizer must accurately transmit at the desired frequency, and it is therefore important to be able to measure whether the output signal of the signal synthesizer is actually at the desired frequency or has drifted away.
In fast frequency hopping applications, the output signal does not dwell at any one frequency for a sufficiently long period of time to utilize the zero-crossing technique effectively to obtain real time, highly accurate measurements of the actual frequency output value. Some available test equipment can record the output signal for a period of time and download it for subsequent processing and analysis. This approach does not give real time information and, in addition, signal recording is suspended during the downloading period.
There is a need for an improved technique for accurately finding the frequency of a signal in real time and with a very short observation time. The present invention fulfills this need, and further provides related advantages.
SUMMARY OF THE INVENTION
The present invention provides a technique for finding the frequency of a signal in real time, with high accuracy. The required observation time to determine the frequency is quite short. Signal frequency determination is continuous, with no gaps in the data.
In accordance with the invention, a method for finding the frequency of a signal comprises providing a signal filter having, as a function of frequency, a signal pass band frequency range wherein a high fraction of an initial amplitude of an introduced signal passes through the signal filter, a signal stop band frequency range wherein a low fraction of an initial amplitude of an introduced signal passes through the signal filter, and a signal filter skirt band frequency range between the signal pass band and the signal stop band, wherein an intermediate fraction of an initial amplitude of an introduced signal passes through the signal filter. In the signal filter skirt band frequency range, the intermediate fraction varies with the frequency of the introduced signal.
A test applied signal is introduced into the signal filter. The test applied signal has a frequency which lies within the signal filter skirt band frequency range. If an initial test signal has a frequency which is not within the signal filter skirt band frequency range, it may be converted to that required frequency range by known up-converting or down-converting techniques. The transmitted amplitude of the test applied signal is measured after the test applied signal has passed through the signal filter, and is utilized as a measure of the frequency of the test applied signal. The transmitted amplitude may be used directly, or converted to a frequency value using a calibration of the filter that relates the transmitted amplitude of the signal applied to the signal filter to the frequency of the signal.
The present approach requires only a very short sampling or observation time of the signal to make accurate measurements of the frequency of the signal. Determinations of frequencies in the gigahertz range to within about ±50 Hertz, made in a measurement period of
50
microseconds, have been demonstrated. By comparison, available commercial test equipment achieves accuracies of about ±5000 Hertz in the same 50 microsecond sampling period. The present approach achieves two orders of magnitude improvement in accuracy of the measurement.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.
REFERENCES:
patent: 4302712 (1981-11-01), Pritchard
patent: 4694466 (1987-09-01), Kadin
patent: 4810960 (1989-03-01), Owen et al.
Corrielus Jean Beth
Gudmestad Terje
Hughes Electronics Corporation
Pham Chi H.
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