US 5,691,924 AGranted

Narrow band spectrum analysis method and apparatus

AssigneeComputational Systems, Inc.

InventorsZbigniew Czyzewski, Ronald G. Canada

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Abstract

A zoom processor for increasing the resolution of a band of interest within an acquired analog time domain frequency signal. The signal is converted from an analog time domain signal to a digital time domain signal. Next, the signal is multiplied by a value which will lower the center frequency of the band of interest to zero. The value by which the signal is multiplied is based on both the sine and cosine of a function of the center frequency and the sampling frequency used to digitize the signal. The sine function is solved with the aid of a numerical substitution which simplifies finding the value of the function in a sine table. After the signal is shifted, it is low-pass filtered, which removes all of the signal except the band of interest. During filtering, the band is also decimated by a predetermined factor, which compresses the time domain signal, and proportionally expands the frequency domain signal which is produced at a later step of the method. A transform, such as a Fast Fourier Transform, is performed on the band, to convert the signal from time domain to frequency domain. The signal is analyzed according to the ultimate function of the instrument in which the zoom processor is incorporated.

Claims8

Dependent claims

A method of zoom processing a time domain frequency signal digitized at a sampling frequency comprising:

providing a zoom processor having an input device, a memory, a processor, and an output device,
acquiring the digitized time domain frequency signal with the input device of the zoom processor, to produce an acquired signal,
lowering the frequency of the acquired signal with the processor of the zoom processor, by:
inputting a sine table into the zoom processor using the input device of the zoom processor, where the value of each entry in the sine table is based on one full period being represented by a predetermined number of entries, the entries being twenty-four bit numbers,
storing the sine table in the memory of the zoom processor,
solving the equation .function..sub.c /.function..sub.s =(X+.DELTA.)/.PSI. for X and .DELTA. using the processor of the zoom processor, where X is a positive integer less than or equal to 800, .PSI. is the predetermined number, and .DELTA. is a non-negative number less than one,
selecting the .eta.X sine table entry using the processor of the zoom processor when .DELTA. equals zero, the value of which entry equals sin(2.pi.).eta.(.function..sub.c /.function..sub.s)!,
interpolating by .DELTA. between the .eta.X sine table entry and .eta.(X+1) sine table entry using the processor of the zoom processor, to determine an interpolated value when .DELTA. does not equal zero, which interpolated value equals sin(2.pi.).eta.(.function..sub.c /.function..sub.s)!, and
calculating cos(2.pi.).eta.(.function..sub.c /.function..sub.s)! using the processor of the zoom processor according to the equation cos(.theta.)=sin(.theta.+.pi./2), and
multiplying the acquired signal by {cos(2.pi.).eta.(.function..sub.c /.function..sub.s)!+j*sin(2.pi.).eta.(.function..sub.c /.function..sub.s)!}, where .eta. is a consecutive data point number, j is an imaginary number having a value of (-1).sup.1/2, .function..sub.c is a frequency of the acquired signal, and .function..sub.s is the sampling frequency used to produce the digitized time domain frequency signal, to produce a shifted signal,
low-pass filtering the shifted signal, to produce a low-pass filtered signal, and
outputting the low-pass filtered signal with the output device of the zoom processor, to produce a zoom processed signal.

9 more elements

A method of shifting a digital frequency signal, that was digitized using a predetermined sampling frequency, comprising:

providing a signal shifter having an input device, a memory, a processor, and an output device,
acquiring the digital frequency signal into the signal shifter using the input device of the signal shifter, to produce an acquired signal,
selecting a frequency .function..sub.c of the acquired signal using the input device of the signal shifter,
selecting the predetermined sampling frequency using the signal shifter,
storing the predetermined sampling frequency in the memory of the signal shifter,
dividing the frequency .function..sub.c of the acquired signal by the predetermined sampling frequency .function..sub.s, using the processor of the signal shifter, to produce a ratio .function..sub.c /.function..sub.s,
setting the ratio .function..sub.c /.function..sub.s equal to (X+.alpha.)/.PSI. where X is a positive integer less than or equal to 800, .DELTA. is a non-negative number less than one, and .PSI. is a predetermined number,
solving the equation .function..sub.c /.function..sub.s =(X+.DELTA.)/.PSI. for X and .DELTA. using the processor of the signal shifter,
storing the values of X and .DELTA. in the memory of the signal shifter,
inputting a sine table into the signal shifter using the input device of the signal shifter, the sine table based on .PSI. entries representing one full period,
storing the sine table in the memory of the signal shifter,
calculating the solution of sin(2.pi.).eta.(.function..sub.c /.function..sub.s)!, where .eta. is a consecutive data point number, with the processor of the signal shifter, by using the value of the .eta.X sine table entry as the solution if .DELTA. equals zero, and by using the processor of the signal shifter to interpolate by .DELTA. between the .eta.X and .eta.(X+1) entries if .DELTA. does not equal zero,
calculating the value of cos (2.pi.).eta.(.function..sub.c /.function..sub.s) !, with the processor of the signal shifter, by using the identity cos(.theta.)=sin(.theta.+.pi./2),
multiplying the signal by {cos(2.pi.).eta.(.function..sub.c /.function..sub.s)!+j*sin(2.pi.).eta.(.function..sub.c /.function..sub.s)!}, using the processor of the signal shifter, where j is an imaginary number having a value of (-1).sup.1/2, to shift the signal by .function..sub.c, such that .function..sub.c is shifted to a frequency of zero, to produce a shifted signal, and
outputting the shifted signal with the output device of the signal shifter.

12 more elements

A zoom processor for acting on a time domain frequency signal which has been digitized at a predetermined sampling frequency comprising:

an interface for;
inputting a value .function..sub.c into the zoom processor, representing a frequency within the time domain frequency signal,
inputting a sine table into the zoom processor, where the value of each entry in the sine table is based on one full period being represented by a predetermined number of entries,
inputting low-pass filter parameters into the zoom processor, and
inputting the predetermined sampling frequency .function..sub.s into the zoom processor,
a signal input device for receiving the time domain frequency signal into the zoom processor, and producing an acquired signal,
a memory for;
storing .function..sub.c,
storing .function..sub.s,
storing the sine table, and
storing the low-pass filter parameters,
a processor for;
solving the equation .function..sub.c /.function..sub.s =(X+.DELTA.)/.PSI. for X and .DELTA., where X is a positive integer less than or equal to 800, .PSI. is the predetermined number, and .DELTA. is a non-negative number less than one,
selecting the .eta.X sine table entry when .DELTA. equals zero, the value of which sine table entry equals sin(2.pi.).eta.(.function..sub.c /.function..sub.s)!, where .eta. is a consecutive data point number,
interpolating by .DELTA. between the .eta.X sine table entry and .eta.(X+1) sine table entry to determine an interpolated value when .DELTA. does not equal zero, which interpolated value equals sin (2.pi.).eta.(.function..sub.c /.function..sub.s)!,
calculating cos (2.pi.).eta.(.function..sub.c /.function..sub.s)! by using the equation cos (.theta.)=sin(.theta.+.pi./2),
multiplying the acquired signal by {cos(2.pi.).eta.(.function..sub.c /.function..sub.s)!+j*sin(2.pi.).eta.(.function..sub.c /.function..sub.s)!}, where j is an imaginary number having a value of (-1).sup.1/2, to shift the acquired signal by .function..sub.c, and producing a shifted signal, and
filtering the shifted signal using the low-pass filter parameters to pass a portion of the shifted signal extending from a low frequency of the negative of the value of the low-pass filter parameter, to a high frequency of the value of the low-pass filter parameter, and producing a low-pass filtered signal, centered at a frequency of zero,
a signal output device for outputting the low-pass filtered signal, and
a power supply for supplying power to the interface, input device, memory, processor, and output device.

17 more elements

Dependencies are inferred from the claim text.