# US 4,379,295 A · Low sidelobe pulse compressor

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- Number: US 4,379,295 A
- Document ID: US4379295A
- Status: Granted
- Kind: A
- Application number: 06/230,984
- Filed: 1981-02-03
- Granted: 1983-04-05
- Earliest claimed: 1981-02-03
- Assignees: The United States of America as represented by the Secretary of the Navy
- Inventors: Bernard L. Lewis; Frank F. Kretschmer
- Web page: https://patentgrep.com/patent/US4379295A

## CPC

- G01S 13/286: {frequency shift keyed}
- G01S: RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES

## Abstract

A pulse compression system based on the transmission of a step approximat to linear FM pulse, and the subsequent processing of echos therefrom by converting the echo pulses to I and Q baseband signals and then sampling these I and Q signals at the Nyquist sampling rate. It has been discovered that this processing yields a signal with a spectrum substantially identical to the spectrum of a properly sampled Frank coded pulse. These sampled I and Q signals are then compressed by a Fast Fourier Transform compressor to yield a pulse with low sidelobes.

## Claims

### Claim 1

A method of processing for use in a pulse-compression system which will yield low sidelobes comprising the steps of:
generating a step-approximation to linear FM signal with N frequency steps;
transmitting said signal;
receiving echos from said transmitted signal;
converting said received echos to baseband signals including the step of narrow band filtering said signal;
digitizing said baseband signals at the Nyquist sampling rate for the baseband;
Fast Fourier Transforming a set of N.sup.2 of said digitized samples and generating N outputs therefrom representing the passbands for the N transmitted frequency steps;
differentially delaying said N passband outputs such that the passband outputs occur simultaneously; and
adding said N differentially delayed outputs to form a single short pulse of a desired length, such that when an echo signal is properly indexed within said set of N.sup.2 digitized samples, then the resultant pulse has a peak amplitude.

### Claim 2 (depends on 1)

A method as defined in claim 1, wherein said converting step is further comprised of the step of converting each echo signal to I and Q baseband signals; and wherein said digitizing step comprises the step of individually digitizing both said I and said Q baseband signals; and wherein said Fast Fourier Transforming step comprises the step of Fast Fourier Transforming a set of N I and Q complex signal words and generating N passband outputs therefrom.

### Claim 3

A pulse compression system which will yield low sidelobes comprising:
means for generating a step-approximation to linear FM signal with N frequency steps;
means for transmitting said signal;
means for receiving echos from said transmitted signal;
means for converting said received echo signal to a baseband signal and including means for narrow band filtering said signal;
means for digitizing said baseband signal at the Nyquist sampling rate for the baseband;
Fast Fourier Transform means for weighting a discrete set of N.sup.2 digitized samples from said digitizing means and generating N outputs therefrom representing the passbands for the N transmitted frequency steps;
means for differentially delaying said N passband outputs such that the N passband outputs occur simultaneously; and
means for adding said differentially delayed outputs to form a single pulse, such that when an echo is properly indexed within a discrete set of N.sup.2 digitized samples, then a short pulse will be generated with a peak amplitude.

### Claim 4 (depends on 3)

A pulse compression system as defined in claim 3, wherein said converting means comprises means for converting said echo signal to I and Q baseband signals; and
wherein said digitizing means comprises means for individually digitizing said I and Q baseband signals; and
wherein said Fast Fourier Transform means comprises means for forming N individual sets of N I and Q complex signal words and generating N passband outputs therefrom.

### Claim 5 (depends on 4)

A pulse compression system as defined in claim 4, wherein said differential delay means comprises a set of N differential delay elements, with a different delay element connected to each of the N passband outputs from said Fast Fourier Transform means, with the delay between adjacent delay elements being equal to the time interval in which a given frequency in said stepped signal is transmitted.

### Claim 6 (depends on 5)

A pulse compression system as defined in claim 5, wherein
said generating means includes means for generating an I.F. pulse which varies in N steps linearly from a frequency f.sub.1 to a frequency f.sub.N with each step lasting for a time .tau., for applying the frequency f.sub.1 and the frequency f.sub.1 shifted in phase by 90.degree. to said converting means, and for generating and applying a timing signal fc=N/.tau. to control the operation of said digitizing means and said Fast Fourier Transform means in accordance therewith; and
wherein said means for converting said echo to I and Q baseband signals comprises means for converting the received echo from the transmitted signal to a stepped I.F. pulse, narrow band amplifying means for amplifying and narrow-band filtering said stepped I.F. pulse, first multiplier and low pass filter means for multiplying said stepped I.F. pulse with the frequency f.sub.1 from said generating means and low pass filtering the product to obtain said I baseband signal, and second multiplier and low pass filter means for multiplying said stepped I.F. pulse with the frequency f.sub.1 from said generating means shifted in phase by 90.degree. and low pass filtering the product to obtain said Q baseband signal.