Noise suppressor for pulsed signal receivers

• US 4,891,649 A
• Filed: 09/02/1988
• Issued: 01/02/1990
• Est. Priority Date: 09/02/1988
• Status: Expired due to Fees

• Current Assignee: TRW Incorporated One Space Park Redondo Beach CA
• Original Assignee: TRW Inc. (Northrop Grumman Corporation)
• Inventors: Lance, deceased, Algie L., Labaar, Frederik
• Primary Examiner(s): Tarcza, Thomas H.
• Assistant Examiner(s): Hellner, Mark
• Application Number:
  US07/239,918
• Time in Patent Office: 487 Days
• Field of Search: 342/175, 342/202, 342/203, 342/204
• US Class Current: 342/203
• CPC Class Codes: G01S 7/2921 based on data belonging to one radar period

Abstract

A pulsed radar system has improved signal to noise characteristics obtained by suppressing phase noise. Means are provided to store a copy of the transmitted RF signal pulse, which copy contains any phase noise of the RF carrier that is transmitted. The signal copy is retained and following a delay interval the signal is mixed in a high frequency electronic RF mixer with the received RF pulse signal, the latter of which represents a reduced level signal as reflected by an object back to the system receiver'"'"'s antenna following the lapse of an interval of time. The RF mixer produces an intermediate frequency signal output in which the phase noise is suppressed thereby enhancing the receiver'"'"'s signal to noise ratio. A novel pulsed signal delay circuit is employed in the foregoing which retains the copy by continuously recirculating a fraction of that RF input pulse over time until it is needed; as when the pulse is to be applied to the RF mixer. The delay circuit includes a fiber optic cable that functions as a delay line. In a related invention the pulsed signal delay circuit is used as an element of a RF receiver to stretch the width of incoming pulses in the time domain to permit a more narrowband signal filter thereby increasing the signal to noise ratio.

Claims

1. An improved pulse type radar system, comprising:

RF transmitter means for generating RF energy pulses, said pulses being repeated at regular intervals defining a pulse repetition time;

RF receiver means for receiving and processing RF energy pulses;

antenna means for radiating RF energy pulses supplied by said transmitter means into space and for receiving RF energy pulses propagated from space and coupling said received RF energy pulses to said receiver means;

said transmitter means including;

RF generator means for supplying an RF carrier of principal frequency, F_o, and including phase noise, φ

(t), represented by slight random phase changes with time of said RF carrier from said principal frequency;

pulse generating means for enabling said RF generator means for a predetermined pulse width interval and at a predetermined pulse repetition rate, whereby RF pulses of said carrier frequency spaced apart in time by said pulse repetition time are consecutively generated by said transmitter means and are propagated into space by said antenna means;

said receiver means for receiving such portion of said transmitted RF energy pulses as may be reflected from an RF energy reflecting object located within the space upon which object said transmitted pulses are incident, whereby said reflected RF pulse is received following an interval of time, T, said time being the period taken for a transmitted RF pulse to travel from said transmitter to said object and return to said receiver means;

said receiver means, including;

front end means for detecting RF energy at said antenna means and supplying said detected RF energy at an output;

RF mixer means, having at least two inputs and an output, for combining RF energy applied at said inputs to an intermediate frequency (IF) RF, F_if, at an output;

said intermediate frequency energy having a frequency which is the difference of the frequencies of the two RF signals applied to said respective inputs;

said receiver front end means output being coupled to an input of said mixer means for supplying said receive detected RF pulses to a first input of said mixer means;

local oscillator means for supplying an RF signal to one input of said mixer of a frequency equal to the sum of said carrier and IF comprising;

frequency upconverter means for upconverting each transmitted RF energy pulse coupled from the transmitter means to produce a copy of the transmitted RF pulse, including the latter'"'"'s phase noise;

duplicate pulse generating means coupled to the output of said upconverter for providing a contiguous train of RF pulses that are copies of the RF pulse inputted by said upconverter means, said train extending over an interval of time, T₂, said interval T₂ being at least as great as said time T, said pulse width interval, but less than said transmitter means pulse repetition time;

and coupling means for coupling the output of said local oscillator means to said local oscillator input of said RF mixer means;

whereby said phase noise contained in said duplicate RF pulse essentially cancels the corresponding phase noise contained in the received reflected RF pulse to provide an intermediate frequency RF output from said mixer of reduced phase noise content;

amplifier and display means for processing and displaying pulses appearing in the IF frequency output from said RF mixer means, representing received RF pulses;

range gating means for enabling said processing said display means at selected time intervals, T_a, T_b, T_c . . . T_n, which intervals covers partially or wholly said pulse repetition interval, following the transmission of each RF pulse by said transmitter means, whereby pulse information displayed may be limited to those received within a period commencing with or in an interval of time in which a reflected RF pulse is received; and

control means for controlling said selected time intervals of said range gating means and said time period T₂ of said duplicate RF pulse generating means.

2. The invention as defined in claim 1 wherein said duplicate RF pulse generating means comprises:

a delay system for maintaining presence at an output for a predetermined interval of pulsed RF signals representative of a pulsed RF signal of predetermined duration received at an input, including;

linear amplifier means having an input and an output for amplifying a pulsed RF signal applied at an input;

limiting amplifier means for providing a constant level RF output responsive to multilevel RF signals applied at an input, said limiting amplifier means having its input coupled to said output of said linear amplifier means, whereby a constant peak level RF output is obtained irrespective of amplitude level variations of the RF signal applied at said input;

feedback circuit means coupled between the output and input of said linear amplifier means;

said feedback circuit means having an attenuative characteristic for reducing the level of the feedback signal to a level less than the level of the input signal to said linear amplifier means and for delaying passage of RF pulses introduced into said feedback circuit by said predetermined interval of said input RF pulse, said interval being at least equal to said RF pulse width.

3. The invention as defined in claim 2 wherein said duplicate RF pulse generating means comprises:

circuit means for producing a contiguous train of RF pulses responsive to a single RF pulse signal of predetermined width received at an input to effectively store the received RF pulse, including;

first and second pulse storing circuit means for storing RF pulses, each of said RF pulse storing circuit means comprising;

linear amplifier means for amplifying RF signals applied at an input and supplying an amplified RF signal at an output;

feedback circuit means, said feedback circuit means being connected between the output and input of said linear amplifier means;

said feedback circuit means having a attenuative characteristic for reducing the level of the feedback signals to a level less than the level of the input signal to said associated linear amplifier means and for delaying RF pulses introduced into said circuit for said predetermined duration of said input RF pulse;

first coupling circuit means responsive to a control signal for coupling a portion of the output of said first RF pulse storing circuit means to the input of said second delay circuit means for a predetermined interval to hand over an RF pulse to the second RF pulse storing circuit means;

second coupling circuit means responsive to a control signal for coupling the output of said second RF pulse storing circuit means to the input of said first RF pulse storing circuit means for a predetermined interval to hand over an RF pulse to the first RF pulse storing circuit means;

first control means for interrupting said feedback circuit means of said first RF pulse storing circuit means for interrupting said feedback loop following the lapse of a predetermined interval to prevent further storing of RF pulses until an RF pulse is thereafter again applied to said associated first linear amplifier means input;

second control means for enabling said first coupling circuit means to pass the final RF pulse stored in said first RF pulse storing circuit means over to said second pulse storing circuit means;

third control means for interrupting said feedback circuit means of said second RF pulse storing circuit means for interrupting said associated feedback loop following the lapse of a predetermined interval to prevent further storage of RF pulses by said second pulse storing means until an RF pulse is thereafter again passed on by said associated second control means; and

fourth control means for enabling said second coupling circuit means to pass the final RF pulse stored in said second RF pulse storing circuit means to the input of said first RF pulse storing circuit means.

4. A delay system for maintaining presence at an output for a predetermined interval of an RF signal representative of a pulsed RF signal of predetermined duration received at an input, comprising:

linear amplifier means having an input and an output for amplifying a pulsed RF signal applied at an input;

limiting amplifier means for providing a constant peak level RF output responsive to multilevel RF signals applied at an input, said limiting amplifier means having its input coupled to said output of said linear amplifier means, whereby a constant peak level RF output is obtained irrespective of amplitude level variations of the RF signal applied at said input;

feedback circuit means coupled between the output and input of said linear amplifier means;

said feedback circuit means having an attenuative characteristic for reducing the level of input signals to a level at the output less than the level of said input signal to said linear amplifier means and for delaying passage of pulses introduced into said feedback circuit by said predetermined interval of said input RF pulse, said interval being at least equal to said pulse width.

5. The invention as defined in claim 4 wherein said feedback circuit means includes:

first coupler means;

second coupler means;

delay line means for providing an extended signal propagation path between an input and an output, the length of said delay line means being of a length sufficient to produce a delay essentially equal to the duration of said RF input pulse;

switch means, said switch means being normally closed to complete a circuit between an input and output and for opening said feedback circuit means responsive to energization;

attenuator means;

said aforementioned means first and second coupler means, delay line, switch means and attenuator means being connected in series between said output and input of said linear amplifier means to permit feedback of an RF pulse of essentially equal duration to said received RF pulse when said switch means is in the closed condition;

whereby said linear amplifier means continues to amplify RF signals to provide a train of RF output pulses of decreasing amplitude and whereby a constant peak level RF output is provided at the output of said limiting amplifier means; and

further comprising;

control means for energizing said switch means to interrupt said feedback circuit means upon the expiration of a predetermined interval, said control means including means for energizing said switch means.

6. The invention as defined in claim 5 wherein said delay line means comprises:

RF modulated laser means for generating an amplitude modulated light output;

light waveguide means for receiving the output of said laser means and propagating said light over a predetermined length to an output, said light waveguide means being in the form of a fiber optic cable of extended length;

photodetector means coupled to the output of said light waveguide means for converting light coupled from said light waveguide into electrical signals; and

amplifier means for amplifying the output from said photodetector means.

7. A circuit for producing a contiguous train of RF pulses responsive to a single RF pulse signal of predetermined width received at an input to effectively store the received RF pulse, comprising:

first and second pulse storing circuit means for storing RF pulses, each of said pulse storing circuit means comprising;

linear amplifier means for amplifying RF signals applied at an input and supplying an amplified RF signal at an output;

feedback circuit means, said feedback circuit means being connected between the output and input of said linear amplifier means;

said feedback circuit means having a attenuative characteristic for reducing the level of input signals to a level at the output less than the level of said input signal to sad associated linear amplifier means and for delaying RF pulses introduced into said circuit for said predetermined duration of said RF input pulse;

first coupling circuit means, responsive to a control signal for coupling a portion of the output of said first RF pulse storing circuit means to the input of said second delay circuit means for a predetermined interval to hand over an RF pulse to the second RF pulse storing circuit means;

second coupling circuit means responsive to a control signal for coupling the output of said second RF pulse storing circuit means to the input of said first RF pulse storing circuit means for a predetermined interval to hand over an RF pulse to the first RF pulse storing circuit means;

first control means for interrupting said feedback circuit means of said first RF pulse storing circuit means for interrupting said feedback loop following the lapse of a predetermined interval to prevent further storage of RF pulses until a RF pulse is thereafter again applied to said associated first linear amplifier means input;

second control means for enabling said first coupling circuit means to pass the final RF pulse stored in said first RF pulse storing circuit means over to said second RF pulse storing circuit means;

third control means for interrupting said feedback circuit means of said second RF pulse storing circuit means for interrupting said feedback loop following the lapse of a predetermined interval to prevent further storage of RF pulses by said second pulse storing means until an RF pulse is passed again to said associated first linear amplifier means input of said second pulse storing means; and

fourth control means for enabling said second coupling circuit means to pass the final RF pulse stored in said second RF pulse storing circuit means to the input of said first RF pulse storing circuit means.

8. A circuit for producing a contiguous train of RF pulses responsive to a single RF pulse signal of predetermined width received at an input to effectively store the received RF pulse, comprising:

first and second RF pulse storing circuit means for storing RF pulses, each of said RF pulse storing circuit means comprising;

linear amplifier means for amplifying RF signals applied at an input and supplying an amplified RF signal at an output;

feedback circuit means, said feedback circuit means being connected between the output and input of said linear amplifier means;

said feedback circuit means having a attenuative characteristic for reducing the level of input signals to a level at the output less than the level of said input signal to said associated linear amplifier means and for delaying pulses introduced into said circuit for said predetermined duration of said input pulse;

said feedback circuit means further including;

first coupler means;

second coupler means;

delay line means for providing a signal propagation path of extended length between an input and an output to produce a propagation delay, the length of said propagation delay being essentially equal to the duration of said input RF pulse;

switch means, said switch means being normally closed to complete a circuit between an input and output and for opening said circuit responsive to energization;

attenuator means;

said aforementioned coupler, switch and attenuator means being connected in series between said output and input of said linear amplifier means for feeding back a RF pulse of essentially equal duration to said received RF pulse and of lesser amplitude level, whereby said amplifier means may continue to amplify signals to provide a serial train of output RF pulses of decreasing amplitude;

first coupling circuit means responsive to a control signal for coupling a portion of the output of said first RF pulse storing circuit means to the input of said second delay circuit means for a predetermined interval to hand over an RF pulse to the second RF pulse storing circuit means;

said first coupling circuit means comprising;

splitter means coupled to the output of said first RF pulse delay circuit;

normally open switch means coupled between the output of said splitter means and said input of said second RF pulse storage circuit for completing an RF path therebetween responsive to energization to permit input of a RF pulse to said second RF pulse delay circuit;

second coupling circuit means responsive to a control signal for coupling the output of said second RF pulse storing circuit means to the input of said first RF pulse storing circuit means for a predetermined interval to hand over an RF pulse to the first RF pulse storing circuit means, said second coupling circuit means comprising;

second splitter means coupled between the output of said second RF delay circuit means;

switch means coupled between the output of said second splitter means and said input of said first RF pulse storing circuit for completing an RF path therebetween responsive to energization to permit a RF pulse to the input of said first RF pulse storing circuit means;

final limiting amplifier means for providing an RF signal output of relatively constant peak level responsive to RF signals applied to an input;

combiner means coupled to a second output of each of said splitter means of said first and second RF pulse storing circuit means and having an output coupled to the input of said limiting amplifier means for supplying RF signals outputted from each of said RF pulse storing circuit means to said final limiting amplifier input, whereby a RF pulse train of constant peak amplitude is provided;

first control means for energizing said switch means in said feedback circuit means of said first RF pulse storing circuit means for interrupting said feedback loop following the lapse of a predetermined interval to prevent further storage of RF pulses until an RF pulse is thereafter again applied to said associated first linear amplifier means input;

second control means for temporarily energizing said switch means in said first coupling circuit means to pass the final RF pulse in said first pulse storing circuit means over to said second RF pulse storing circuit means;

third control means for energizing said switch means in said feedback circuit means of said second RF pulse storing circuit means for interrupting said associated feedback loop following the lapse of a predetermined interval to prevent further storage of RF pulses by said second RF pulse storing means until an RF pulse is thereafter again applied to said associated first linear amplifier means input of said second RF pulse regenerating means;

fourth control means for temporarily energizing said switch means in said second coupling circuit means to pass the final RF pulse stored in said second RF pulse storage circuit means to the input of said first RF pulse storing circuit means.

9. The invention as defined in claim 8 further comprising:

a third delay line;

said third delay line being coupled in series circuit in said first coupling circuit for delaying passage of pulsed RF signals through said circuit;

a fourth delay line;

said fourth delay line being coupled in series circuit in said second coupling circuit for delaying passage of pulsed RF signals through said circuit, whereby the transition time of said switch means may be reduced.

10. The invention as defined in claim 8 wherein said first delay circuit includes:

limiting amplifier means coupled to the input of said power combiner for providing a constant peak level RF output responsive to RF supplied at its input, said limiting amplifier means having its input coupled to the output of said linear amplifier means for amplifying RF output signals from said linear amplifier means.

11. The invention as defined in claim 8 wherein each said coupling circuit includes delay line means for increasing the propagation time for passage of pulsed RF signals through said coupling circuit, whereby transition time of said switch means may be increased.

12. A delay circuit for use with RF pulses having a duration of T₁, for producing a contiguous train of RF pulses over an interval greater than T₁ comprising:

linear amplifier means for producing at an output an amplified RF pulse responsive to an RF pulse applied at an input;

at least one delay feedback loop coupled between the output and input of said amplifier means for producing a RF pulse at said amplifier input a predetermined duration of time following the appearance of said amplified initial RF pulse at said amplifier means output, said duration being essentially equal to the width of the input pulse T₁ and the level of said pulse being less than the level of said preceding pulse;

said loop having a loop gain of less than unity;

switch means in said feedback loop responsive to energization for interrupting said feedback loop for preventing operation of said feedback loop during said interruption, whereby said linear amplifier means ceases to produce pulse outputs and resumes a quiescent condition awaiting another RF pulse at the input thereof;

at least one limiting amplifier for producing an output of a constant RF level, irrespective of variations of the input RF signal;

said limiting amplifier having an input coupled to the output of said linear amplifier means for amplifying RF pulses provided by said amplifier and providing said amplified signal at an output.

13. The invention as defined in claim 12 further including:

control means for energizing said switch means upon the lapse of an interval of time T₂, where T₂ is equal or greater than 3T₁ and equal or less than 10T₁.

14. The invention as defined in claim 12 wherein said delay feedback loop includes:

a fiber optic cable.

15. The method of suppressing electronic phase noise contained within RF pulse signals receives at an RF receiver of a transmitter and receiver system with said receiver containing IF amplifiers for amplifying signals of an IF frequency, said RF pulse signals originating in and being propagated from said transmitter, comprising the steps of:

storing a copy of the pulsed RF signal transmitted by the system transmitter in the form of an RF signal that differs from the transmitted RF signal by an amount equal to said IF frequency in an analog storing device, said transmitted pulse RF signal containing any phase noise introduced therein at the moment of transmission by the transmitter;

receiving the reflected pulsed RF signal at the receiver following a delay interval, T, subsequent to the transmission of said RF signal; and

combining said received pulsed RF signal and said stored copy in an RF mixer to provide a phase noise free intermediate frequency pulse signal of the duration of said transmitted RF pulse.

16. The method of suppressing electronic phase noise contained within RF pulse signals received at an RF receiver of a transmitter and receiver system, said RF pulse signals originating in and being propagated from said transmitter, comprising the steps of:

storing a copy of the pulsed RF signal transmitted by the system transmitter in an analog storing device, said transmitted pulse RF signal containing any phase noise introduced therein at the moment of transmission by the transmitter;

receiving the reflected pulsed RF signal at the receiver following a delay interval, T, subsequent to the transmission of said RF signal; and

combining said received pulsed RF signal and said stored copy in an RF mixer to provide a phase noise free intermediate frequency pulse signal of the duration of said transmitted RF pulse and wherein the step of storing a copy of said pulsed RF signal includes the steps of;

applying a portion of said RF signal to a frequency up-converter and combining said signal with an IF frequency signal to provide a combined signal at an output of a frequency equal to the sum of the two frequencies at the output of said up-converter;

applying said combined signal output of said frequency up-converter to the input of a pulse delay system, said combined signal forming a copy of said pulsed RF signal translated in frequency, to maintain said combined signal available for an interval of time;

and wherein the step of combining said pulsed RF signal and said stored copy includes the steps of;

applying said received signal and the signal output of said analogue storing device to the inputs of an RF mixer means to provide an output pulse at a frequency equal to the difference in frequency between said two signals.

17. In an RF receiver of the kind adapted to receive and process RF pulses and which contains means for receiving an RF pulse having a predetermined width and predetermined frequency in the RF region and means for converting the RF pulse from the high frequency region to an RF pulse in an intermediate frequency (IF) region and processing circuits for demodulating and displaying information represented in or by said RF pulse, the improvement comprising:

pulse stretching means for storing copies of a received RF pulse to form a train of N contiguous RF pulses, said N being an integer number, providing a pulse that is of greater width or stretched in the time domain corresponding to said received pulse, said stretched pulse being greater in width than said predetermined width of said RF pulse;

means for applying said input RF pulse to said pulse stretching means;

filter means coupled to the output of said RF pulse stretching means for passing RF in a predetermined frequency bandwidth, said frequency bandwidth being the optimal frequency bandwidth for a filter for a RF pulse of said predetermined width and shape, in terms of optimal signal to noise ratio; and

means coupling said filter means output to said signal processing circuits of said receiver means;

whereby the signal to noise ratio of said receiver is enhanced by reduction of electronic noise due to the difference between the optimum frequency bandwidth for said received RF pulse and the optimum frequency bandwidth of said stretched RF pulse, the optima in terms of signal to noise ratio.

18. The invention as defined in claim 17, wherein said pulse stretching means comprises:

linear amplifier means for producing at an output an amplified RF pulse responsive to an RF pulse applied at an input;

at least one delay line feedback loop coupled between the output and input of said amplifier means for producing contiguous pulses at said amplifier output for a predetermined duration of time following the appearance of said initial pulse at said amplifier means input, and the level of each said pulses being less than the level of its preceding pulse;

said loop having a loop gain of less than unity;

switch means in said feedback loop responsive to energization for interrupting said feedback loop for preventing operation of said feedback loop during said interruption, whereby said linear amplifier means ceases to produce pulse outputs and resumes a quiescent condition awaiting another RF pulse at the input thereof;

whereby responsive to input of an RF pulse having a duration of T₁, a contiguous train of RF pulses is produced extending over an interval NT₁, N being an integer and T₁ being the RF pulse width.

19. The invention as defined in claim 17, wherein said pulse stretching circuit comprises:

RF pulse delay system means for maintaining presence at an output for a predetermined interval of an RF signal representative of a pulsed RF signal of predetermined duration received at an input.

20. The invention as defined in claim 19 wherein said pulse delay system means comprises:

linear amplifier means having an input and an output for amplifying a pulsed RF signal applied at an input;

feedback circuit means coupled between the output and input of said linear amplifier mean;

said feedback circuit means having an attenuative characteristic for reducing the level of input signals to a level at the output less than the level of said input signal to said linear amplifier means and for delaying passage of pulses introduced into said feedback circuit by a predetermined interval, said interval being at least equal to said pulse width.

21. The invention as defined in claim 20 wherein said RF pulse delay circuit means further includes:

limiting amplifier means for providing a constant peak level RF output responsive to multilevel RF signals applied at an input, said limiting amplifier means having its input coupled to said output of said linear amplifier means, whereby a constant peak level RF output is obtained irrespective of amplitude level variations of the RF signal applied at said input.

22. The invention as defined in claim 20 wherein said feedback circuit means includes:

first coupler means;

second coupler means;

delay line means for providing an extended signal propagation path between an input and an output, the length of said delay line means being of a length sufficient to produce a delay essentially equal to the duration of said input RF pulse;

switch means, said switch means being normally closed to complete a circuit between an input and output and for opening said feedback circuit means responsive to energization;

attenuator means;

said aforementioned means first and second coupler means, delay line, switch means and attenuator means being connected in series between said output and input of said linear amplifier means to permit feedback of an RF pulse of essentially equal duration to said received RF pulse when said switch means is in the closed condition;

whereby said linear amplifier means continues to amplify RF signals to provide a serial train of RF output pulses of decreasing amplitude and whereby a constant peak level RF output is provided at the output of said limiting amplifier means; and

further comprising;

control means for energizing said switch means to interrupt said feedback circuit means upon the expiration of a predetermined interval, said control means including means for energizing said switch means.