Hybrid analog and digital comb filter for clutter cancellation
First Claim
1. A system for cancelling clutter from an analog input signal and producing a digital output signal, comprising:
- a first summer receiving the analog input signal and producing an output, an analog to digital converter receiving the output of said first summer and producing a corresponding digital output comprising the digital output signal of the system, a filter receiving the digital output of said analog to digital converter and producing as an output a filtered signal having a predetermined mathematical relationship to the output signal of the analog to digital converter, a digital to analog converter receiving the output of said filter and producing as an output an equivalent analog signal wherein, said first summer receives and sums said filtered signal in subtractive relationship with the analog input signal.
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Abstract
A comb filter operative as a clutter canceller in a radar system is implemented as a hybrid analog and digital system, and provides a digital output signal from an analog input, radar beam return signal. The analog input signal is applied to a summer, the latter providing its output to an analog to digital (A/D) converter from which the canceller output is obtained. The A/D converter output also is applied to a digitally implemented filter, which produces a digital, time delayed replica of the input signal. A digital to analog (D/A) converter receives the digital replica signal and produces an equivalent analog signal for supply to the input summer for subtraction from the input analog signal. The summer output provides cancellation of clutter from the input signal, thereby reducing the requisite dynamic range of the A/D converter. The hybrid design of the invention thus optimally combines the advantages of analog and digital techniques while avoiding defects thereof, in affording an encoding interface between analog input and digital output signals.
49 Citations
10 Claims
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1. A system for cancelling clutter from an analog input signal and producing a digital output signal, comprising:
- a first summer receiving the analog input signal and producing an output, an analog to digital converter receiving the output of said first summer and producing a corresponding digital output comprising the digital output signal of the system, a filter receiving the digital output of said analog to digital converter and producing as an output a filtered signal having a predetermined mathematical relationship to the output signal of the analog to digital converter, a digital to analog converter receiving the output of said filter and producing as an output an equivalent analog signal wherein, said first summer receives and sums said filtered signal in subtractive relationship with the analog input signal.
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2. A system as recited in claim 1, wherein said first summer comprises an analog summer, and said filter comprises a digital filter.
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3. A system as recited in claim 1, wherein said analog to digital converter includes means for sampling the analog input signal at a predetermined sampling rate, and wherein said filter produces a time delay corresponding to the period of the sampling rate.
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4. A system as recited in claim 1, wherein said digital to analog converter includes adjustable gain control means for adjusting the response of the system.
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5. A system as recited in claim 1, wherein the filter characteristics h(nT) are defined in terms of the system transfer function w(nT) in accordance with:
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6. A system as recited in claim 1, wherein said filter comprises a further summer receiving the digital output of said analog to digital converter and producing an output, a delay line receiving as an input the output of said further summer and producing as an output a time delayed replica of the input thereto, said further summer receiving and summing the delay line output in additive relationship with the digital output of said analog to digital converter in producing the output to said delay line.
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7. A system as recited in claim 6, wherein said further summer and said delay line respectively comprise a digital summer and a digital delay line.
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8. A clutter canceller comprising:
- a summer receiving an input signal x(nT) and producing an output signal y(nT), a feedback loop responsive to the output y(nT) of said summer and producing the function u(nT-T) x(nT-T) supplied as a feedback signal to said summer, said summer summing the feedback signal u(nT-T) in subtractive relationship with the input signal x(nT) to produce as the output y(nT) x(nT) - x(nT-T) and wherein said feedback loop includes an analog to digital converter receiving the analog output of said summer and producing an equivalent digital output y(nT);
a digital filter responsive to the digital output y(nT) of said analog to digital converter and producing the function u(nT-T), and a digital to analog converter responsive to the digital function u(nT-T) output of said filter to supply the equivalent analog function u(nT-T) x(nT-T) as the feedback signal to said summer.
- a summer receiving an input signal x(nT) and producing an output signal y(nT), a feedback loop responsive to the output y(nT) of said summer and producing the function u(nT-T) x(nT-T) supplied as a feedback signal to said summer, said summer summing the feedback signal u(nT-T) in subtractive relationship with the input signal x(nT) to produce as the output y(nT) x(nT) - x(nT-T) and wherein said feedback loop includes an analog to digital converter receiving the analog output of said summer and producing an equivalent digital output y(nT);
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9. A clutter canceller as recited in claim 8, wherein said filter comprises:
- a digital summer receiving the digital output y(nT) of said analog to digital converter and producing the function u(nT) as an output, and a digital delay line receiving the u(nT) output of said digital summer and producing the function u(nT-T) as an output, and said digital summer receiving and combining the filter output function u(nT-T) in additive relationship with the digital output y(nT) whereby u(nT) y(nT) + u(nT-T).
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10. A clutter canceller as recited in claim 8, wherein for the system transfer function w(nT) y(nT)/x(nT), said filter has characteristics h(nT) defined in accordance with
Specification