Apparatuses and methods of suppressing a narrow-band interference with a compensator and adjustment loops

• US 6,219,376 B1
• Filed: 02/21/1998
• Issued: 04/17/2001
• Est. Priority Date: 02/21/1998
• Status: Expired due to Term

• Current Assignee: Topcon GPS, LLC
• Original Assignee: Topcon Positioning Systems Incorporated (Topcon Corporation)
• Inventors: Zhodzishsky, Mark I., Ashjaee, Javad, Prasolov, Victor A., Vorobiev, Michail Y.
• Primary Examiner(s): Chin, Stephen
• Assistant Examiner(s): Ha, Dac V.
• Application Number:
  US09/027,550
• Time in Patent Office: 1,151 Days
• Field of Search: 375/144, 375/148, 375/285, 375/346, 375/348, 375/350, 455/296, 455/304, 455/305, 455/306
• US Class Current: 375/148
• CPC Class Codes: G01S 19/21 Interference related issues ; Issues related to cross-correlation, spoofing or other methods of denial of service
• CPC Class Codes: H04B 1/1036 with automatic suppression of narrow band noise or interference, e.g. by using tuneable notch filters H04B1/123 takes precedence; filter circuits H03H

Abstract

Method of suppression of narrow-band interferences attending at the receiver input added to the useful broadband signal and noise.

There is a disclosed compensator rejecting narrow-band interferences by means of adjustment loops. Two general methods of construction of such loops are considered. The first general method is based on filtration of the in-phase and quadrature components of the error vector—difference of the interference vector and compensating vector. The second method is based on filtration of the amplitude and full phase of the interference signal. Automatic tuning of the compensator to the mean frequency and effective interference band is ensured.

Claims

1. A method of suppressing an interference signal from an input signal which includes a useful signal and the interference signal, the interference signal having a bandwidth more narrow than the bandwidth of the useful signal and having a mean frequency value ω

_INT,T, said method comprising the steps of;

(a) generating a first signal as a function of the input signal;

(b) generating a first quadrature component and a second quadrature component of said first signal with respect to a first periodic reference signal, said quadrature components being orthogonal to one another, each said quadrature component having a frequency spectrum;

(c) filtering each of said first and second quadrature components in time using at least one accumulator to attenuate a portion of said component'"'"'s frequency spectrum which is outside a selected frequency band, said selected frequency band including a portion of the interference signal spectrum corresponding to the mean frequency value of the interference signal, said filtering step generating a first filtered component and a second filtered component; and

(d) generating a difference signal which is related to the difference between said input signal and said filtered components.

2. The method of claim 1 wherein said first reference signal used in generating said quadrature components in step (b) comprises a sinusoidal signal having a substantially constant frequency.

3. The method of claim 1 wherein said selected frequency band of said step (c) includes the peak of the interference signal and substantially all of the frequency components of the interference signal which have power levels above the power level of the useful signal.

5. The method of claim 2 wherein said step (c) comprises the step of filtering each of said quadrature components with a respective filter having two serially coupled integrators.

6. The method of claim 2 wherein said step (c) comprises the step of filtering each of said quadrature components with a respective filter having a double pole at zero frequency.

7. The method of claim 2 wherein said step (c) comprises the step of filtering each of said quadrature components with a respective filter, said filter providing a structure which is equivalent to two parallel signal processing paths which receive the signal to be filtered and separately process the signal, the outputs of said paths being combined by a summator and provided to a first integrating accumulator (Σ

₁), said first path multiplying the signal by a first gain factor (K₁), said second path multiplying the signal by a second gain factor (K₂) and integrating the signal with a second integrating accumulator(Σ

₂).

8. The method of claim 7 further comprising the step of measuring the frequency spectrum of the difference signal in a frequency band around the mean interference frequency value ω

_INT,T, and the step of adjusting said first and second gain factors (K₁,K₂) from a set of values where the interference signal is detected in said frequency band to a set of values where the interference signal disappears from said frequency band.

9. The method of claim 7 further comprising the step of measuring the frequency spectrum of the difference signal in a frequency band around the mean interference frequency value ω

_INT,T, and the step of adjusting said first and second gain factors (K₁,K₂) from a set of values where the interference signal is not detected in said frequency band to a set of values where the interference signal is detected in said frequency band.

10. The method of claim 7 wherein the ratio α

of the square of said first gain factor (K₁²) divided by the second gain factor (K₂) is in a range between approximately 0.5 and approximately 16 (α

=K₁²/K₂).

11. The method of claim 10 wherein the quantity α

is in a range between approximately 1 and approximately 4.

12. The method of claim 2 wherein said step (c) comprises the step of filtering each of said quadrature components with a respective digital filter which receives sampled signal values at a rate of F_S, said filter having a frequency transfer function K_F(f) as a function of frequency f comprising the terms $\frac{K_1}{p}+\frac{K_2}{p^2},\mathrm{where}$

 



p=j2π



(fFS),

in a frequency band between approximately −

F_S/20 and approximately +F_S/20.

13. The method of claim 2 wherein said step (c) comprises the step of filtering each of said quadrature components with a respective filter, said filter providing a structure which is equivalent to two parallel signal processing paths which receive the signal to be filtered and separately process the signal, the outputs of said paths being combined by a summator and provided to a first integrating accumulator (Σ

₁), said first path multiplying the signal by a first gain factor (K₁), said second path multiplying the signal by a second gain factor (K₂) and integrating the signal with a second integrating accumulator(Σ

₂), said filter step being part of the feedback loop through the processing steps;

wherein each step in said feedback loop modifies the magnitude of the feedback signal by a respective gain/attenuation factor;

wherein all of the steps of said feedback loop except said filtering step provide a collective gain/attenuation factor of β

_T; and

wherein the quantity R of the collective factor β

_T times the square of the filter'"'"'s first gain factor (K₁²) divided by the filter'"'"'s second gain factor (K₂) (R=β

_T·

K₁²/K₂) is in a range between approximately 0.5 and approximately 16.

14. The method of claim 13 wherein the quantity R is in a range between approximately 1 and approximately 4.

4. A method of suppressing an interference signal from an input signal which includes a useful signal and the interference signal, the interference signal having a bandwidth more narrow than the bandwidth of the useful signal and having a mean frequency value ω

_INT,T, said method comprising the steps of;

(a) generating a first signal as a function of the input signal;

(b) generating a first quadrature component and a second quadrature component of said first signal with respect to a first periodic reference signal, said quadrature components being orthogonal to one another, each said quadrature component having a frequency spectrum;

(c) filtering each of said first and second quadrature components to attenuate a portion of said component'"'"'s frequency spectrum which is outside a selected frequency band, said selected frequency band including a portion of the interference signal spectrum corresponding to the mean frequency value of the interference signal, said filtering step generating a first filtered component and a second filtered component; and

(d) generating a difference signal which is related to the difference between said input signal and said filtered components; and

wherein step (a) comprises the step of providing the difference signal as said first signal, said step thereby forming a feedback loop which continuously conveys a feedback signal around the sequence of processing steps included by steps (a)-(d).

15. A method of suppressing an interference signal from an input signal which includes a useful signal and the interference signal, the interference signal having a bandwidth more narrow than the bandwidth of the useful signal and having a mean frequency value ω

_INT,T, said method comprising the steps of;

(a) generating a first signal as a function of the input signal;

(b) generating a first quadrature component and a second quadrature component of said first signal with respect to a first periodic reference signal, said quadrature components being orthogonal to one another, each said quadrature component having a frequency spectrum;

(c) filtering each of said first and second quadrature components to attenuate a portion of said component'"'"'s frequency spectrum which is outside a selected frequency band, said selected frequency band including a portion of the interference signal spectrum corresponding to the mean frequency value of the interference signal, said filtering step generating a first filtered component and a second filtered component; and

(d) generating a difference signal which is related to the difference between said input signal and said filtered components; and

wherein said step (b) comprises the step of quadrature converting said first signal using said first reference signal to generate said quadrature components;

wherein said step (d) comprises the step of inverse quadrature converting said filtered components using a second periodic reference signal to generate an interference copy signal (u_INT), said second periodic reference signal having the same frequency as said first reference signal and being advanced in phase by a predetermined amount of time; and

wherein said step (d) further comprises the step of forming said first signal as the difference between the input signal and said interference copy signal.

16. The method of claim 15 wherein said step (a) comprises the step of generating a digital form of the input signal;

wherein said step (b) further comprises the step of generating said quadrature coordinates in digital form;

wherein said step (c) further comprises the step of generating said filtered components in digital form.

17. The method of claim 16 wherein said step (b) comprises the step of generating a quantized version of said first signal having a plurality M of quantization levels, and the step of quadrature converting said quantized version.

18. The method of claim 17 wherein M is at least 255.

19. The method of claim 17 wherein M is at least 63.

20. The method of claim 17 wherein said step (b) further comprises the step of generating a quantized version of said first reference signal having a plurality of L₁ of quantization levels, and wherein L₁ is greater than 1 and is in the range of approximately M/16 to approximately 16*M.

21. The method of claim 17 wherein said step (d) further comprises the step of generating said interference copy signal (u_INT) in digital form having a plurality of N quantization levels, and wherein N is greater than 1 and is in the range of approximately M/16 to approximately 16*M.

22. The method of claim 21 wherein N is greater than 1 and is in the range of approximately M/2 to approximately 2*M.

23. The method of claim 17 wherein said step (d) further comprises the step of generating a quantized version of said second reference signal having a plurality L₂ of quantization levels, and wherein L₂ is greater than 1 and is in the range of approximately M/16 to approximately 16*M.

24. The method of claim 23 wherein L₂ is greater than 1 and is in the range of approximately M/2 to approximately 2*M.

25. The method of claim 15 wherein said step of quadrature converting provides a first gain/attenuation factor β

₁ between said first signal and the quadrature components of said first signal;

wherein said step of inverse quadrature converting provides a second gain/attenuation factor β

₂ between said filtered components and said interference copy signal; and

wherein said step (c) comprises the step of filtering each of said quadrature components with a respective filter, said filter providing a structure which is equivalent to two parallel signal processing paths which receive the signal to be filtered and separately process the signal, the outputs of said paths being combined by a summator and provided to a first integrating accumulator (Σ

₁), said first path multiplying the signal by a first gain factor (K₁), said second path multiplying the signal by a second gain factor (K₂) and integrating the signal with a second integrating accumulator(Σ

₂); and

wherein the quantity R of the first factor β

₁ times the second factor β

₂ times the square of said filter'"'"'s first gain factor (K₁²) divided by the filter'"'"'s second gain factor (K₂) (R=β

₁·

β

₂·

K₁²/K₂) is in a range between approximately 0.5 and approximately 16.

26. The method of claim 25 wherein the quantity R is in a range between approximately 1 and approximately 4.

27. A method of suppressing an interference signal from an input signal which includes a useful signal and the interference signal, the interference signal having a bandwidth more narrow than the bandwidth of the useful signal and having a mean frequency value ω

_INT,T, said method comprising the steps of;

(a) sampling the input signal at periodic intervals and generating a digital representation thereof having M quantization levels of resolution;

(b) generating an approximate copy of the interference signal in a digital form having N quantization levels of resolution;

(c) generating a difference signal which is related to the difference between said digital representation of the input signal and said approximate copy of the interference signal;

wherein said step (b) of generating said approximate copy of the interference signal comprises the steps of;

(d) generating a first quadrature component and a second quadrature component of said difference signal by quadrature converting said difference signal with respect to a first periodic reference signal having a frequency ω

_INT approximately equal in magnitude to the mean frequency value ω

_INT,T of the interference signal, said quadrature components being orthogonal to one another, each said quadrature component having a frequency spectrum;

(e) filtering each of said first and second quadrature components to attenuate a portion of said component'"'"'s frequency spectrum which is outside a selected frequency band, said selected frequency band including a portion of the interference signal spectrum corresponding to the mean frequency value of the interference signal, said filtering step generating a first filtered component and a second filtered component; and

(f) generating said approximate copy of the interference signal by inverse quadrature converting said filtered components using a second periodic reference signal, said second periodic reference signal having a frequency equal in magnitude to the frequency of said first reference signal.

28. The method of claim 27 wherein the phase of said second reference signal is advanced with respect to the first reference signal by a predetermined amount of time.

29. The method of claim 27 wherein the method generates a rejection bandwidth Δ

<sub>SUP,−

3 dB</sub> in said difference signal to signals centered about the frequency magnitude of said first reference signal ω

_INT, and wherein the difference between the mean interference frequency ω

_INT,T and the frequency magnitude ω

_INT of said first periodic reference signal is not more than one-half the value of the rejection bandwidth Δ

<sub>SUP,−

3 dB</sub>.

30. The method of claim 27 wherein said step (e) comprises the step of filtering each of said quadrature components with a respective filter having two serially coupled integrators.

31. The method of claim 27 wherein said step (e) comprises the step of filtering each of said quadrature components with a respective filter having a double pole at zero frequency.

32. The method of claim 27 wherein said step (e) comprises the step of filtering each of said quadrature components with a respective filter, said filter providing a structure which is equivalent to two parallel signal processing paths which receive the signal to be filtered and separately process the signal, the outputs of said paths being combined by a summator and provided to a first integrating accumulator (Σ

₁), said first path multiplying the signal by a first gain factor (K₁), said second path multiplying the signal by a second gain factor (K₂) and integrating the signal with a second integrating accumulator(Σ

₂).

33. The method of claim 32 wherein said steps (b) and (c) provide a feedback loop which conveys a feedback signal around the loop of processing steps included by steps (b) and (c);

wherein each step in said feedback loop modifies the magnitude of the feedback signal by a respective gain/attenuation factor;

wherein all of the steps of said feedback loop except said filtering step provide a collective gain/attenuation factor of β

_T; and

wherein the quantity R of the collective factor β

_T times the square of the filter'"'"'s first gain factor (K₁²) divided by the filter'"'"'s second gain factor (K₂) (R=β

_T·

K₁²/K₂) is in a range between approximately 0.5 and approximately 16.

34. The method of claim 33 wherein the quantity R is in a range between approximately 1 and approximately 4.

35. The method of claim 27 wherein M is at least 63.

36. The method of claim 27 wherein said step (d) further comprises the step of generating a quantized version of said first reference signal having a plurality of L₁ of quantization levels, and wherein L₁ is in the range of approximately M/16 to approximately 16*M.

37. The method of claim 27 wherein N is greater than 1 and is in the range of approximately M/16 to approximately 16*M.

38. The method of claim 27 wherein N is greater than 1 and is in the range of approximately M/2 to approximately 2*M.

39. The method of claim 27 wherein said step (f) further comprises the step of generating a quantized version of said second reference signal having a plurality L₂ of quantization levels, and wherein L₂ is greater than 1 and is in the range of approximately M/16 to approximately 16*M.

40. The method of claim 37 wherein L₂ is in the range of approximately M/2 to approximately 2*M.

41. A method of suppressing an interference signal from an input signal which includes a useful signal and the interference signal, the interference signal having a bandwidth more narrow than the bandwidth of the useful signal and having a mean frequency value ω

_INT,T, said method comprising the steps of;

(a) generating an approximate copy of the interference signal in analog form;

(b) generating a difference signal which is related to the difference between the input signal and said approximate copy of the interference signal;

(c) sampling the difference signal at periodic intervals and generating a digital representation thereof having a plurality of M quantization levels;

wherein said step (a) of generating said approximate copy of the interference signal comprises the steps of;

(d) generating a first quadrature component and a second quadrature component of said difference signal in digital form by quadrature converting said difference signal with respect to a first periodic reference signal having a frequency ω

_INT approximately equal in magnitude to the mean frequency value ω

_INT,T of the interference signal, said quadrature components being orthogonal to one another, each said quadrature component having a frequency spectrum;

(e) filtering each of said first and second quadrature components to attenuate a portion of said component'"'"'s frequency spectrum which is outside a selected frequency band, said selected frequency band including a portion of the interference signal spectrum corresponding to the mean frequency value of the interference signal, said filtering step generating a first filtered component and a second filtered component; and

(f) generating a digital form of said approximate copy of the interference signal by inverse quadrature converting said filtered components using a second periodic reference signal, said second periodic reference signal having a frequency equal in magnitude to the frequency of said first reference signal; and

(g) converting the digital form of said approximate copy of the interference signal to analog form with a resolution having a plurality N of quantization levels, wherein N is greater than 1 and in a range of approximately M/16 to approximately 16*M.

42. The method of claim 41 wherein said step (c) comprises the step of quantizing said difference signal to two levels.

43. The method of claim 41 wherein the phase of said second reference signal is advanced with respect to the first reference signal by a predetermined amount of time.

44. The method of claim 41 wherein the method generates a rejection bandwidth Δ

<sub>SUP,−

3 dB</sub> in said difference signal to signals centered about the frequency magnitude of said first reference signal ω

_INT, and wherein the difference between the mean interference frequency value ω

_INT,T and the frequency magnitude ω

_INT of said first periodic reference signal is not more than one-half the value of the rejection bandwidth Δ

<sub>SUP,−

3 dB</sub>.

45. A method of suppressing an interference signal from an input signal which includes a useful signal and the interference signal, the interference signal having a bandwidth more narrow than the bandwidth of the useful signal and having a mean frequency value ω

_INT,T, said method comprising the steps of;

(a) quadrature down-converting the input signal using a quadrature local oscillator having frequency value ω

_H to produce a first quadrature component U_K,C and a second quadrature component U_K,S, said quadrature components being orthogonal, the mean interference frequency value (ω

_INT,T) being shifted to an intermediate value Ω

_INT,T (Ω

_INT,T=|ω

_INT,T−

ω

_H|), each of said quadrature components having a frequency spectrum which includes said intermediate value Ω

_INT,T;

(b) generating an approximate copy of the down-converted interference signal in quadrature form having a first quadrature component U_INT,C and a second quadrature component U_INT,S;

(c) generating a difference signal in quadrature form which is related to the difference between the down-converted input signal and said approximate copy of the down-converted interference signal, said difference signal having a first quadrature component U_D,C which is related to the difference between the first quadrature components of the down-converted input signal and said approximate copy (U_D,C∝

U_K,C−

U_INT,C) and a second quadrature component U_D,S which is related to the difference between the second quadrature components of the down-converted input signal and said approximate copy (U_D,S∝

U_K,S−

U_INT,S), said first and second quadrature components (U_D,C,U_D,S) being a first set of quadrature components of the difference signal, each of said quadrature components (U_D,C,U_D,S) having a frequency spectrum that includes said intermediate value Ω

_INT,T of the mean interference frequency; and

wherein said step (b) of generating said approximate copy of the interference signal comprises the steps of;

(d) generating a second set of quadrature components (C_D, S_D) of the difference signal with respect to a first sinusoidal quadrature reference signal, said step comprising the step of frequency shifting said first set of quadrature components (U_D,C,U_D,S) of the difference signal in a first direction such that said intermediate value Ω

_INT,T is shifted to a value of approximately zero frequency;

(e) filtering each of said quadrature components (C_D, S_D) of said second set to generate respective filtered quadrature components (U_C, U_S), each of said filtered quadrature components (U_C, U_S) having a frequency spectrum which includes said shifted value of said intermediate value Ω

_INT,T;

(f) generating the quadrature components (U_INT,C, U_INT,S) of said approximate interference copy by frequency shifting said filtered quadrature components (U_C, U_S) in a second direction opposite to the first direction such that said shifted value of the mean interference frequency is shifted back to said intermediate value Ω

_INT,T, wherein said step of frequency shifting in the second direction uses a second sinusoidal quadrature reference signal having substantially the same magnitude of frequency as said first reference signal, the full phase of said second quadrature reference signal being shifted by a substantially constant amount of time with respect to the full phase of said first quadrature reference signal such that the magnitude of the full phase of said first reference signal is delayed in time with respect to the magnitude of the full phase of said second reference signal;

said steps (b) and (c) thereby providing two feedback adjustment loops, each of which modifies a respective quadrature component of said difference signal to suppress the amount of the interference signal present within the difference signal.

46. The method of claim 45 wherein the step of frequency shifting in said step (d) comprises the step of conjugate-complex multiplying said first set of quadrature components (U_D,C,U_D,S) of the difference signal with said first quadrature reference signal, and wherein said first quadrature reference signal has a frequency value approximately equal to said intermediate value Ω

_INT,T.

47. The method of claim 45 wherein the step of frequency shifting in said step (d) comprises the step of complex multiplying said first set of quadrature components (U_D,C,U_D,S) of the difference signal with said first quadrature reference signal, and wherein said first quadrature reference signal has a frequency value approximately equal to the negative of said intermediate value Ω

_INT,T.

48. The method of claim 45 wherein the step of frequency shifting in said step (f) comprises the step of complex multiplying said filtered quadrature components (U_C, U_S) with said second quadrature reference signal, and wherein said second quadrature reference signal has a frequency value approximately equal to said intermediate value Ω

_INT,T.

49. The method of claim 45 wherein the step of frequency shifting in said step (f) comprises the step of conjugate-complex multiplying said filtered quadrature components (U_C, U_S) with said second quadrature reference signal, and wherein said second quadrature reference signal has a frequency value approximately equal to the negative of said intermediate value Ω

_INT,T.

50. The method of claim 45 wherein said step (c) further comprises the step of sampling said first set of quadrature components (U_D,C,U_D,S) of the difference signal at periodic intervals and generating a digital representation thereof having M quantization levels for each quadrature component;

wherein steps (d), (e), and (f) are performed with the signals in digital form; and

wherein said step (b) further comprises the step (g) of converting the digital form of the quadrature components of said approximate interference copy (U_INT,C, U_INT,S) to analog form with a resolution of N quantization levels in each of said components (U_INT,C, U_INT,S), wherein N is greater than 1 and is in a range of approximately M/16 to approximately 16*M.

51. The method of claim 50 wherein said first quadrature reference signal provided in step (d) is in digital form having a resolution of L₁ quantization levels, wherein L₁ is greater than 1 and is in a range of approximately M/16 to approximately 16*M.

52. The method of claim 50 wherein said second quadrature reference signal provided in step (f) is in digital form having a resolution of L₂ quantization levels, wherein L₂ is greater than 1 and is in a range of approximately M/16 to approximately 16*M.

53. The method of claim 45 wherein said step (a) comprises the step of direct quadrature converting the input signal by the use of two analog multipliers and two low-pass filters, each said multiplier receiving the input signal and a respective quadrature component of a quadrature reference signal, the output of each said multiplier being provided to a respective one of said low-pass filters.

54. The method of claim 45 wherein said step (e) comprises the step of filtering each of said quadrature components (C_D, S_D) of said second set with a respective filter having two serially coupled integrators.

55. The method of claim 45 wherein said step (e) comprises the step of filtering each of said quadrature components (C_D, S_D) of said second set with a respective filter having a double pole at zero frequency.

56. The method of claim 45 wherein said step (e) comprises the step of filtering each of said quadrature components (C_D, S_D) of said second set with a respective filter, said filter providing a structure which is equivalent to two parallel signal processing paths which receive the signal to be filtered and separately process the signal, the outputs of said paths being combined by a summator and provided to a first integrating accumulator (Σ

₁), said first path multiplying the signal by a first gain factor (K₁), said second path multiplying the signal by a second gain factor (K₂) and integrating the signal with a second integrating accumulator(Σ

₂).

57. The method of claim 56 wherein each step in each said feedback loop modifies the magnitude of the feedback signal of the loop by a respective gain/attenuation factor;

wherein all of the steps of each said feedback loop except said filtering step of said feedback loop provide a collective gain/attenuation factor of β

_T; and

wherein, for each said feedback loop, the quantity R of the collective factor β

_T of the feedback loop times the square of the first gain factor (K₁²) of the filter in the feedback loop divided by the second gain factor (K₂) of the filter in the feedback loop (R=β

_T·

K₁²/K₂) is in a range between approximately 0.5 and approximately 16.

58. The method of claim 57 wherein the quantity R is in a range between approximately 1 and approximately 4.

59. A method of suppressing an interference signal from an input signal which includes a useful signal and the interference signal, the interference signal having a bandwidth more narrow than the bandwidth of the useful signal and having a mean frequency value Ω

_INT,T, said method comprising the steps of;

(a) quadrature down-converting the input signal using a quadrature local oscillator having a frequency value ω

_H to produce a first quadrature component U_K,C and a second quadrature component U_K,S, said quadrature components being orthogonal, the mean interference frequency value (ω

_INT,T) being shifted to an intermediate value Ω

_INT,T (Ω

_INT,T=|ω

_INT,T−

ω

_H|);

(b) generating an approximate copy of the down-converted interference signal in quadrature form having a first quadrature component U_INT,C and a second quadrature component U_INT,S;

(c) generating a difference signal in quadrature form which is related to the difference between the down-converted input signal and said approximate copy of the down-converted interference signal, said difference signal having a first quadrature component U_D,C which is related to the difference between the first quadrature components of the down-converted input signal and said approximate copy (U_D,C∝

U_K,C−

U_INT,C) and a second quadrature component U_D,S which is related to the difference between the second quadrature components of the down-converted input signal and said approximate copy (U_D,S∝

U_K,S−

U_INT,S), said first and second quadrature components (U_D,C,U_D,S) being a first set of quadrature components of the difference signal, and

wherein said step (b) of generating said approximate copy of the interference signal comprises the steps of;

(d) conjugate complex multiplying the first set of quadrature components (U_D,C,U_D,S) of the difference signal by a first quadrature reference signal to generate a second set of quadrature components (C_D, S_D) of the difference signal with respect to the first quadrature reference signal;

(e) filtering said second set of quadrature components (C_D, S_D) of the difference signal; and

(f) complex multiplying the filtered second set of quadrature components (C_D, S_D) of the difference signal by a version of said first quadrature reference signal to produce the quadrature components of said approximate interference copy;

said steps (b) and (c) thereby providing two feedback adjustment loops, each of which modifies a respective quadrature component of said difference signal to suppress the amount of the interference signal present within the difference signal.

60. The method of claim 59 wherein the phase of said version of said first quadrature reference signal is advanced in time with respect to said quadrature reference signal by an amount which is less than one-half period of the intermediate frequency value Ω

_INT,T.

61. A method of suppressing an interference signal from an input signal which includes a useful signal and the interference signal, the interference signal having a bandwidth more narrow than the bandwidth of the useful signal and having a mean frequency value ω

_INT,T, said method comprising the steps of;

(a) quadrature down-converting the input signal using a quadrature local oscillator having frequency value ω

_H to produce a first quadrature component U_K,C and a second quadrature component U_K,S, said quadrature components being orthogonal, the mean interference frequency value (ω

_INT,T) being shifted to an intermediate value Ω

_INT,T (Ω

_INT,T=|ω

_INT,T−

ω

_H|);

(b) generating an approximate copy of the down-converted interference signal in quadrature form having a first quadrature component U_INT,C and a second quadrature component U_INT,S;

(c) generating a difference signal in quadrature form which is related to the difference between the down-converted input signal and said approximate copy of the down-converted interference signal, said difference signal having a first quadrature component U_D,C which is related to the difference between the first quadrature components of the down-converted input signal and said approximate copy (U_D,C∝

U_K,C−

U_INT,C) and a second quadrature component U_D,S which is related to the difference between the second quadrature components of the down-converted input signal and said approximate copy (U_D,S∝

U_K,S−

U_INT,S), said first and second quadrature components (U_D,C,U_D,S) being a first set of quadrature components of the difference signal, and

wherein said step (b) of generating said approximate copy of the interference signal comprises the steps of;

(d) complex multiplying the first set of quadrature components (U_D,C,U_D,S) of the difference signal by a first quadrature reference signal to generate a second set of quadrature components (C_D, S_D) of the difference signal with respect to the first quadrature reference signal;

(e) filtering said second set of quadrature components (C_D, S_D) of the difference signal; and

(f) conjugate complex multiplying the filtered second set of quadrature components (C_D, S_D) of the difference signal by a version of said first quadrature reference signal to produce the quadrature components of said approximate interference copy;

said steps (b) and (c) thereby providing two feedback adjustment loops, each of which modifies a respective quadrature component of said difference signal to suppress the amount of the interference signal present within the difference signal.

62. The method of claim 61 wherein the phase of said version of said first quadrature reference signal is advanced in time with respect to said quadrature reference signal by an amount which is less than one-half period of the intermediate frequency value Ω

_INT,T.

63. A method of suppressing an interference signal from an input signal which includes a useful signal and the interference signal, the interference signal having a bandwidth more narrow than the bandwidth of the useful signal and having a mean frequency value ω

_INT,T, said method comprising the steps of;

(a) quadrature down-converting the input signal using a quadrature local oscillator having a frequency value ω

_H to produce a first quadrature component U_K,C and a second quadrature component U_K,S, said quadrature components being orthogonal, the mean interference frequency value (ω

_INT,T) being shifted to an intermediate value Ω

_INT,T (Ω

_INT,T=|ω

_INT,T−

ω

_H|);

(b) generating an approximate copy of the down-converted interference signal in quadrature form having a first quadrature component U_INT,C and a second quadrature component U_INT,S;

(c) generating a difference signal in quadrature form which is related to the difference between the down-converted input signal and said approximate copy of the down-converted interference signal, said difference signal having a first quadrature component U_D,C which is related to the difference between the first quadrature components of the down-converted input signal and said approximate copy (U_D,C∝

U_K,C−

U_INT,C) and a second quadrature component U_D,S which is related to the difference between the second quadrature components of the down-converted input signal and said approximate copy (U_D,S∝

U_K,S−

U_INT,S), said first and second quadrature components (U_D,C,U_D,S) being a first set of quadrature components of the difference signal, and

wherein said step (b) of generating said approximate copy of the interference signal comprises the steps of;

(d) complex multiplying the first set of quadrature components (U_D,C,U_D,S) of the difference signal by a first quadrature reference signal to generate a second set of quadrature components (C_D, S_D) of the difference signal with respect to the first quadrature reference signal;

(e) filtering said second set of quadrature components (C_D, S_D) of the difference signal; and

(f) complex multiplying the filtered second set of quadrature components (C_D, S_D) of the difference signal by a second quadrature reference signal to produce the quadrature components of said approximate interference copy, said second quadrature reference signal substantially being the complex conjugate of said first quadrature reference signal;

said steps (b) and (c) thereby providing two feedback adjustment loops, each of which modifies a respective quadrature component of said difference signal to suppress the amount of the interference signal present within the difference signal.

64. The method of claim 63 further comprising the step of advancing the phase of said second quadrature reference signal with respect to said first quadrature reference signal by an amount which is less than one-half period of the intermediate frequency value Ω

_INT,T.

65. The method of claim 63 wherein said step (d) of complex multiplying comprises the step of conjugate complex multiplying said first set of quadrature components (U_D,C,U_D,S) of the difference signal by said first quadrature reference signal, and wherein said step (f) of complex multiplying comprises the step of conjugate complex multiplying the filtered second set of quadrature components (C_D, S_D) of the difference signal by a second quadrature reference signal.

66. The method of claim 65 further comprising the step of advancing the phase of said second quadrature reference signal with respect to said first quadrature reference signal by an amount which is less than one-half period of the intermediate frequency value Ω

_INT,T.

67. A method of suppressing an interference signal from an input signal which includes a useful signal and the interference signal, the interference signal having a bandwidth more narrow than the bandwidth of the useful signal and having a mean frequency value Ω

_INT,T, said method comprising the steps of;

(a) generating an approximate copy of the interference signal;

(b) generating a difference signal which is related to the difference between the input signal and said approximate copy of the interference signal;

wherein said step (a) of generating said approximate copy of the interference signal comprises the steps of;

(c) generating a phase coordinate signal having substantially constant amplitude and having a frequency and phase which substantially follow the frequency and phase of the interference signal, said phase coordinate signal being generated with a phase-lock loop;

(d) generating a magnitude coordinate signal by synchronous detection of said difference signal with said phase coordinate signal followed by filtering;

(e) generating said approximate interference copy by multiplying said phase coordinate signal and said magnitude coordinate signal, said steps (a) and (b) thereby providing a feedback adjustment loop which reduces the amplitude of the interference signal in the difference signal.

68. The method of claim 67 wherein there is substantially no phase difference in the phase coordinate signal used in steps (d) and (e).

69. The method of claim 67 wherein said step (b) further comprises the step of sampling the input signal at periodic intervals and generating a digital representation thereof having a plurality M of quantization levels;

and

wherein said step (a) further comprises the step of generating the approximate copy of the interference signal at said periodic intervals in digital form having N quantization levels, wherein N is greater than 1 and is in a range of approximately M/16 to approximately 16*M.

70. The method of claim 67 wherein said step (b) further comprises the step of sampling the input signal at periodic intervals and generating a digital representation thereof having a plurality M of quantization levels;

wherein said step (c) generates said phase coordinate signal at said periodic intervals in digital form having L₂ quantization levels, wherein L₂ is greater than 2 and is in a range of approximately M/16 to approximately 16*M; and

wherein said step (e) further comprises the step of multiplying said digital form of said phase coordinate signal with said magnitude coordinate signal.

71. The method of claim 70 wherein said step (d) further comprises the step of synchronously detecting said difference signal with said digital form of said phase coordinate signal.

72. The method of claim 67 wherein said step (b) further comprises the step of sampling the input signal at periodic intervals and generating a digital representation thereof having a plurality M of quantization levels;

wherein said step (c) generates said phase coordinate signal at said periodic intervals in digital form having L₂ quantization levels, wherein L₂ is greater than 2 and is in a range of approximately M/16 to approximately 16*M; and

wherein said step (d) further comprises the step of synchronously detecting said difference signal with said digital form of said phase coordinate signal.

73. The method of claim 67 wherein said step (c) generates said phase coordinate signal in quadrature format, wherein step (d) generates said magnitude coordinate signal in non-quadrature format, and wherein said step (e) multiplies said magnitude coordinate signal by only one of the quadrature components of said phase coordinate signal.

74. The method of claim 67 wherein said step (c) generates said phase coordinate signal in quadrature format, wherein step (d) generates said magnitude coordinate signal in non-quadrature format by synchronously detecting said difference signal with only one of the quadrature components of said phase coordinate signal.

75. The method of claim 67 wherein the frequency bandwidth of the difference signal synchronously detected by said step (d) is substantially larger than the bandwidth of the interference signal.

76. The method of claim 67 wherein step (b) generates said difference signal in proportion to the difference between the input signal and said approximate copy of the interference signal;

and

wherein step (d) comprises the steps of receiving said difference signal in unfiltered form and of synchronously detecting the unfiltered difference signal.

77. The method of claim 67 wherein said step (e) generates said approximate interference copy without filtering the multiplication product of said phase coordinate signal and said magnitude coordinate signal.

78. The method of claim 67 wherein said phase-lock loop comprises a filter having adjustable filtering characteristics which change the bandwidth of the PLL loop, wherein said method further comprises the step of measuring the frequency spectrum of the difference signal in a frequency band around the mean interference frequency value ω

_INT,T, and the step of adjusting the characteristics of said filter to substantially match the bandwidth of the PLL loop within a factor of two of the bandwidth of the interference signal.

79. The method of claim 67 wherein said phase-lock loop comprises a filter which provides a structure which is equivalent to two parallel signal processing paths which receive the signal to be filtered and separately process the signal, the outputs of said paths being combined by a summator and provided as the filter output, said first path multiplying the signal by a first gain factor (K₁), said second path multiplying the signal by a second gain factor (K₂) and integrating the signal with an integrating accumulator(Σ

).

80. The method of claim 79 further comprising the step of measuring the frequency spectrum of the difference signal in a frequency band around the mean interference frequency value ω

_INT,T, and the step of adjusting said first and second gain factors (K₁,K₂) from a set of values where the interference signal is detected in said frequency band to a set of values where the interference signal disappears from said frequency band.

81. The method of claim 79 further comprising the step of measuring the frequency spectrum of the difference signal in a frequency band around the mean interference frequency value ω

_INT,T, and the step of adjusting said first and second gain factors (K₁,K₂) from a set of values where the interference signal is not detected in said frequency band to a set of values where the interference signal is detected in said frequency band.

82. The method of claim 67 wherein said step of filter in said step (d) comprises the step of filtering with a filter having adjustable filtering characteristics, wherein said method further comprises the step of measuring the frequency spectrum of the difference signal in a frequency band around the mean interference frequency value ω

_INT,T, and the step of adjusting the characteristics of said filter to substantially match the bandwidth of PLL within a factor of two of the bandwidth of the interference signal.

83. The method of claim 67 wherein said phase-lock loop comprises a filter which provides a structure which is equivalent to two parallel signal processing paths which receive the signal to be filtered and separately process the signal, the outputs of said paths being combined by a summator and provided as the filter output, said first path multiplying the signal by a first gain factor (K₁), said second path multiplying the signal by a second gain factor (K₂) and integrating the signal with an integrating accumulator(Σ

).

84. A method of suppressing an interference signal from an input signal which includes a useful signal and the interference signal, the interference signal having a bandwidth more narrow than the bandwidth of the useful signal and having a mean frequency value ω

_INT,T, said method comprising the steps of;

(a) generating a first signal as a function of the input signal;

(b) generating a first quadrature component and a second quadrature component of said first signal with respect to a first periodic reference signal, said quadrature components being orthogonal to one another, each said quadrature component having a frequency spectrum;

(c) filtering each of said first and second quadrature components in the time domain to attenuate a portion of said component'"'"'s frequency spectrum which is outside a selected frequency band, said selected frequency band including a portion of the interference signal spectrum corresponding to the mean frequency value of the interference signal, said filtering step comprising the step of adding together present and past values of the first quadrature component and the step of adding together present and past values of the second quadrature component, said filtering step generating a first filtered component and a second filtered component; and

(d) generating a difference signal which is related to the difference between said input signal and said filtered components.

85. A method of suppressing an interference signal from an input signal which includes a useful signal and the interference signal, the interference signal having a bandwidth more narrow than the bandwidth of the useful signal and having a mean frequency value ω

_INT,T, said method comprising the steps of;

(a) generating a first signal as a function of the input signal;

(b) generating a first quadrature component and a second quadrature component of said first signal with respect to a first periodic reference signal, said quadrature components being orthogonal to one another, each said quadrature component having a frequency spectrum;

(c) filtering each of said first and second quadrature components in time with a respective filter having at least one pole to attenuate a portion of said component'"'"'s frequency spectrum which is outside a selected frequency band, said selected frequency band including a portion of the interference signal spectrum corresponding to the mean frequency value of the interference signal, said filtering step generating a first filtered component and a second filtered component; and

(d) generating a difference signal which is related to the difference between said input signal and said filtered components.