Automatic direction finding system

• US 4,219,821 A
• Filed: 09/07/1978
• Issued: 08/26/1980
• Est. Priority Date: 09/07/1978
• Status: Expired due to Term

• Current Assignee: Uniden Corporation (Cornwall Capital Management LP)
• Original Assignee: Regency Electronics Incorporated (BK Technologies Corp.)
• Inventors: Selim, Harold N.
• Primary Examiner(s): Tubbesing, T. H.
• Application Number:
  US05/940,254
• Time in Patent Office: 719 Days
• Field of Search: 343/113 R, 343/728, 343/867
• US Class Current: 342/445
• CPC Class Codes: G01S 3/143 by vectorial combination of signals derived from differently oriented antennae
• CPC Class Codes: G01S 3/16 using amplitude comparison of signals derived sequentially from receiving antennas or antenna systems having differently-oriented directivity characteristics or from an antenna system having periodically-varied orientation of directivity characteristic

Abstract

An automatic direction finding (ADF) system including a direction finding antenna array which connects to a radio receiver to process an audio output signal selectively illuminating a segment of a segmented circular visual directional display indicating the direction from which the received signal arrived. In an FM version, the ADF system electronically switches between antennas in the direction finding antenna array alternately incorporating vector information onto the received carrier in a manner that can be phase detected to produce an audio frequency signal within the normal pre-audio bandpass of the receiver. The audio frequency is then demultiplexed to produce separate vector signals. These separate vector signals are then processed for selectively illuminating a segment of the segmented circular visual directional display. When a sufficiently strong signal is not being received, the ADF system sequentially illuminates each segment of the segmented circular visual directional display in a clockwise direction at a rate which can be readily and visably perceived by an operator. This operation conveys the image of circuit activity, is exciting for an operator to watch, and may have the effect of simulating a radar sweep, although it is in no way related to the searching by the receiver. An additional segment is positioned in the center of the display to indicate the processing of a received radio frequency signal by the ADF system.

Claims

1. In apparatus for use in an automatic direction finding system the combination comprising:

a. indicator means suitable for display of directional information of a received radio frequency signal;

b. direction determining means connected to said indicator means for responding to a received signal of a sufficient level and for causing the indicator means to indicate the direction of arrival of the received radio frequency signal; and

c. sweep means connected to said indicator means for causing said indicator means to sequentially display directions independently of any direction determined by said direction determining means, the sequential display being at a visually perceivable rate.

2. The apparatus of claim 1 comprising control means for automatically preventing the sweep means from operating said indicator means when said direction means indicates the direction of arrival of the received radio frequency signal.

3. The apparatus of claim 1 in which said sweep means includes means for causing sequential display of directions at a rate to sequentially display all directions in from 0.1 to 10 seconds.

4. The apparatus of claim 1 in which said sweep means includes means for causing sequential display of directions at a rate to sequentially display all directions in from about 0.5 to about 1 second.

5. The apparatus of claim 1 in which said sweep means includes means for displaying each direction for from 2.7 to 270 milliseconds.

6. The apparatus of claim 5 wherein said indicator means comprises a segmented visual directional display having a different segment for each direction displayed.

7. The apparatus of claim 6 wherein said indicator means comprises several concentric conductor circles for connection to the segments.

8. The apparatus of claim 1 in which said sweep means includes means for displaying each direction for about 15 milliseconds.

9. The apparatus of claim 8 wherein said indicator means comprises a segmented visual directional display having a different segment for each direction displayed.

10. The apparatus of claim 1 comprising function indicating means for indicating, independently of the indicated direction, whether the direction determining means is causing the indicator to indicate the direction of arrival of a received radio frequency signal.

11. The apparatus of claim 10 in which said function indicating means comprises a visual indicator and which apparatus additionally comprises means for causing said visual indicator to flash when a high level radio frequency signal is being received.

12. The apparatus of claim 10 in which said function indicating means includes a light emitting diode.

13. The apparatus of claim 12 which additionally comprises means for causing said light emitting diode to flash when a high level of radio frequency signal is being received.

14. The apparatus of claim 1 wherein said indicator means comprises a segmented visual directional display in which said segments are positioned on the circumference of a circle.

15. The apparatus of claim 14 in which each of said segments comprises a light emitting diode.

16. The apparatus of claim 14 comprising function indicating means, located at the center of said circle, for indicating, independently of the indicated direction, whether the direction determining means is causing the indicator to indicate the direction of arrival of a received radio frequency signal.

17. The apparatus of claim 14 wherein said segmented visual directional display comprises a thirty-six segment circular visual directional display.

18. The apparatus of claim 17 in which each of said segments comprises a light emitting diode.

19. The apparatus of claim 14 in which said sweep means includes means for causing sequential display of directions at a rate to sequentially display all directions in from 0.1 to 10 seconds.

20. The apparatus of claim 19 comprising control means for automatically preventing the sweep means from operating said indicator means when said direction means indicates the direction of arrival of the received radio frequency signal.

21. The apparatus of claim 19 in which visual indicating elements are light emitting diodes.

22. The apparatus of claim 21 in which said display includes 36 visual indicating elements.

23. The apparatus of claim 14 in which said sweep means includes means for causing sequential display of directions at a rate to sequentially display all directions in from about 0.5 to about 1 second.

24. Apparatus for use in an automatic direction finding system, the combination comprising:

a. a radio receiver for receiving audio signals from modulated radio frequency signals, said receiver having an antenna input terminal and an audio output terminal;

b. a first directionally sensitive antenna means for providing information about a received signal for a first vector, said first vector being a component of the direction from which said received signal arrived;

c. a second directionally sensitive antenna means for providing information about a received signal for a second vector, said second vector being a component of the direction from which said received signal arrived, said second directionally sensitive antenna means being oriented to be directionally sensitive in a different direction than said first directionally sensitive antenna means;

d. means for alternately, at a first rate, making said first antenna means and said second antenna means operatively coupled to the antenna input terminal of said radio receiver;

e. means for modulating received signals from said first and second antenna means to incorporate said directional vector information as modulation which is at an audio frequency rate, said audio frequency rate being substantially greater than said first rate;

f. detection and storage means coupled to said audio output terminal for alternately, and in synchrony with said first rate, detecting the audio frequency information and separately storing it to establish individual vector values for each said antenna means; and

g. indicator means responsive to said stored information to indicate the actual direction represented by the stored values.

25. The apparatus of claim 24 comprising a narrow bandpass filter at said audio frequency rate connected to said audio output terminal and coupling the signal there to said detection and storage means.

26. Apparatus for use in an automatic direction finding system, the combination comprising:

a. first directionally sensitive antenna means for providing information about a received signal for a first vector, said first vector being a component of the direction from which said received signal arrived;

b. second directionally sensitive antenna means for providing information about said received signal for a second vector, said second vector being a component of the direction from which said received signal arrived;

c. multiplexing means for processing said received signal (1) to be modulated to include include a first constant frequency audio component at a level corresponding to said first vector and having at least several adjacent audio frequency cycles corresponding to said first vector and (2) to be modulated to include a second constant frequency audio component at a level corresponding to said second vector and having at least several adjacent audio frequency cycles corresponding to said second vector;

d. audio detecting means to detect an audio signal from said multiplexed received signal;

e. vector detector means to separately detect from said audio signal signals corresponding to said first and second vectors; and

f. indicator means responsive to said vector detector means to indicate the actual direction from which said received radio frequency signal arrived.

27. The apparatus of claim 26 wherein said vector detector includes a narrow bandpass, high Q, filter at said audio frequency of said first constant frequency audio component.

28. The apparatus of claim 26 in which

(a) said second directionally sensitive antenna means comprises means for providing information about said received signal for a second vector which is perpendicular to said first vector;

(b) said multiplexing means includes means for modulating the received signal so that said second constant frequency audio component is at a level corresponding to the perpendicular second vector; and

(c) said vector detector includes means for separately detecting from said audio signal, signals that correspond to said first vector and said second vector that is perpendicular thereto.

29. The apparatus of claim 26 in which said multiplexing means includes means for modulating said received signal to include a first constant frequency audio component at a frequency of between 30 Hz and 3,000 Hz at a level corresponding to said first vector and includes means for modulating said received signal to include a second constant frequency audio component at a frequency of between 30 Hz and 3,000 Hz at a level corresponding to said second vector.

30. The apparatus of claim 26 in which said multiplexing means includes means for periodically alternating between;

(a) modulating said received signal during a first time period to include a first constant frequency audio component at a first frequency and at a level corresponding to said first vector and

(b) modulating said received signal during a second time period to include a second constant frequency audio component at said first frequency and at a level corresponding to said second vector.

31. The apparatus of claim 30 in which the multiplexing means includes means for causing each of said first and said second time periods to be from 10 to 500 milliseconds.

32. The apparatus of claim 30 wherein said first antenna means includes two spaced antennas alternately chopped and periodically switched at an audio frequency switching rate to phase modulate said received signal and said second direction antenna means includes two spaced antennas, one antenna which is not shared by said first direction antenna means, alternately chopped and periodically switched at an audio frequency switching rate to phase modulate said received signal.

33. The apparatus of claim 30 wherein said first antenna means includes a first loop antenna and a sense antenna in which said first loop antenna received signal is alternately and periodically added in opposite phase to said sense antenna received signal at an audio frequency switching rate to amplitude modulate said received signal and said second antenna means includes a second loop antenna sharing said sense antenna in which said second loop antenna received signal is alternately and periodically added in opposite phase to said sense antenna received signal at an audio frequency switching rate to amplitude modulate said received signal.

34. The apparatus of claim 30 wherein said vector detector means includes storage means for storing said first and second vectors.

35. The apparatus of claim 34 wherein said vector detector means includes means for adjusting the value of a signal corresponding to said first vector only during the second half of said first time period and for adjusting the value of a signal corresponding to said second vector only during the second half of said second time period.

36. A direction finding antenna array for use with a frequency modulation receiver having an antenna terminal to receive a frequency modulated audio frequency signal and for use with an FM automatic direction finding system having first and second antenna commutation switching signals, said direction finding antenna array comprising:

a. first pair of antennas means, said pair of antennas means spaced less than one half wavelength apart and first switching means for switching between said first pair of antennas means in response to said first antenna commutating switching signal;

b. second pair of antennas means orthogonally opposed to said first pair of antennas means, said second pair of antennas means spaced a distance equal to the spacing of said first pair of antennas means and second switching means for switching between said second pair of antennas means in response to said second antennas commutating switching signal; and

c. means for connecting together the outputs of (1) said first antenna means switched by said first antenna commutating switching signal and (2) said second antenna means switched by said second antenna commutating switching signal, whereby said switched rf signals may be connected to said antenna terminal of said frequency modulation receiver through a single coaxial cable.

37. The direction finding array of claim 36 in which each antenna is a dipole antenna and all of the dipoles are parallel.

38. The direction finding array of claim 36 in which each dipole is at the corner of a square and which additionally includes support members extending along the diagonals of the square.

39. The direction finding array of claim 38 which additionally includes a mast which is perpendicular to said support members and extends to the center of the square.

40. A direction finding antenna array for use with an amplitude modulation receiver having an antenna terminal to receive an AM radio frequency signal and for use with an amplitude modulated automatic direction finding system having first and second antenna commutating switching signals, said direction finding antenna array comprising:

a. first and second loop antenna means orthogonally positioned with respect to each other including electrostatic shield means surrounding each of said loop antenna means and insulator means electrically breaking each of said electrostatic shield means at the loop apexes; and

b. sense antenna means positioned through the apex of said first and second loop antenna means.

41. The direction finding antenna array of claim 40 in which said loops are rectangular in shape and said sense antenna means includes vertical whip having a loading coil immediately above the apex.

42. An automatic direction finding system used with an FM receiver having an antenna terminal to receive a frequency modulated radio frequency signal and an audio output terminal said combination comprising:

a. oscillator means to generate a clock source of frequency;

b. divider means connected to said oscillator means to selectively divide said frequency of said oscillator means and generate chopper signals;

c. latch means connected to said divider means to retain a selected count of said divider means;

d. segmented circular display means connected to said latch means to display the retained count;

e. switching signal means connected to said divider means to generate antenna commutating switching signals and synchronous detector switching signals;

f. sweep display means connected between said oscillator means and said switching signal means and to said latch means to sequentially illuminate said display means in a circular direction when said receiver is not receiving a radio frequency signal of sufficient strength;

g. direction finding antenna array means including antenna means orthogonally positioned with respect to each other, switching means for switching said antenna means and said antenna means switched by said antenna commutating switching signals of said switching signal means and said antenna means connected to said antenna terminal of said FM receiver;

h. first filter means connected to said audio output terminal of said FM receiver;

i. synchronous detector means serially connected with buffer means to said first filter means to process vector information for each of said antenna means and said synchronous detector means switched by said synchronous detector switching signals of said switching signal means;

j. chopper means connected to each of said buffer means and switched by said chopper signals of said divider means;

k. second filter means connected to said chopper means to integrate said filter said signals from said chopper means and outputting a signal having a phase component; and

l. sweep signal control means connected between said second filter means and said latch means to detect a zero crossover of the phase component of said signal from said second filter means whereby said sweep signal control means latches said latch means on a count of said divider means and said divider means phased locked to said chopper signals thereby selectively illuminating a segment of said display means indicating the direction of arrival of the rf signal received by said direction finding antenna array means.

43. The automatic direction finding system of claim 42 wherein said oscillator means comprises a 10.240 MHz crystal synthesized source of clock frequency from said FM receiver of an FM marine radio telephone transceiver.

44. The automatic direction finding system of claim 42 wherein said divider means comprises a divide-by-36 divider.

45. The automatic direction finding system of claim 44 wherein said divider means comprises a divide-by-nine binary counter and a divide-by-four shift register connected to said divide-by-nine binary counter, said divide-by-four shift register outputting a 1.11 kHz signal.

46. The automatic direction finding system of claim 44 wherein said display means comprises a nine-by-four light emitting diode matrix connected to said latch means whereby said diodes are arranged on the circumference of a circle thereby forming a thirty-six segment circular visual directional display to selectively illuminate a segment diode of said matrix indicating the direction of arrival of the received radio frequency signal.

47. The automatic direction finding system of claim 42 which additionally includes means for producing said first and second antenna commutating switching signals at a frequency of about 1 kHz.

48. The automatic direction finding system of claim 42 wherein said switching signal means comprises a divide-by-four circuit connected to the output of said divider means, a counter connected to said divide-by-four circuit, and a plurality of logic gate means connected to said divider means and said divide-by-four circuit means whereby said plurality of logic gate means outputs first and second antenna commutating switching signals, and first and second synchronous detector signals.

49. The automatic direction finding system of claim 48 which additionally includes means for producing said first and second antenna commutating switching signals at about 1 kHz and for alternately switching between said first and second antenna commutating switching signals at about 15 Hz.

50. The automatic direction finding system of claim 49 wherein said switching signal means comprises means for alternately producing said first and second synchronous detector signals at about 1 kHz only during the second halves of the periods of a 15 Hz signal.

51. The automatic direction finding system of claim 42 wherein said sweep display means comprises a pulse swallower means and a counter means whereby said counter means produces an output in phase with said divider means for a predetermined number of counts and then shifts in phase for one count when said pulse swallower swallows a pulse thereby advancing said latch means one count and thereby sequentially advancing a segment of said segmented display means in a circular direction.

52. The automatic direction finding system of claim 42 wherein said latch means comprises quad D clocked latches connected to said divider means.

53. The automatic direction finding system of claim 42 wherein said sweep display means comprises means for advancing said segmented display means at 0.1 to 10 revolutions per second when no radio frequency signal is present.

54. The automatic direction finding system of claim 42 wherein said antenna means comprise pairs of dipole antennas having the center conductor of each dipole antenna spaced a fractional wave length from each of said other dipole antennas.

55. The automatic direction finding system of claim 42 wherein said switching means for each pair of antenna means comprises two PIN switching diodes connected between each of said pair of antenna means and each of said antenna commutating switching signals.

56. The automatic direction finding system of claim 42 wherein said first filter means comprises a 1.11 kHz narrow bandpass filter.

57. The automatic direction finding system of claim 56 wherein said first filter means comprises an active filter and alternating current amplifier connected to said active filter.

58. The automatic direction finding system of claim 42 wherein each of said synchronous detector means comprises a bilateral switch serially connected with an operational amplifier whereby said bilateral switch is switched by said synchronous detector signal of said switching signal means.

59. The automatic direction finding system of claim 58 wherein each of said bilateral switches are CMOS.

60. The automatic direction finding system of claim 42 wherein each of said chopper means comprises a bilateral switch whereby said bilateral switch is switched by said chopper signal of said divider means.

61. The automatic direction finding system of claim 42 wherein said second filter means comprises an active low pass filter whereby said second filter filters out high frequency components.

62. The automatic direction finding system of claim 61 comprising a manually adjustable electronic directional calibration means connected between said second filter means and said sweep signal control means whereby said directional calibration means provides the range of ten degrees of adjustment.

63. The automatic direction finding system of claim 42 wherein said sweep signal control means comprises a crossover detector network means whereby said network means latches said latch means thereby causing said display means to selectively illuminate a segment of said display means to indicate the direction of arrival of the radio frequency signal received by the direction finding antenna array means.

64. The automatic direction finding system of claim 42 comprising signal illumination means connected to said sweep control means whereby said signal illumination means illuminates during reception of a frequency modulated signal.

65. The automatic direction finding system of claim 42 comprising means to hold said latch means in a latched state for a predetermined time period after said received radio frequency signal ceases whereby said predetermined time period allows said display means to hold said selective illuminated segment illuminative thereby holding the direction finding fix on said display means.

66. The automatic direction finding system of claim 65 wherein said hold means comprises a capacitor.

67. The automatic direction finding system of claim 42 wherein said automatic direction finding system is hard wired and mechanically secured with said FM receiver.

68. The automatic direction finding system of claim 42 wherein a coaxial transmission line connects said direction finding antenna array means to said antenna terminal of said FM receiver and two conductors connect said antenna commutating switching signals to said switching means of said direction finding antenna array means.

69. The automatic direction finding system of claim 42 wherein said sweep display means comprises means for causing said sequential illumination of each segment of said display means to operate at a visually perceivable rate when no signal is being received.

70. In a combination of an automatic direction finding system used with an AM receiver having an antenna terminal to receive an amplitude modulated audio frequency signal, having an audio output terminal and generating an automatic gain control signal, said combination comprising:

a. oscillator means to generate a clock source of frequency;

b. divider means connected to said oscillator means to selectively divide said frequency of said oscillator means and generate chopper signals;

c. latch means connected to said divider means to retain a selected count of said divider means;

d. segmented display means connected to said latch means to display the retained count of said latch means;

e. switching signal means connected to said divider means to generate loop antenna commutating switching signals, synchronous detector signals, and a loop commutating signal;

f. sweep display means connected between said oscillator means and said switching signal means and to said latch means to sequentially illuminate said display means in a circular direction when said receiver is not receiving a radio frequency signal of sufficient strength;

g. direction finding antenna array means including two loop antennas means orthogonally positioned with respect to each other and a sense antenna positioned at the intersection of said loop antenna means;

h. loop antenna selector means connected to each of said loop antenna means and switched by said loop antenna commutating switching signals of said switching signal means;

i. loop antenna phase splitter means serially connected to said loop selector means;

j. loop commutator means connected to said phase splitter means and switched by said loop commutating signal generated by said switching signal means;

k. automatic gain control and threshold adjustment means connected to said AM receiver;

l. first filter means connected to said audio output terminal of said AM receiver;

m. synchronous detector means serially connected with buffer means to said first filter means to process phase information for each of said loop antenna means and said synchronous detector means switched by said synchronous detector switching signals of said switching signal means;

n. chopper means connected to each of said buffer means and switched by said chopper signals of said divider means;

o. second filter means connected to said chopper means to integrate and filter said signals from said chopper means outputting a signal having a phase component; and

p. sweep signal control means connected between said second filter means and said latch means to detect a zero crossover of the phase component of said signal from said second filter means whereby said sweep signal control means latches said latch means on a count of said divider means and said divider means phased locked to said chopper signals thereby selectively illuminating said display means indicating the direction of arrival of the rf signal received by said direction finding antenna array means.

71. The automatic direction finding system of claim 70 wherein said divider means comprises a divide-by-36 divider outputting a 1 kHz signal.

72. The automatic direction finding system of claim 70 wherein said switching signal means comprises means for alternately producing said first and second loop antenna commutating switching signals at about 250 Hz during alternate periods of a 15 Hz signal.

73. The automatic direction finding system of claim 72 wherein each of said chopper means comprises a bilateral switch connected to said chopper signal of said divider means.

74. The automatic direction finding system of claim 70 wherein said switching signal means comprises means for alternately producing said first and second synchronous detector signals at about 250 Hz only during the second halves of the periods of a 15 Hz switching signal.

75. The automatic direction finding system of claim 70 wherein said loop antenna means comprises a tuning capacitor connected across each said loop antenna means.

76. The automatic direction finding system of claim 70 wherein said sense antenna means comprises a vertical antenna and a loading coil on said vertical antenna whereby said loading coil provides for a reduced length of said vertical antenna.

77. The automatic direction finding system of claim 70 wherein said automatic gain control and threshold adjustment means comprises an operational amplifier whereby said amplifier adjusts the automatic gain control signal from said AM receiver.

78. The automatic direction finding system of claim 70 which additionally includes a summation amplifier means for summing received ratio signals from said sense antenna with said loop commutated signal from said loop commutator means, with said loop commutated signal being at a reduced injection level to said radio frequency signal from said sense antenna means.

79. The automatic direction finding system of claim 78 wherein said summation amplifier means comprises means for causing said injection level of said loop commutated signal to be about 20 percent of the level of the radio frequency signal from said sense antenna means.

80. The automatic direction finding system of claim 70 wherein said first filter means comprises a 250 hertz low pass filter.

81. The automatic direction finding system of claim 70 wherein said first filter means comprises an active filter in series with an alternating current amplifier connected to said active filter.

82. The automatic direction finding system of claim 70 wherein said automatic direction finding system is hard wired and mechanically secured with said AM receiver.

83. The automatic direction finding system of claim 82 wherein said AM receiver is in a Citizens Radio Service amplitude modulated transceiver.

84. The automatic direction finding system of claim 70 wherein said display means comprises a nine-by-four light emitting diode matrix connected to said latch means whereby said diodes are arranged on the circumference of a circle thereby forming a thirty-six segment circular visual directional display for selective illumination of a segment diode of said matrix indicating the direction of arrival of the received radio frequency signal.

85. The automatic direction finding system of claim 70 wherein said switching signal means comprises a binary counter and a plurality of logic gate means connected to said binary counter to output

(a) first and second loop antenna commutating switching signals, and

(b) first and second synchronous detector signals at the outputs of said logic gate means.

86. The automatic direction finding system of claim 85 wherein said switching signal means comprises means for alternately producing said first and second loop antenna commutating switching signals at about 250 Hz during alternate periods of a 15 Hz signal.

87. The automatic direction finding system of claim 86 wherein said switching signal means further comprises means for alternately producing said first and second synchronous detector signals at about 250 Hz only during the second halves of the periods of a 15 Hz signal.

88. The automatic direction finding system of claim 70 wherein said sweep display means comprises a pulse swallower means and a counter means whereby said counter means produces an output in phase with said divider means for a predetermined number of counts and then shifts in phase for one count when said pulse swallower swallows a pulse thereby advancing said latch means one count and thereby sequentially advancing a segment of said segmented display means in a circular direction.

89. The automatic direction finding system of claim 70 wherein said latch means comprises quad D clocked latches connected to said divider means.

90. The automatic direction finding system of claim 70 wherein said sweep display means comprises means for advancing said segmented display means at 0.1 to 10 revolutions per second when no radio frequency signal is present.

91. The automatic direction finding system of claim 70 wherein each of said synchronous detector means comprises a bilateral switch serially connected with an operational amplifier whereby said bilateral switch is switched by said synchronous detector signal of said switching signal means.

92. The automatic direction finding system of claim 91 wherein each of said bilateral switches are CMOS.

93. An automatic direction finding signal processor, for use in an automatic direction finding system having synchronous detector and chopper switching signals phase locked, which processes a demodulated audio output signal from a receiver having direction finding antenna means, said signal processor comprising:

a. first filter means connected to said audio output terminal of said receiver to filter said carrier phase shifted signal from said audio signal;

b. synchronous detector means serially connected with buffer means to said first filter means to process phase information for each of said means and switched by said synchronous detector antenna means and said synchronous detector switching signals of said automatic direction finding system;

c. chopper means connected to each of said buffer means and switched by said chopper switching signals of said automatic direction finding system;

d. second filter means connected to said chopper means to integrate and filter said signals from said chopper means outputting a signal having a phase shift component indicative of the direction of arrival of the received rf signal; and

e. control means connected to said second filter means to detect a zero crossover of said phase component of said signal whereby said zero crossover point is indicative of the direction of arrival of the radio frequency signal.

94. The automatic direction finding signal processor of claim 93 wherein said first filter means comprises an active L-C filter.

95. The automatic direction finding signal processor of claim 94 wherein said center frequency of said active filter is equal to the frequency of said synchronous detector means.

96. The automatic direction finding signal processor of claim 95 wherein each of said synchronous detector means comprises a bilateral switch serially connected to an operational amplifier whereby said bilateral switches are switched by said synchronous detector switching signals of said automatic direction finding system.

97. The automatic direction finding signal processor of claim 96 wherein said control means comprises an operational amplifier and a diode connected to the output of said operational amplifier whereby said diode is forward biased by said operational amplifier on detecting a zero crossover.

98. The automatic direction finding signal processor of claim 93 which additionally includes means for adjusting the phase of the signal from said second filter means.

99. In apparatus for use in an automatic direction finding system, the combination comprising:

a. indicator means suitable for display of directional information of a received radio frequency signal and including;

(1) an oscillator means to generate a clock source of frequency;

(2) divider means connected to said oscillator means to selectively divide said frequency of said oscillator means and generate phase coherent chopper signals;

(3) latch means connected to said divider means to retain a selected count of said divider means; and

(4) display means connected to said latch means to display the retained count of said latch means; and

b. direction determining means including;

(1) means for obtaining two individual vector value signals which are representative of vector components of the direction of arrival of the received radio frequency signals;

(2) chopper means for chopping said vector value signals in response to said chopper signals and for combining the chopped signals; and

(3) phase detector means responsive to the phase of the combined chopped signal for latching said latch means in synchrony with the phase of the combined chopped signal.

100. The apparatus of claim 99 in which said divider means comprises means for producing said chopper signals in the form of two square wave signals with 90°

phase difference.

101. The apparatus of claim 100 which additionally includes a band pass filter at the frequency of said chopper signals between said chopper means and said phase detector.

102. The apparatus of claim 101 which additionally includes a zero crossing detector in said phase detector.

103. The apparatus of claim 102 in which said divider means comprises means for producing said chopper signals at a frequency of about 1 kHz.

104. The apparatus of claim 99 in which said phase detector means includes a signal threshold detector with feedback which changes the threshold point from a zero crossover point to a defined offset whereby a diminished sine wave input will not vary the result and a direction signal squelch point is achieved for very low input signals.

105. The apparatus of claim 104 in which said threshold detector comprises an operational amplifier having the sine wave input at the inverted input, and having, at the non-inverted input, two equal value resistors and one much smaller resistor, one of said two equal value resistors connecting to the output of said operational amplifier and the other to a supply voltage, and the smaller resistor connecting to a voltage of the zero crossing value.