Programmable pulse capture device with automatic gain control

• US 20040051859A1
• Filed: 09/12/2002
• Published: 03/18/2004
• Est. Priority Date: 09/12/2002
• Status: Active Grant

• Current Assignee: Lockheed Martin Corporation (Martin Marietta Corporation)
• Original Assignee: Lockheed Martin Corporation (Martin Marietta Corporation)
• Inventors: Flockencier, Stuart W.
• Granted Patent: US 7,312,856 B2
• Time in Patent Office: Days
• US Class Current: 356/4.01
• CPC Class Codes: G01S 17/89 for mapping or imaging
• CPC Class Codes: G01S 7/4868 Controlling received signal intensity or exposure of sensor
• CPC Class Codes: G01S 7/487 Extracting wanted echo signals , e.g. pulse detection
• CPC Class Codes: G01S 7/489 Gain of receiver varied automatically during pulse-recurrence period

Abstract

The invention an optical system and a method for automatically controlling the gain of a receiver in an optical system. The optical system includes an optical receiver, a pulse capture unit, and an automatic gain control. The pulse capture unit includes a capture unit capable of capturing an optical signal received by the optical receiver; and, a process unit capable of processing the captured optical signal. The automatic gain control is capable of controlling the gain of the optical receiver responsive to the content of the processed optical signal. The method includes comparing the intensity of at least one returned pulse, and typically a plurality of returned pulses, to a predetermined value; and controlling the gain of an optical detector responsive to the comparison. In addition, the maximum gain is controlled by a noise limit in some implementations.

Claims

1. An apparatus, comprising:

an optical detector;

a threshold unit capable of converting an analog optical signal received by the optical detector to a digital representation thereof;

a capture unit capable of capturing the digital representation of the received optical signal;

a process unit capable of processing the captured digital representation; and

an automatic gain control capable of controlling the gain of the optical detector responsive to the content of the processed digital representation.

2. The apparatus of claim 1, wherein the optical detector comprises an array of photodiodes.

3. The apparatus of claim 1, wherein the threshold unit includes a bank of voltage comparators.

4. The apparatus of claim 1, wherein the capture unit samples the digitized representation of the received optical signal and encodes the resultant samples proportionally to the peak of the digitized representation at the time it is sampled.

5. The apparatus of claim 1, wherein the capture unit includes:

a time demultiplexer capable of demultiplexing the digitized representation; and

a line encoder capable of encoding the demultiplexed digitized representation.

6. The apparatus of claim 1, further comprising a memory in which the capture unit is capable of buffering the captured digital representation and from which the process unit is capable of reading the buffered captured digital representation.

7. The apparatus of claim 1, wherein the process unit includes:

a convolution circuit capable of generating a filtered signal; and

a peak detect circuit capable of detecting the peak amplitude of the filtered signal.

8. The apparatus of claim 1, wherein the automatic gain control drives the gain higher responsive to determining that the intensity of a return pulse in the optical signal is lower than a predetermined value and drives the gain lower responsive to determining that the intensity of the return pulse in the optical signal is higher than a predetermined value.

9. The apparatus of claim 8, where the automatic gain control includes:

an intensity median computation circuit capable of comparing the intensity of the return pulse to a predetermined value and outputting a first signal indicating the result of the comparison;

an up/down counter capable of receiving the first signal, incrementing and decrementing responsive to the first signal, and outputting a second signal proportional to the count therein;

a digital to analog converter capable of converting the second signal to an analog signal; and

an attenuator receiving the analog signal and attenuating a gain signal responsive to the analog signal.

10. The apparatus of claim 8, further comprising a constant false alarm rate circuit capable of detecting a noise event and clamping the content of the up/down counter.

11. The apparatus of claim 8, wherein the predetermined value is the median intensity value of the processed return pulses.

12. An apparatus, comprising:

means for detecting an optical signal;

means for converting an analog optical signal received by the optical detector to a digital representation thereof;

means for capturing the digital representation of the received optical signal;

means for processing the captured digital representation; and

means for automatically controlling the gain of the detecting means responsive to the content of the processed digital representation.

13. The apparatus of claim 12, wherein the detecting means comprises an array of photodiodes.

14. The apparatus of claim 12, wherein the converting means includes a bank of voltage comparators.

15. The apparatus of claim 12, wherein the capturing means samples the digitized representation of the received optical signal and encodes the resultant samples proportionally to the peak of the digitized representation at the time it is sampled.

16. The apparatus of claim 12, further comprising means for buffering the captured digital representation and from which the processing means is capable of reading the buffered captured digital representation.

17. The apparatus of claim 12, wherein the automatic gain control means drives the gain higher responsive to determining that the intensity of a return pulse in the optical signal is lower than a predetermined value and drives the gain lower responsive to determining that the intensity of the return pulse in the optical signal is higher than a predetermined value.

18. An optical system, comprising:

an optical receiver;

a pulse capture unit, including;

a capture unit capable of capturing an optical signal received by the optical receiver; and

a process unit capable of processing the captured optical signal; and

an automatic gain control capable of controlling the gain of the optical receiver responsive to the content of the processed optical signal.

19. The optical system of claim 18, wherein the optical receiver comprises an array of photodiodes.

20. The optical system of claim 18, wherein the capture unit samples the digitized representation of the received optical signal and encodes the resultant samples proportionally to the peak of the digitized representation at the time it is sampled.

21. The optical system of claim 18, further comprising a memory in which the capture unit is capable of buffering the captured digital representation and from which the process unit is capable of reading the buffered captured digital representation.

22. The optical system of claim 18, wherein the process unit includes:

a convolution circuit capable of generating a filtered signal; and

a peak detect circuit capable of detecting the peak amplitude of the filtered signal.

23. The optical system of claim 18, wherein the automatic gain control drives the gain higher responsive to determining that the intensity of a return pulse in the optical signal is lower than a predetermined value and drives the gain lower responsive to determining that the intensity of the return pulse in the optical signal is higher than a predetermined value.

24. The optical system of claim 18, wherein the optical system comprises a portion of a LADAR system.

25. The optical system of claim 18, wherein the optical receiver comprises a portion of an optical transceiver.

26. The optical system of claim 18, further comprising an automatic target recognition system.

27. A method for automatically controlling the gain of a receiver in an optical system, comprising:

comparing the intensity of a returned pulse to a predetermined value; and

controlling the gain of an optical detector responsive to the comparison.

28. The method of claim 27, wherein comparing the intensity of the returned pulse to the predetermined value includes comparing the intensity to a median value of a plurality of returned pulses.

29. The method of claim 27, wherein controlling the gain of the optical detector includes:

incrementing and decrementing an up/down counter;

converting an output of the counter proportional to the content thereof to an analog signal; and

attenuating a power supply signal proportionally to the amplitude of the analog signal.

30. The method of claim 27, further comprising gating the comparison.

31. The method of claim 30, wherein gating the comparison includes:

enabling the comparison in a time period in which a true return pulse is expected; and

disabling the comparison otherwise.

32. The method of claim 27, further comprising clamping the upper bound of the controlled gain.

33. The method of claim 32, wherein clamping the upper bound includes:

detecting a noise event; and

decreasing the gain.

34. The method of claim 33, wherein decreasing the gain includes:

decrementing an up/down counter;

converting an output of the counter proportional to the content thereof to an analog signal; and

attenuating a power supply signal proportionally to the amplitude of the analog signal.

35. The method of claim 33, further comprising gating the detection.

36. The method of claim 35, wherein gating the comparison includes:

enabling the detection in a time period in which a false returned pulse is expected; and

disabling the detection otherwise.

37. A method comprising:

receiving a plurality of return pulses;

generating an optical signal including the received return pulses;

capturing the return pulses in the optical signal;

processing the captured return pulses; and

automatically controlling the gain in generating the optical signal responsive to the intensity of the processed return pulses and responsive to the detected noise event.

38. The method of claim 37, wherein automatically controlling the gain in generating the optical signal includes:

comparing the intensity of each true return pulse to a predetermined value; and

adjusting the gain of an optical detector responsive to the comparison.

39. The method of claim 37, wherein automatically controlling the gain includes comparing the intensity of the returned pulses to a predetermined value.

40. The method of claim 39, wherein comparing the intensity of the returned pulses to the predetermined value includes comparing the intensity to a median value of a plurality of returned pulses.

41. The method of claim 40, wherein automatically controlling the gain includes:

incrementing and decrementing an up/down counter responsive to the comparison;

converting an output of the counter proportional to the content thereof to an analog signal; and

attenuating a power supply signal proportionally to the amplitude of the analog signal.

42. The method of claim 37, further comprising gating the comparison.

43. The method of claim 37, further comprising clamping the upper bound of the controlled gain.

44. The method of claim 37, further comprising:

generating a train of optical pulses;

scanning a field of view while transmitting the train of optical pulses such that, upon encountering an object in the field of view, they are reflected as the return pulses.

45. The method of claim 37, wherein processing the captured return pulses generates a three-dimensional data set.

46. The method of claim 45, further comprising identifying a target from the three-dimensional data set.

47. The method of claim 37, wherein generating the optical signal includes:

detecting the received return pulses; and

digitizing the detected return pulses.

48. The method of claim 37, wherein capturing the return pulses includes:

sampling the optical signal; and

encoding the samples of the optical signal.

49. The method of claim 48, wherein capturing the return pulses includes buffering the encoded samples.

50. The method of claim 37, wherein processing the captured return pulses includes filtering the optical signal through a finite impulse response filter.

51. The method of claim 50, further comprising linearizing the captured optical signal.