NOISE REJECTION CIRCUIT

• US 3,747,053 A
• Filed: 08/20/1971
• Issued: 07/17/1973
• Est. Priority Date: 08/20/1971
• Status: Expired due to Term

• Current Assignee: Garland R. Austin
• Original Assignee: Garland R. Austin
• Inventors: Austin, Garland R.
• Primary Examiner(s): Farley, Richard A.
• Application Number:
  US05/173,479
• Time in Patent Office: 697 Days
• Field of Search: 340/1R,3R,5R 343/17.1R 328/111
• US Class Current: 367/109
• CPC Class Codes: G01R 29/0273 the pulse characteristic being duration, i.e. width (indicating that frequency of pulses is above or below a certain limit)
• CPC Class Codes: G01S 7/2921 based on data belonging to one radar period
• CPC Class Codes: G01S 7/527 Extracting wanted echo signals Doppler systems G01S15/50
• CPC Class Codes: Y10S 367/901 Noise or unwanted signal reduction in nonseismic receiving system

Abstract

This invention is concerned with a rejection circuit for discriminating between short radio frequency pulses of different lengths. The circuit operates to delay the leading edges of the pulses by an adjustable time, which is set to be greater than the time duration of the short noise pulses, which are to be rejected, but shorter than the longer signal pulses which are to be passed. The radio frequency pulses are detected and converted to square pulses. These pulses control a switching means that is conducting at all times, except when the pulse occurs. When the switch becomes non-conducting, a capacitor connected across the switch begins to charge. This voltage controls a Schmitt trigger circuit, such that when the voltage reaches a selected threshold value, the Schmitt trigger starts putting out a square pulse. This pulse ends when the detected pulse ends, and the switch starts to conduct, and the capacitor voltage rapidly drops to a low value. By the use of an adjustable resistance, the charging of the capacitor can be slowed down to the point where its voltage never reaches the threshold value in the short time duration of the noise pulses.

Claims

1. In a pulse-width discrimination circuit in which radiofrequency signal pulses of time duration T1 are received at a signal receiving means, and in which radio-frequency noise pulses of time duration T2, are also received, where T1 is greater that T2, and where said noise pulses are discriminated against by pulse-width discrimination means in which the starting times of said pulses are delayed by selected time increments, the improvemnet, comprising:

a. detector means for converting said radio frequency signal and noise pulses into positive, square pulses of time duration T1 and T2 respectively;

b. first switch means connected directly to s,id detector means and responsive to said square pulses, and adapted to be conducting at all times between first and second terminals, except when said positive square-pulses are present;

c. capacitor means connected between said first and second terminals, said first terminal connected to ground and said second terminal connected by a first resistance to a positive voltage, said capacitance adapted to be charged at a selected rate from said voltage through said first resistance while said first switch is not conducting, and to be discharged rapidly through said first switch when said first switch is conducting;

d. Schmitt trigger means, the input thereof responsive to the potential of said capacitance; and

e. means to vary said first resistance;

Whereby the selected rate of charging of said capacitance is varied.

2. The apparatus as in claim 1 in which said Schmitt trigger, in response to the rising potential of said capacitance during charging while said positive square pulse is present, and responsive to its own threshold potential, produces a square pulse output of shorter time duration than said positive square pulse, and including means responsive to said output square pulse to provide a visual indication of the time of occurrence of said output square pulse. 3. The apparatus as in claim 2 including transducer means adapted to receive a sonic radio frequency signal and to produce a corresponding electrical radio frequency signal and first amplifier meas for amplifying said radio frequency signal, said detector means connected to said first amplifier means.

4. The apparatus as in claim 3 including electrical second radio frequency pulse generating means, and means to apply aid second pulses to said transducer means.

5. The apparatus as in claim 4 including means to adjust the time duration of said second generated electrical radio frequency pulses.

6. The apparatus as in claim 5 including rotating means to initiate said pulse generating means and to display said visual indication.

7. A sonar locator apparatus comprising:

a. electrical means to generate repetitive first electrical radio frequency signals of time duration T1, b. transducer means connected to said electrical generating means to produce, in a body of water, sonic radio frequency signals, and to receive and transduce portions of said sonic signals returned from objects in said water, back into second electrical radio frequency signals of time duration T1, and noise radio frequency signals of time duration T2, where T1 is greater than T2, c. detector means for converting said second radio frequency signals and said noise signals into positive square pulses of time duration T1 and T2 respectively, d. first switch means connected directly to said detector means and responsive to said square pulses, and adapted to be conducting at all times between first and second terminals except when said positive sqaure pulses are present;

e. capacitor means connected between said first and second terminals, said first terminal connected to ground and said second terminal connected by a first resistance to a positive voltage, said capacitor adapted to be charged at a selected rate from said voltage through said first resistance while said first switch is not conducting, and to be discharged rapidly through said first switch when said first switch is conducting;

f. Schmitt trigger means, the input thereof responsive to the potential of said capacitor; and

g. means to vary said first resistance, whereby the rate of charging of said capacitor is varied.