Coherent processing tunnel diode ultra wideband receiver

• US 5,337,054 A
• Filed: 05/18/1992
• Issued: 08/09/1994
• Est. Priority Date: 05/18/1992
• Status: Expired due to Fees

• Current Assignee: ANRO Engineering Incorporated
• Original Assignee: ANRO Engineering Incorporated
• Inventors: Mara, Richard M., Ross, Gerald F.
• Primary Examiner(s): Sotomayor, John B.
• Application Number:
  US07/885,300
• Time in Patent Office: 813 Days
• Field of Search: 342/93, 342/89, 342/91, 342/92, 342/205
• US Class Current: 342/93
• CPC Class Codes: G01S 7/34 Gain of receiver varied automatically during pulse-recurrence period, e.g. anti-clutter gain control

Abstract

A four-terminal network in tandem with a tunnel diode (TD) threshold receiver currently used in radar or communications improves its sensitivity. Previous inventors have shown that the temperature and sensitivity properties of a conventional TD threshold device used for detecting very short duration bursts of microwave energy would be enhanced by appropriately biasing the TD by a current derived from the thermal noise; the current sets the TD operating point. The magnitude of the current is determined by a constant false alarm rate (CFAR) feedback loop. The subject invention recognizes that a TD changes states (i.e., a detection event) when the area under the current vs. time curve or the charge passing through the device exceeds a prescribed number of picocoulombs. To maximize the charge and improve detection, a form of superheterodyne conversion is introduced to convert the oscillatory short pulse microwave signal received by an antenna to a monopolar baseband signal. This is done by mixing the incoming signal with a CW carrier whose frequency is precisely chosen so that the resulting beat frequency is one-half of an rf cycle for the given duration of the microwave burst. This maximizes the charge available to trigger the TD. Proper gain and filtering is provided in the adjunct four-terminal network to establish minimum noise figure and appropriate gain to drive the TD held in a CFAR loop. It is shown both mathematically and experimentally that the subject invention can increase the receiver sensitivity by a factor of 20 dB or greater over the prior art. Proper microwave gain is provided to establish the noise figure prior to mixing in the added four-terminal network and a bandpass filter is employed to discard frequency components and the gain of a wideband IF (e.g., 0-500 MGz) is selected to appropriately drive the TD.

Claims

1. A system for receiving ultra-wideband (UWB) signals, said system comprising:

means for receiving oscillatory short pulse microwave signal bursts;

preamplifier means coupled to said receiving means for amplifying each of said oscillatory short pulse microwave signal bursts;

oscillator means for generating an attenuated microwave signal at a predetermined frequency;

mixing means, coupled to said preamplifier means and said oscillator means, and responsive to said oscillatory short pulse microwave signal bursts and said attenuated microwave signal at said predetermined frequency for generating an envelope signal equal to the sum and difference between said attenuated microwave signal bursts wherein said predetermined frequency is selected such that a video or baseband pulse at the output of said mixing means has a maximum positive area at some time;

bandpass filter means coupled to said mixing means and responsive to said envelope signal to generate a baseband unipolar pulse for said each of said oscillatory short pulse microwave signal bursts;

IF amplifier means coupled to said bandpass filter means for amplifying each of said baseband unipolar pulses;

tunnel diode means coupled to said IF amplifier means and responsive to each of said amplified baseband unipolar pulses for maximizing the sensitivity of said tunnel diode means by maximizing the charge through a junction since said each of said amplified baseband unipolar pulses provides an envelope of said amplified burst of microwave energy which in its net charge through the junction exceeds a prescribed number of picocoulombs.

2. The system of claim 1 wherein said each of said oscillatory short pulse microwave signal bursts has a one nanosecond or less duration with a nominal band center frequency of 2.5 GHz.

3. The system of claim 1 wherein said predetermined frequency is 2 GHz and the frequency of said baseband unipolar pulses is 500 MHz.

4. The system of claim 1 wherein amplified baseband unipolar pulses have a positive peak in the order of 2 volts.

5. A radar system for detecting a target comprising:

transmitter means for sending oscillatory radar signal bursts toward said target;

antenna means for receiving reflected oscillatory radar signal bursts from said target;

preamplifier means coupled to said antenna means for amplifying said reflected oscillatory radar signal bursts;

envelope generator means coupled to said preamplifier means and responsive to said reflected oscillatory radar signal bursts for generating a unipolar pulse for each of said reflected oscillatory radar signal bursts, said envelope generator means including means for maximizing said unipolar pulse;

a tunnel diode constant false alarm rate (CFAR) receiver coupled to said envelope generator means for receiving each of said unipolar pulses for greater system sensitivity.

6. The system of claim 5 wherein each of said oscillatory radar signal bursts has a one nanosecond or less duration with a nominal band center frequency of 2.5 GHz and has a 10 kHz or greater system pulse repetition rate.

7. A microwave system for detecting a target comprising:

a constant false alarm rate receiver (CFAR) includes,

clock means for generating a free running cyclical clock signal,

timing means coupled to said clock means and responsive to said clock signal for generaling an odd cyclical timing signal representing even cycles,

transmitting means having an active state and a disabled state, said transmitting means being coupled to said timing means and responsive to said odd cyclical timing signal for sending out oscillatory short pulse microwave signal bursts having a predetermined duration on odd cycles when in said active state,

threshold detecting means generating noise spikes when said transmitting means is in said disabled state during said even cycles,

first register means coupled to said timing means and said threshold detecting means and having a first predetermined number of stages for receiving said noise spikes during said first predetermined number of consecutive even cycles and having first means for generating a count of said noise spikes received during said first predetermined number of consecutive even cycles, said threshold detecting means coupled to said first register means for receiving said count, said threshold detecting means including threshold adjusting means for setting said predetermined amplitude so that said count is at a second predetermined number thereby setting a constant bias current; and

envelope generator means coupled to said transmitting means for receiving bursts of several cycles of microwave energy having said predetermined duration when said transmitting means is in said active state and generating an amplified baseband unipolar pulse for each of said bursts, said envelope generator means including a tunnel diode responsive to said unipolar pulse for maximizing the sensitivity of said tunnel diode since the net charge through the junction exceeds a prescribed number of picocoulombs, said envelope generator means including means for maximizing said unipolar pulse;

said threshold detector means coupled to said envelope generator means for receiving each of said unipolar pulses and generating a signal for said each of said unipolar pulses having an amplitude greater than said constant bias current;

second register means, having said predetermined number of stages, coupled to said timing means and said threshold detector means and responsive to said odd cyclical timing signal for receiving said signals during said first predetermined number of said signals is received during said first predetermined number of said odd cycles.

8. The microwave system of claim 7 wherein said first predetermined number is 32, said second predetermined number is 3 or less on an average, and said third predetermined number is arbitrarily set at 27.

9. A microwave system for detecting a target comprising:

means for generating a sequence of oscillatory short pulse microwave signal bursts;

antenna means coupled to said generating means, said antenna means including means for transmitting said sequence of oscillatory short pulse microwave signal bursts to a target area and means for amplifying a sequence of reflected oscillatory short pulse microwave signal bursts reflected from said target;

envelope generator means coupled to said antenna means for converting said amplified sequence of reflected oscillatory short pulse microwave signal bursts received from said antenna means to a sequence of unipolar pulses, each of said unipolar pulses corresponding to a respective reflected oscillatory short pulse microwave signal burst, said envelope generator means including means for maximizing said unipolar pulse;

CFAR means, including tunnel diode means, coupled to said envelope generator means for receiving said sequence of unipolar pulses wherein said each of said unipolar pulses encompasses a greater area than its corresponding oscillatory burst, thereby increasing system sensitivity.

10. A microwave communication system, comprising:

a CFAR receiver for generating a cyclical timing signal;

a microwave transmitter coupled to said CFAR receiver and responsive to said cyclical timing signal for sending information in the form of a sequence of oscillatory short pulse microwave signal bursts;

a microwave link coupled to said transmitter for carrying said sequence of oscillatory short pulse microwave signal bursts;

an envelope generator coupled to said microwave link for receiving said sequence of oscillatory short pulse microwave signal bursts for conversion to a sequence of unipolar pulses;

said CFAR receiver coupled to said envelope generator for receiving said sequence of unipolar pulses and having means for converting said sequence of unipolar pulses to said information, wherein each of said unipolar pulses generates more energy than its corresponding oscillatory burst, thereby increasing system sensitivity.