US 3787854 A
A noise cancelling self-mixing Doppler type microwave motion detector with greater detecting range and/or lower false alarm rate than prior art self-mixing techniques using equal radiated power is described. The device includes a three terminal microwave oscillator/mixer and a wave guide launched dielectric antenna in a single triaxial structure and a dual input signal processor that provides improved signal to noise ratio as a result of effective signal enhancement and noise cancellation. Separate, out of phase, Doppler output signals are derived from both the emitter and the base of the oscillator/mixer transistor and processed by a balanced amplifier which rejects in-phase noise components.
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United States Patent Friedman et al.
NOISE CANCELLING SELF MIXING DOPPLER RADAR Inventors: Robert L. Friedman, 51-25 Van Kleeck St., Elmhurst, NJ. 11373;
Zeev Lieser, 1048 Coolidge Rd., Elizabeth, NJ. 07208 Filed: Aug. 31, 1972 Appl. No.: 285,251
References Cited UNITED STATES PATENTS Gupta 343/8 Primary Examiner-Benjamin A. Borchelt Assistant-Examiner-G. E. Montone Attorney, Agent, or Firm-Charles E. Temko  ABSTRACT A noise cancelling self-mixing Doppler type microwave motion detector with greater detecting range and/or lower false alarm rate than prior art self-mixing techniques using equal radiated power is described. The device includes a three terminal microwave oscillator/mixer and a wave guide launched dielectric antenna in a single triaxial structure and a dual input signal processor that provides improved signal to noise ratio as a result of effective signal enhancement and noise cancellation. Separate, out of phase, Doppler output signals are derived from both the emitter and the base of the oscillator/mixer transistor and processed by a balanced amplifier which rejects in-phase noise components.
3 Claims, 2 Drawing Figures PAIENIEBJANZZIBH SHEET 1 ll? 2 PAIENIED JAN22 I974 SHEET 2 0F 2 BRIEF DESCRIPTION OF THE PRIOR ART The state of the art in high performance microwave Doppler motion detectors generally utilizes a two or three terminal conventional oscillator, separate radiating and receiving antennas, or a single antenna with a circulator, mixer diode and processing amplifier circuitry arranged into a homodyne or zero IF system.
Economy, lower performance systems utilize the oscillator as a self-mixer. The non-linearity of the oscillator device itself performs the mixing function and allows a single radiating/receiving antenna to be used. Sensitivity of the system in this mode is inferior to the conventional method described above, partially due to the higher noise figure of the oscillating detector which contains greater l/f or flicker noise as compared to a high quality Schottky diode, and also because of the AM noise content of the oscillator which is no longer isolated from the mixer by the injection decoupling of the higher performance system. The derived Doppler output signal is thus superimposed upon these noise components which degrade the signal to noise ratio and so limit the operational distance capability.
BRIEF DESCRIPTION OF THE PRESENT INVENTION Briefly stated, the invention contemplates a low-loss, high dielectric antenna structure which simultaneously serves as a transmitting antenna for a coupled oscillator, CW or pulsed, and as a receiving antenna coupled into the field of said oscillator. Target motion within the range of the disclosed embodiment causes an exhibiting of the Doppler effect such that the reflected and returned signal is shifted in frequency from the fundamental frequency produced by the oscillator and the coupling of the received signal energy into the oscillator field results in a heterodyne action in the oscillator device junctions.
The resultant output produced from said mixing action is available across the impedance presented by any element of the oscillator device, such as the base to ground and emitter to ground resistances normally required for forward biasing purposes, and is at the Doppler frequency resultant of the target motion rate and the transmit carrier frequency. By proportioning the length of the oscillator line elements of the base and the emitter to be one quarter wave length each, at the transmit frequency, it may be seen that a phase inversion is maintained between the base and the emitter for the coupled receive signal as a result of the standing wave pattern on the total one half wave length line element thus created. A balanced, push-pull output is then derived from the self-oscillator/mixer for receive signal Doppler, whereas internal noise components such as l/f noise and power supply ripple develop in-phase output components.
Coupling the derived Doppler and noise voltages into a balanced, push-pull amplifier results in an enhanced output for the Doppler signal, and a cancellation of the noise components. The net result of this action is an improved signal to noise ratio, which allows either greater motion detection distance range or a lower false alarm rate for equal transmit power and receive sensitivity as compared to a conventional system not utilizing the invention.
The invention is also useful for increasing the performance of separate mixer and oscillator systems by improving the signal to noise ratio through the elimination of the oscillator noise power. Either a two or three terminal oscillator may be compensated for by deriving one half of the balanced Doppler output from across an oscillator element and the other half of the balanced Doppler signal from an opposite polarity output separate diode mixer.
BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, to which reference will be made in the specification, similar reference characters have been employed to designate corresponding parts throughout the several views.
FIG. 1 is a schematic wiring diagram of an embodiment of the invention.
FIG. 2 is a schematic sectional view of the antenna structure forming a part of the embodiment, coupled to and from a triaxial structure oscillator.
DETAILED DESCRIPTION OF THE DISCLOSED EMBODIMENT In accordance with the invention, with reference to FIG. 1 of the drawings, the device, generally indicated by reference character 10 includes a step-down transformer 11 powered by a 1 10 volt primary winding 12 and a secondary winding 13 connected to a bridge rectifier 14 to produce 12 volts DC. Current flows through a surge limiting resistor 15 to a ripple filter capacitor 16 and an integrated circuit voltage regulator 17. A resistor 18 is connected to the emitter of a transistor 32, and through a feed through capacitor 20 through the antenna structure 21 to a microwave oscillator transistor 22.
Referring to FIGS. 1 and 2, a coaxial transmission structure includes a collector tube 23, a base tube 24 and an outer shield 25. Feed through capacitors 26 and 27 are interconnected with forward biasing resistors 28 and 29 for the transistor 22. A coupling capacitor 30 and variable resistor 31 form part of a network for the transistor 32, which network includes resistors 33, 29, 38 and 31. A coupling capacitor 35 couples signals from transistor 32 to transistor 19 which is enabled by a forward biasing network including resistors 35a and 35b for the transistor 19.
The common emitter resistor 18 serves the microwave oscillator transistor 22 and the first amplifier transistor 32. A variable resistor 39 provides range i.e., distance control the resistor 31 providing proper D.C. operating level for the above mentioned transistors 19 and 32 Reference characters 38 and 43 designates a low pass filter resistor capacitor network which limits operating bandwidth at llOHz.
The transistor 42 of emitter follower type interconnects with a coupling capacitor 41 to a triggering circuit generally indicated by reference character 44. This circuit includes a resistor 45, a transistor 46, a driver transistor 47, a noise threshold diode 48, resistors 49, 50 and 51; coupling capacitor 52, bias resistor 53, load diode 54 and relay 55. Reference numeral 56 designates a feed back resistor, and resistor 57 and capacitor 57a form a regenerative feed back RC combination for the triggering circuit.
Referring to FIG. 2, the structural aspects of the combination coaxial transmission structure and transmitting and receiving antenna structure are illustrated in greater detail. The effective length of the collector tube 23 is regulated by the inner end 59 of an adjustment screw 60, so that it can be made exactly one quarter wave length (of the operating frequency) long. The opposite end of the collector tube is physically mounted on the casing of the transistor 22, and the base lead 61 is connected to the base tube 24. The emitter lead 62 extends directly into the cavity 63 of the housing 64 which mounts a dielectric rod 65, the length of the emitter lead to the exit point 66 at which it connects with the feed throughcapacitor is also one quarter wave length long. The outer shield slips into a circular mounting member 67 interconnected to the outer surface 68 of the housing, permitting convenient assembly or disassembly, if required. The tubes 23-25 are insulated from each other by spacers 70 and 71 which maintain coaxial alignment.
We wish it to be understood that we do not consider the invention limited to the precise details of structure shown and set forth in this specification, for obvious modifications will occur to those skilled in the art to which the invention pertains.
1. In a self-mixing Doppler radar construction including a coaxial transmission element and a rod antenna extending from a transmitting and receiving cavity, the improvement comprising: said coaxial transmitting element including a first outer coaxial shield, a cavityforming housing, means on a outer surface of said housing for mounting said shield at substantially right angles with respect thereto, a microwave oscillator transistor, a second tube coaxially disposed within said outer shield interconnected to the base of said transistor, a third tube coaxially disposed within said second tube and interconnected to the collector of said transistor; said transistor being disposed within the base of said third tube, and having an emitter lead extending into said cavity in said housing.
2. Structure in accordance with claim 1, further characterized in that said third tube has a length, and said dielectric cavity has a diameter equal to one quarter of the wave length of the operating frequency.
3. Structure in accordance with claim 1, including a second transistor connected in emitter to emitter and base to base relation with respect to said first mentioned transistor, the followers of both transistors being connected to a relay circuit, whereby a received Doppler signal may be of push-pull form, thereby cancelling inherent noise from said signal.