Communications: directive radio wave systems and devices (e.g. – Base band system
Reexamination Certificate
1999-01-28
2001-02-20
Sotomayor, John B. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Base band system
C342S022000, C342S027000, C342S028000, C342S118000, C342S124000, C342S135000, C342S175000, C342S195000
Reexamination Certificate
active
06191724
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to wide bandwidth pulsed microwave transmitters and receivers, and more particularly to short-range, sub-nanosecond pulse, phase-coherent K-band radars.
2. Description of Related Art
Range measurement of close-range targets is of great interest to a number of industries. Automotive backup warning radar, fluid level sensing in tanks and vats, material level sensing in silos, safety systems, home “do-it-yourself” projects, and aids to the blind are but a few of the applications for short-range non-contact range measurement. Radar range measurement appears to be the technology of choice but has yet to make significant penetration into these markets. The main shortcoming with radar has been the difficulty in realizing a low-cost short-pulse radar with a narrow antenna beam.
A potentially suitable radar is ultra-wideband (UWB) radar, for example, ground penetrating radar as disclosed in U.S. Pat. No. 4,698,634 by Alongi or Micropower Impulse Radar (MIR) in U.S. Pat. No. 5,774,091 by McEwan. UWB radar emits suitably short pulses (<Ins) but has serious drawbacks; its low frequency spectrum can create interference with countless other spectrum users below 3 GHz, and its low frequency spectrum (i.e., long wavelength) prevents narrow antenna beam formation with a compact antenna.
Ultrasound is a potential technology that is both simple and inexpensive. Unfortunately, it is of limited accuracy since the speed of sound varies 10% over outdoor temperatures. Accuracy is of central importance in tank level measurements and construction applications, and 10% accuracy is simply not consistent with modern requirements. Accuracies of 1% to 0.01% are needed. These accuracies can be met with pulse-echo radar using precision timing techniques as will be described herein.
In addition to limited accuracy, ultrasound is susceptible to extraneous acoustic noise, and water or dirt overcoatings on its transducers can disable it. In spite of these limitations, ultrasound has been a popular ranging technology due to its simplicity and its ability to form a narrow beam with a small transducer. A narrow beam is needed to reduce clutter reflections from off-axis objects, such as a tank wall. A narrow beam also implies high antenna gain, which improves signal to noise (S/N) ratio.
While both limited and antiquated, ultrasonic rangefinding remains the dominant non-contact range measurement technology since there have been no real alternatives. One might consider an optical approach to rangefinding, such as a laser rangefinder or a video system. However, optical systems also lack environmental ruggedness—the optics cannot be located behind a decorative panel and can be disabled by an overcoating of water, snow, ice or dirt. Clearly, a better technology is needed.
Radar rangefinders are environmentally rugged: the speed of light (at which radar waves travel) does not vary with temperature (for all practical purposes), and radar waves propagate freely through wood walls, gypsum walls and plastic panels, even with an overcoating of water, ice, snow or dirt.
Pulse-echo radars operating in the 24 GHz band have a wavelength of 12.5 mm, which is almost exactly the same wavelength as 25 KHz ultrasound. Since antenna beamwidth is determined by the wavelength to antenna aperture ratio, radar and ultrasound will have comparably narrow beamwidths with the same antenna/transducer footprint.
An ultrasonic rangefinder may typically transmit a burst of 12 sinusoidal cycles of acoustic energy with a corresponding pulse width that defines the two-object resolution of the system. Of course, its incremental resolution is not a function of emitted pulse width, but that of the timing system. A 24 GHz radar with the same two-object resolution as the 12-cycle ultrasound system needs to transmit a 12 cycle, 0.5-nanosecond sinusoidal burst at 24 GHz, since the wavelengths are comparable. Clearly, the radar needs to have a wide bandwidth, on the order of 1-2 GHz.
Prior art pulse echo radars do not exhibit the combination of 1) K-band RF operation, e.g., 24 GHz, 2) sub-nanosecond RF pulse width, 3) extreme phase coherence (<10-picoseconds for the entire transmit-receive system, 4) expanded time output with ultrasonic parameters, 5) simple assembly with low cost surface mount technology (SMT) components, and 6) commercially appealing size and cost. Clearly, a new technology is needed.
Attempts by the present inventor to develop a 24 GHz radar rangefinder using SMT components were met with frustration and failure—a quarter wavelength at 24 GHz is 3 mm or even less when material dielectric constants are included. Since SMT components have dimensions on the order of 3 mm, wavelength effects are a severe limitation.
One approach to counter the effect of diminishing wavelength is to decrease component size with monolithic technology such as GaAs MMIC (monolithic microwave integrated circuit). Unfortunately, the high cost of GaAs MMIC, about $10 per chip, puts radar in an uncompetitive position relative to ultrasound, which can be fully implemented on a single low cost silicon chip. A pulse-echo radar system with transmit and receive MMICs, and support circuitry might cost $50 to manufacture, after factoring-in expensive assembly techniques for very small high bandwidth components and special circuit board materials. In contrast, a complete ultrasound system can be manufactured for under $5.
SUMMARY OF THE INVENTION
One solution to the cost problem of a radar rangefinder is to employ SMT components with as few microwave semiconductors as possible. To implement this approach, unique wide-bandwidth harmonic techniques were developed for the present invention. Accordingly, a pulsed transmit oscillator operates at a sub-multiple of a transmit frequency and a strong harmonic is extracted for transmission. Similarly, a pulsed local oscillator in the receiver operates at one-half or one-quarter the transmit frequency and drives a harmonic sampler operating at the transmit frequency. Thus, all the critical microwave components operate at frequencies where SMT components are viable, typically at less than 15 GHz.
In a typical radar configuration, the transmit oscillator is connected to an antenna which radiates a short RF burst at a harmonic frequency. Echo bursts are received by a receive antenna and sampled by a harmonic sampler that is driven by a sub-harmonic (to the radiated frequency) RF burst. The timing of the RF burst is slowly swept to produce an equivalent time analog replica of the received echo burst, which can be used to determine target characteristics such as size and range.
In a preferred embodiment, the transmitter of the present invention uses a single, pulsed GaAsFET transistor operating at 12 GHz and frequency doubled to 24 GHz using a resonant antenna and a waveguide beyond cutoff to extract the desired second harmonic. The GaAsFET is the same as that used by “Dish” TV systems, so its cost has been driven down by this popular consumer electronics technology to about $1 in volume.
The receiver in the preferred embodiment uses a silicon transistor operating in a short pulse mode at 6 GHz and effectively frequency quadrupled to 24 GHz by a harmonic sampler. The silicon transistor cost is 15 cents in volume. The only other microwave semiconductor component in the system is the detector diode. The present invention uniquely employs a simple technique to double the bandwidth of commercially available SMT detector diodes, thereby allowing the use of a 70-cent detector diode. With an RF lineup costing under $2, it is quite feasible to manufacture a complete 24 GHz rangefinder for under $5, or about ten times lower than the GaAs MMIC approach.
The present invention emits a short sinusoidal RF burst containing a limited number of cycles, such as 12 RF cycles. Thus, there is a need to generate 500-picosecond wide RF bursts at 24 GHz. As a further constraint, the sinusoidal cycles within the RF burst must be phase coherent with the timing pulses that trigg
Haynes Mark A.
Haynes & Beffel LLP
Sotomayor John B.
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