Waveform synthesis for imaging and ranging applications

Details Waveform synthesis for imaging and ranging applications Waveform synthesis for imaging and ranging applications

2003-05-13

2004-12-07

Lobo, Ian J. (Department: 3662)

Communications: directive radio wave systems and devices (e.g.,

With particular circuit

Complex signal

C342S02500R, C342S195000

Reexamination Certificate

active

06828933

ABSTRACT:

FIELD OF THE INVENTION
The present invention is generally related to signal processing. More particularly, the present invention is related to methods and systems providing waveform synthesis for imaging and ranging applications such as RADAR, SONAR, LIDAR, medical imaging, tomography, and communications applications utilizing spread spectrum modulation/demodulation techniques.
BACKGROUND OF THE INVENTION
Although the present background describes the functionality and limitations of synthetic aperture radar systems or a particular class of communications, such description is merely provided to exemplify a problem capable of resolution with the present invention. Any discussion herein directed to specific radars or communications protocols should not be taken by those skilled in the art as a limitation on the applicability of the invention described herein.
Modern high-performance radar systems often generate signals of extraordinarily wide bandwidth. For example, the General Atomics Lynx Synthetic Aperture Radar (SAR) employs a Linear-FM (LFM) chirp waveform and can operate over 3 GHz bandwidth at a 16.7 GHz center frequency. Furthermore, maximum exploitation of these radar signals requires the generated waveforms to be of very high quality, possessing exceptional spectral purity.
To facilitate high-quality LFM chirp generation, a programmable Digital Waveform Synthesizer (DWS) can often be employed. Use of a GaAs ASIC has been shown to implement well known double accumulator architectures to generate a phase that is quadratic with time, a phase that is generally converted by a memory look-up table to a digital sinusoidal signal and is ultimately converted to an analog signal by a Digital to Analog Converter (DAC). Furthermore, an ability to predistort the phase of the output as a function of instantaneous frequency to correct for unspecified nonlinearities of subsequent components in the signal path has previously been explored. No calibration scheme, however, has been presented for determining correction factors.
Two principal architectures are presently employed for achieving LFM chirp generation. The first architecture
100
is referred to as single-ended output operation, and is illustrated in
FIG. 1
(labeled as prior art). With this architecture a single signal output is generated by the DWS
110
and presented to subsequent components in the signal path. After mixing
115
with a Local Oscillator (LO)
120
signal the nature of a single-ended DWS signal is to generate the desired signal as well as an undesired mirror-image signal, which must be filtered by analog components in the signal path. This filter is often called a sideband filter
130
. Consequently, desired and undesired signals are separated by frequency; limiting the usable bandwidth for a generated signal to something less than half the DWS clock frequency. Equivalently, a clock frequency of more than twice the highest output waveform frequency is required. Proper final system bandwidth is achieved through a frequency multiplier
140
.
Multiplexing of multiple parallel chirp generators can allow wide bandwidth single-ended chirps to be generated with commercial silicon Field Programmable Gate Array (FPGA) components. Frequency multiplication can also be employed to widen the bandwidth of a single-ended DWS output signal, but often to the detriment of spectral purity. It is also well known that frequency multiplication raises undesired frequency spurs by 6 dB per doubling with respect to the desired signal level. Frequency spurs, however, are undesired signal perturbations caused by quantization effects and DAC residual nonlinearities. Consequently, minimizing the frequency multiplication factor that can be applied to a DWS output can enhance spectral purity.
A second architecture that has been used for achieving LFM chirp generation quality can be referred to as balanced or quadrature modulator operation. Such architecture is generally illustrated in
FIG. 2
(also labeled as prior art). With this architecture
200
, two output signals are generated by the Quadrature DWS (QDWS)
210
and presented to a Single Sideband (SSB) mixer
220
where they are combined
215
to form a single signal to the subsequent signal path. In a perfect system, the two signals generated by the QDWS
210
will differ by a constant 90 degrees of phase, and are termed the In-phase (I) and Quadrature-phase (Q) signals. The signal pair together can be generally referred to in the art as Quadrature signals. In a perfect SSB mixer
220
, no mirror-image signal will be generated, obviating the need for a sideband filter. Furthermore, no spectral separation between desired and a nonexistent undesired signal would need to be maintained. Consequently, the QDWS
210
output bandwidth of the desired signal would be able to approach the QDWS clock frequency itself, which is twice the bandwidth of the single-ended DWS system. This, in turn, would require half the frequency multiplication
230
when compared to a single-ended DWS system to achieve a final system bandwidth, and include attendant 6 dB lower spur levels and better spectral purity.
SSB signal generation techniques, including the employment of quadrature signals, are generally known in the art. Quadrature signals can be generated by a variety of techniques, including Hilbert filters that generate a 90-degree phase shift for all input waveform frequencies, and directly by separate memory look-up tables within the digital signal generation portion of the QDWS. The precision with which quadrature signals can be generated and combined in a SSB mixer, however, is problematic, particularly for high-dynamic-range applications such as imaging radar systems. Imperfections in quadrature signal generation or their combination within a SSB mixer results in the non-cancellation of the undesired mirror-image sideband signal. Such imperfections can result in a relative phase error or an amplitude imbalance. Additionally, the LO
120
may undesirably leak through the mixer and be present in the mixer output in addition to the desired signal. Any of these errors reduce the spectral purity of the resulting SSB mixer output signal, and degrades a SAR image with ghosts and other artifacts. Consequently, quadrature modulators in high-performance radar systems require some form of error cancellation or other mitigation scheme.
In the field of communications, a quadrature modulator for wireless CDMA systems has been described wherein amplitude and phase of the quadrature component signals are predistorted to provide perfect quadrature signals to the SSB mixer. Furthermore, DC biases are added to the quadrature output signals to mitigate LO leakage. The corrections, however, are derived for a single QDWS output frequency and do not allow for frequency dependent errors. While this may be reasonable for applications such as wireless communications, it is inadequate for high-performance SAR systems. Furthermore, no attempt is made to compensate imbalances in the SSB mixer itself, and the stated procedure precludes this.
An iterative procedure to adjust for LO leakage, and phase and gain imbalance in a quadrature modulator based on an envelope detection-of a transmitted signal has also been described by the prior art. Other prior art techniques include: use of a similar technique to compensate an SSB mixer-which degrades a resultant output signal and adaptive techniques for achieving quadrature signal balance using a test tone. The adaptation procedures currently described in the prior art is not adequate for wideband LFM chirps. Furthermore, verified wideband frequency-dependent errors are not addressed by the prior art.
A quadrature modulator that allows frequency-dependent phase and amplitude corrections to be made to the output of the QDWS has been proposed. All corrections are made to analog signals after the DACs. These corrections, however, neglect problematic frequency dependent errors in the SSB mixer. Furthermore, the nature of the errors to be corrected is presumed to be predetermined—that is, no

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