Synthetic aperture radar (SAR) systems, typically located on board an aircraft or satellite platform, provide SAR imagery of the radar return signals in both the range dimension and the cross-range or azimuth dimension. Range resolution is achieved in a well known manner by using either a high bandwidth fixed frequency transmit pulse or a frequency modulated (FM) transmit pulse. Resolution in the cross-range dimension is achieved by synthesizing a large antenna aperture using the motion of the radar platform. The key to SAR is the data processing of reflected return data. For an overview of SAR, reference is made to “An Introduction To Synthetic Aperture Radar” by W. M. Brown and L. J. Porcello, IEEE Spectrum (September, 1969), pages 52–62.
For optimal performance, the frequency content of relatively high frequency communication signal processing systems, such as those used for generating wideband chirps for SAR, should be as pure as possible, in particular, they should exhibit phase continuity or coherency through the entire output frequency range. Analog synthesizer-based systems, which offer a relatively wide tuning range, suffer from arbitrary phase steps when switching between local oscillators. A direct digital synthesizer (DDS), on the other hand, provides phase continuity with low noise when switching, but is capable of operation within a relatively narrow tuning range (e.g., 100 MHz).
One technique currently used to generate a wideband chirp involves multiplying up the output chirp of a DDS to realize the desired output frequency range of the system. Unfortunately, successive multiplications also multiply noise by the same factor. This problem is compounded because radiation requirements typically limit the choice of DDS to those having relatively low frequency rates, which means that even higher multiplication factors are required. Another conventional approach is to limit the frequency range (width) of the chirp and use receiver processing to resolve phase errors associated with the discontinuities.
One conventional approach is disclosed in U.S. Pat. No. 5,878,335 to Kushner which is directed to a low-power digital frequency synthesizer that combines direct digital frequency synthesis techniques with serrodyne frequency translation principles to produce a wideband frequency response with high spectral purity. A DDS is used to generate a high-resolution analog carrier signal from a low-speed digital clock signal. The carrier signal is phase modulated by a low-resolution signal generated from a high-speed digital clock signal. The modulation signal is a higher frequency signal than the carrier signal, and the phase modulation is accomplished by exact decoded gain elements.