Characterization of Passive Spectral Regrowth in Radio Frequency Systems

Passive spectral regrowth is the generation of nonlinear frequency content as a signal passes through a passive system. The nonlinearity may be a result of the intermodulation between individual signal components or signal components coupling to and mixing with energy in non-electromagnetic domains. Passive spectral regrowth is much weaker than regrowth in active devices due to the lack of gain, however characterization of the new spectral content is growing in importance as the sensitivity of radio-frequency (RF) receivers improves. Passive components at the end of an RF chain, including the antenna, can produce distortion that is difficult to remove using conventional filtering techniques. Alternatively, understanding of the nonlinear mixing may be exploited in RF sensing for unique characterization of remote or difficult-to-reach objects. To better understand passive spectral regrowth, an investigation is undertaken to experimentally and analytically characterize the nonlinear effects of spectral-regrowth-generating phenomena on an RF signal. Detection of low-level passive spectral regrowth close in frequency to a high-power stimulus signal requires extreme dynamic range measurement capability not available on commercial, off-the-shelf equipment. To enable these measurements, a high-dynamic-range nonlinear measurement system is built using analog cancellation. Automated analog cancellation with careful attention to signal path calibration and non-iterative phase calculation is key to achieving up to 140 dB of dynamic range in two-tone characterization with tones separated by at least 1 kHz.;The primary nonlinear mechanism studied is acousto-electromagnetic interaction. Acousto-electromagnetic scattering is conventionally analyzed as introducing a Doppler frequency shift only, but with increasingly sophisticated ability to analyze radar returns, it is necessary to strip away previously held assumptions and characterize other possibly detectable acoustic modulation processes including time-varying path loss, time-varying radar cross section, and amplitude modulation associated with special relativity. The fundamental physical mechanisms and assumptions of RF scattering by a vibrating object are explored for a simple vibrating plate, from which several modulation processes are abstracted and the fundamental limits of what can be sensed are derived. Acousto-electromagnetic analysis is repeated at a higher system level, in determining the modulation appearing on RF signals in an acoustically-stimulated antenna. As RF systems are further integrated, single aperture systems have become more common, thus increasing the importance of understanding passive spectral regrowth in these structures. The dynamic range limits of a single aperture system are explored, and acoustically-induced modulation is demonstrated in a log-periodic dipole array antenna.