| Digital and analog microwave phase shifters have been extensively used in frequency translation applications, but they have rarely been used in microwave coherent measurement systems because they usually lack adequate carrier and sideband suppression. Increasing the carrier and sideband suppression on present day phase shifters, would be very expensive and complicated to integrate into microwave integrated circuits (MICs) and monolithic microwave integrated circuits (MMICs). To overcome these limitations, I have developed the first frequency translator based on a nonlinear transmission line (NLTL) phase shifter, and demonstrated its application in a high performance microwave coherent reflectometer. Rather than the more common practice of forming shock waves on the NLTL with large signal excitation, this work uses its voltage-variable delay, together with both amplitude and phase linearization, to modulate the phase of a small 0.5--3.0 GHz microwave signal.;To demonstrate this concept, I designed and constructed a brassboard scale model. I have calculated some important initial parameters of this scale model and done many computer simulations using PSPICE and Libra. After investigating the characteristics of the scale model NLTL, I designed and built additional circuits to compensate delay (phase) nonlinearity and amplitude variation of the output signal from the scale model NLTL, I applied serrodyne (sawtooth) modulation, in which the phase of the RF signal is varied linearly from 0 to 360° with time, to the scale model NLTL to realize a novel distributed frequency translator. The resultant single sideband modulator exhibits record >45 dBc carrier suppression and >50 dBc spurious sideband suppression.;With this high performance frequency translator, I created a coherent microwave measurement system, resulting in an extremely low cost, lightweight and small size system. I designed and built a heterodyne reflectometer for this purpose using this new frequency translator, and it gave comparable performance to current commercial network analyzers. This approach is expected to support coherent microwave measurement techniques in the future because it offers a clear path toward complete integration into MICs and MMICs. It will also have significant applications in other instrumentation and sensors such as THz reflectometers and gas detection systems. |
