600-GHz Electronically Tunable Vector Measurement System

The design satisfies a complex set of technical and economic requirements.

A compact, high-dynamic-range, electronically tunable vector measurement system that operates in the frequency range from ≈560 to ≈635 GHz has been developed as a prototype of vector measurement systems that would be suitable for use in nearly-real- time active submillimeter-wave imaging. A judicious choice of intermediate frequencies makes it possible to utilize a significant amount of commercial off-the-shelf communication hardware in this system to keep its cost relatively low. The electronic tunability of this system has been proposed to be utilized in a yet-to-bedeveloped imaging system in which a frequency- dispersive lens would be used to steer transmitted and received beams in one dimension as a function of frequency. Then acquisition of a complete image could be effected by a combination of frequency sweeping for scanning in the aforesaid dimension and mechanical scanning in the perpendicular dimension.

As used here, “vector measurement system” signifies an instrumentation system that applies a radio-frequency (RF) excitation to an object of interest and measures the resulting amplitude and phase response, relative to either the applied excitatory signal or another reference signal related in a known way to applied excitatory signal. In the case of active submillimeter- wave imaging, the RF excitation would be a submillimeter-wavelength signal radiated from an antenna aimed at an object of interest, and the response signal would be a replica of the RF excitation as modified in amplitude and phase by reflection from or transmission through the object.

This Vector Measurement System is made from a combination of commercially available and custom components. The complexity of the reference channel is necessitated by a requirement to cancel phase noise that originates in the microwave synthesizers.
The system is depicted schematically in the figure. The ultimate sources of the RF excitation and reference signals and of local-oscillator (LO) signals for use in down-conversion of the response signal are two compact, inexpensive microwave synthesizers that are electronically tunable over the frequency range from 14 to 18 GHz in increments of 250 kHz. The outputs of both synthesizers are multiplied ×6 in frequency. The resulting signals, having frequencies in the neighborhood of 100 GHz, are amplified ≈20 dB by a pair of monolithic microwave integrated-circuit (MMIC) amplifiers. Then one of the amplified signals is further multiplied ×6 in frequency for use as the RF excitation signal, while the other is further multiplied ×3 in frequency for use as the LO signal in a subharmonically pumped mixer.

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