Vector modulators are used to impose baseband modulation on RF signals, but non- ideal behavior limits the overall performance. The non-ideal behavior of the vector modulator is compensated using data collected with the use of an automated test system driven by a LabVIEW® program that systematically applies thousands of control-signal values to the device under test and collects RF measurement data.

The Automated Test System uses computer-controlled digital-to-analog converters and a VNA to systematically apply I and Q signals to the VMUT, while measuring the RF performance.
The technology innovation automates several steps in the process. First, an automated test system, using computer-controlled digital-to-analog converters (DACs) and a computer-controlled vector network analyzer (VNA) systematically can apply different I and Q signals (which represent the complex number by which the RF signal is multiplied) to the vector modulator under test (VMUT), while measuring the RF performance — specifically, gain and phase (see figure). The automated test system uses the LabVIEW software to control the test equipment, collect the data, and write it to a file. The input to the LabVIEW program is either user-input for systematic variation, or is provided in a file containing specific test values that should be fed to the VMUT. The output file contains both the control signals and the measured data.

The second step is to post-process the file to determine the correction functions as needed. The result of the entire process is a tabular representation, which allows translation of a desired I/Q value to the required analog control signals to produce a particular RF behavior. In some applications, “corrected” performance is needed only for a limited range. If the vector modulator is being used as a phase shifter, there is only a need to correct I and Q values that represent points on a circle, not the entire plane.

This innovation has been used to calibrate 2-GHz MMIC (monolithic microwave integrated circuit) vector modulators in the High EIRP Cluster Array project (EIRP is high effective isotropic radiated power). These calibrations were then used to create correction tables to allow the commanding of the phase shift in each of four channels used as a phased array for beam steering of a Ka-band (32-GHz) signal.

The system also was the basis of a breadboard electronic beam steering system. In this breadboard, the goal was not to make systematic measurements of the properties of a vector modulator, but to drive the breadboard with a series of test patterns varying in phase and amplitude. This is essentially the same calibration process, but with the difference that the data collection process is oriented toward collecting breadboard performance, rather than the measurement of output from a network analyzer.

This work was done by James Lux, Amy Boas, and Samuel Li of Caltech for NASA’s Jet Propulsion Laboratory.

The software used in this innovation is available for commercial licensing. Please contact Karina Edmonds of the California Institute of Technology at (626) 395-2322. Refer to NPO-44518.

This Brief includes a Technical Support Package (TSP).
System for Automated Calibration of Vector Modulators

(reference NPO-44518) is currently available for download from the TSP library.

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This article first appeared in the October, 2009 issue of NASA Tech Briefs Magazine.

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