Millimeter-wave frequencies represent tremendous available bandwidth for short-range communications, notably in emerging 5G wireless networks. To enable such communications, compact (mainly integrated) antennas will be needed, and they will require testing at those high frequencies. Many of these antennas will be in the form of monolithic microwave integrated circuits (MMICs), posing new sets of challenges for engineers faced with measuring the performance levels of these integrated antennas.
In search of solutions, several researchers from the Institute of Microwave Engineering at Germany’s Ulm University reviewed some of the challenges in measuring three-dimensional (3D) electromagnetic (EM) fields of on-chip antennas, including the presence of a wafer probe near the fields. Their proposed test system enables measurements of the radiation patterns of integrated antennas at frequencies as high as 280 GHz.
Challenges facing the researchers in developing their measurement system stemmed from the small wavelengths (about 1 mm) of the signals under test. The slightest changes in antenna under test (AUT) and/or probe position will result in phase and amplitude measurement errors. The presence of wafer probes causes reflections in far-field antenna radiation measurements and results in limited scanning areas for near-field radiation measurements. Even bending and movement of test cables can result in changes in test results at millimeter-wave frequencies.
As a solution for positional stability and resolution, a robotic arm was used to control the receive antenna as well as an on-wafer probe for measurements on integrated antennas. The robotic arm works with a vector network analyzer (VNA) and system computer, with the computer coordinating the required positioning data and VNA tuning for each measurement.
The test system is capable of on-the-fly measurements (while an AUT and receive antenna are moving) as well as point-to-point measurements, where the robotic arm moves an AUT to different points along a programmed test path and the VNA is triggered to make measurements at each stopped position. The VNA is equipped with appropriate frequency-converter modules to cover the millimeter-wave frequency band of interest.
The measurement system was used to perform measurements as high as 280 GHz with a horn antenna as the AUT. The robotic control arm provide six degrees of freedom, featuring 350 μm positioning accuracy and 50-μm repeatability. Alignment of the receive AUT can be performed with the aid of a laser range finder. Measurements of the horn antenna at 280 GHz agreed closely with computer simulations. The researchers performed a thorough uncertainty analysis of the measurement system, with calculated measurement uncertainties of less than 0.3 dB—impressive for the frequencies under test.
See “The Challenges of Measuring Integrated Antennas at Millimeter-Wave Frequencies,” IEEE Antennas & Propagation Magazine, Vol. 59, No. 4, August 2017, p. 84.