TDR-Based Antenna Measurements Allow Fast, Accurate and Cost-Effective Testing of Millimeter-Wave Patch Antennas
By Chris Scholz, Ph.D., Field Applications Engineer, LeCroy Corporation
Prof. Haiying Huang, Assistant Professor, University of Dallas at Arlington

Introduction
Time domain methods, such as time-domain reflectometry/transmission (TDR/T), are well established methods for digital designers, signal integrity engineers, and optical scientists. RF and wireless engineers generally prefer frequency domain methods, such as vector network analysis, as a characterization method. Recent advances in semiconductor processing and wireless sensor development have blurred the clear distinction between the digital and the analog world. As a consequence, new methodologies that combine time domain and frequency domain technologies are necessary.

LeCroy’s WaveExpert digital sampling oscilloscopes (Figure 1) combine time domain and frequency domain analysis in a compact and cost-effective package that enables the development of innovative test and measurement techniques to meet the needs of many analog/RF and digital/signal integrity applications. One such application is the characterization of a dual-band millimeter wave patch antenna used in wireless sensing applications.

The TDR/T analysis and S-parameter measurement package for the WaveExpert provides the capability to perform time-domain and frequency domain measurements up to 20 GHz with a fully integrated TDR/T analysis with S-parameter extraction. The package enables single-ended and true differential S-parameter measurements, automated deskew, and time domain measurements with sub-millimeter resolution to be made with the sampling oscilloscope. Advanced open/short/load (OSLT) calibration removes the effects of cables, fixtures, etc. from measurements for improved accuracy.

The device under test (DUT) is a wireless sensor used to monitor changes in load condition of structural elements, such as airframes. To take full advantage of the sensor, it is essential to measure dynamic changes in it. Due to the required off-line processing, it is not feasible to apply conventional vector network analysis techniques under dynamic operating condition. The sensor under test was developed in the Advanced Sensor Technology Laboratory (ASTL) at the University of Texas, Arlington. A key component of the sensor is a millimeter-wave patch antenna that consists of a 2 mm x 4 mm copper patch on a duroid substrate.

Operating Principle
TDRs operate in the time domain by launching a 20 ps step into the antenna and measuring reflected waveforms. The acquisition window is selected by inspecting the received waveform and adjusting the time window so all reflections of the sensor are visible in the oscilloscope trace. After performing an advanced OSLT calibration, frequency domain scattering parameters (S-parameters) are automatically calculated by the sampling oscilloscope and displayed on the screen.

In contrast, Vector Network Analyzers (VNAs) operate in the frequency domain. A sinusoidal wave is launched into the device. The reflected signal is filtered via a very narrowband filter that accurately tracks the source frequency. If the transmitter and receiver are synchronized and swept in frequency, a steady state frequency response of the sensor can be acquired.

Comparison of Antenna Measurements Using TDR and VNA Measurements
Figure 2 shows a comparison of the S11 parameters acquired using a WaveExpert with the TDR/T and S-parameter package (solid red trace) and a conventional VNA (dashed blue trace). Dips in the reflection of the antenna’s spectrum at around 12 GHz and 17.5 GHz indicate the transmission bands of the antenna.

Both methods yield more than adequate measurements results of the sensor. The main difference between the measurements is the smaller dynamic range of the TDR measurements. For the sensor under test, only relative changes in transmission frequency are important; differences in dynamic range of the two measurement techniques are of lesser importance. One advantage of the WaveExpert is that dynamic changes in the sensor’s characteristics can be measured easily by simply monitoring changes in the oscilloscope’s display. To implement similar capabilities using a traditional VNA, tools to have advanced control of the VNA, off-line data analysis, and extensive digital signal processing would be required.

Conclusion
TDR is a powerful tool when characterizing high-speed wireless components and devices. As the clear distinction between the analog and digital world vanishes, time-domain methods will become valuable tools for wireless and RF engineers to characterize dynamic changes in RF performance. In contrast to traditional vector network analysis, TDR measurements are intuitive and thus become valuable tools in enabling communications between digital, signal integrity and RF engineers.

The primary advantage of a VNA over TDR techniques are in the higher dynamic range of VNA measurements. TDR measurement tools, however, are significantly less expensive than VNAs and meet the requirements of many practical measurement needs.

LeCroy Corporation
www.lecroy.com
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