When Advanced RF Calibration is the Answer

WinCal - RF Data Viewer - Golden Die ValidationNetwork-analyzer calibration involves characterizing the entire test fixture so subsequent measurements will pertain to the device under test (DUT) only. To characterize the transmission path of each port, numerous measurements of known standards are performed. An error matrix is computed so it can be applied to subsequent measurements.

When performing an on-wafer RF calibration, there are tradeoffs between the type of standards measured and the mathematical algorithms used to compute the error model. For each technique, certain standards play a role in the overall device characterization. Depending on the particular technique, some standards are known or fully characterized. For some methods, such as SOLR, characteristics of other standards (such as the thru path) are of little significance.

Although there are many on-wafer calibration techniques to choose from, consider the following three techniques and how they differ:

When establishing an on-wafer, RF calibration plan, you must consider what standards will be available within your test environment. For example, it may not be possible to probe a fully characterized, electrically clean, thru path. (In this case, you may wish to select SOLR, as the algorithm does not rely upon the thru for much of its calculations). Circumstances of this sort can quickly help determine which algorithm is optimal for your application.

LRRM (Line Reflect Reflect Match), on the other hand, provides a very high degree of repeatability in one-port and two-port measurements. Parasitic effects associated with the one port standards (Short, Open, Load) are characterized as part of the RF calibration. As a tradeoff, the characteristics of the thru path must not-only be known, but the path itself must be free of significant transmission anomalies.

This advanced-calibration algorithm is best suited for a well-designed probe environment, with proper pad placements on the device. It is advisable to consider the available calibration standards, when choosing a pad layout topology. When pad layout matches available standards, the best on-wafer RF calibration can be achieved. When standards are not available, you may want to consider the tradeoffs of a simpler RF calibration technique.

The LRRM technique is an integral feature of our WinCal XE™ VNA RF calibration software. WinCal provides a unified on-wafer calibration and measurement environment within one software application. After calibration, measurements may be taken from WinCal, or directly from the VNA.

Completing an RF calibration involves numerous steps, including design for calibration, selection of standards, characterization of standards, and finally, the RF calibration step. Depending upon your situation, some steps may be skipped. For example, the characteristics of a known thru path may be available from a datasheet. In this case, steps leading up to thru-path characterization may be bypassed.

For a more detailed analysis of these detailed, advanced RF calibration steps, please download the application note Advanced RF Calibration Techniques with Cascade Microtech’s Pyramid Probe®.

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