May 28, 2013
Whenever engineers discuss Vector Network Analyzers (VNAs), the topic of calibration is sure to come up. That’s because without proper calibration, the S-parameter measurements taken by the instrument will not be accurate due to VNA imperfections such as match, directivity, and frequency response. We discussed calibration in a previous post but one area not touched upon was the importance of the VNA configuration.
In almost every case, the VNA settings are used during calibration. Therefore, making sure the VNA is at the desired setting beforehand can be especially helpful. Here are four settings of interest:
1. Frequency Start, Stop and Number of Points – These are the most obvious settings. Segmented sweep must also be set up in advance if a more custom frequency list is desired.
2. IF Bandwidth & Averaging – These parameters control the digital filtering and post-processing that determine the effective noise floor, amount of trace noise, and in some cases, immunity to interfering signals. There is, however, a trade-off for better noise performance: slower sweep speed.
3. Point-by-Point vs. Sweep-by-Sweep Averaging – Point-by-point averaging incurs additional measurements at each given frequency point and increases sweep time roughly proportionally. Because additional measurements are taken simultaneously, the effect is similar to the proportional change in IFBW. Another benefit is the displayed data is fully optimized during the first sweep.
Sweep-by-sweep averaging acquires additional measurements on subsequent sweeps. The result is a gradual shift in trace amplitude. Before extracting data, you must verify that a fully corrected sweep has occurred. This approach is a rolling average, so the time necessary to fully stabilize from a sudden DUT change is roughly proportional to the average count. Consequently, it is an alternate way to improve lower frequency variations.
4. Power – Port power is not as critical if the VNA has receivers with excellent linearity, such as the VectorStar® from Anritsu. Remember that step-attenuator settings must be selected before calibration. Changing the step-attenuator settings alters the RF match and insertion loss in the measurement path; these changes would invalidate the calibration.
Setting the port power to the maximum level before reaching receiver compression provides the widest possible signal-to-noise floor ratio and, as a result, dynamic range. For this reason, be sure to perform the setting before beginning calibration.
Want to learn more? An application note explaining calibration principles, including the various VNA configurations, is available here.
