Agilent Measurement Journal - Issue 4 - 2008 - (Page 39)

I Understanding systematic errors in network analyzers Figure 1 summarizes the sources of systematic errors within a typical network analyzer. The ability to measure phase enables VNAs to accurately account for all sources of error. Directivity error affects the accuracy of reﬂection measurements. Isolation error affects the accuracy of transmission measurements. Source and load errors relate to the mismatch between the device-under-test (DUT) and the impedance of the analyzer’s measurement ports. Reﬂection and transmission tracking relate to the difference in frequency response of the analyzer’s reference and measurement receivers. In network analysis, “It depends” is often the accurate — but sometimes unsatisfying — answer to the question, “Which calibration technique is most accurate?” The “best” answer depends on factors such as the application, the required accuracy and the care taken during the calibration process itself. Compared to past-generation vector network analyzers (VNAs), today’s instruments offer more choices in calibration methods. While greater choice is beneﬁcial, it also can lead to greater confusion. Fortunately, a few key points of comparison can quickly narrow the ﬁeld and identify the most appropriate calibration technique. This article discusses common network analyzer calibration techniques and their relative accuracies. The focus is on sound measurement practices and other factors that can enhance accuracy. Isolation Source Source mismatch Directivity Load mismatch Types of calibration With the signiﬁcant processing power and ﬂexibility of today’s network analyzers, the availability of targeted applicationspeciﬁc calibration methods is growing. Examples of applicationspeciﬁc calibration types include mixer/converter calibration (for frequency-offset devices), noise ﬁgure calibration and in-ﬁxture measurements. Full one- and two-port vector calibrations are our focus. As such, vector calibration methods targeted at all sources of error in the network analyzer will be reviewed. These methods provide greater accuracy than approaches such as response calibration that do not consider all error terms. For a discussion of calibration accuracy, we can limit the discussion to a few common calibration types from which most modern methods are derived. In general, there are three commonly used families of calibration techniques: SOLT (shortopen-load-thru), TRL (thru-reﬂect-line) and ECal (electronic calibration) modules. Within each, there are implementations usually targeted at speciﬁc measurement requirements such as broadband frequencies or on-wafer probing. These commonly used calibration techniques are summarized in Table 1 along with the key beneﬁts of each. Reflection tracking Transmission tracking Figure 1. Six systematic errors are present in the forward direction of a network analyzer. Exploring SOLT calibration The ﬁrst calibration method most network analyzer users learn is SOLT. When properly implemented, SOLT calibration can provide excellent accuracy and repeatability. This calibration method requires short, open and load calibration standards. If sexed connectors (e.g., male or female) are on the DUT, then one of each standard is required for male and female connections. Connecting the two measurement planes together makes the thru connection. Agilent Measurement Journal 39

Table of Contents Feed for the Digital Edition of Agilent Measurement Journal - Issue 4 - 2008

Agilent Measurement Journal - Issue 4 - 2008 Delivering Confidence through Compliance with Standards Contents Emerging Innovations Trying Early Device Implementations at the IEEE 1588 PlugFest Achieving Greater Confidence in Measurement Accuracy through Consistency in Calibration Services Overcoming the Challenges of RFID Component Testing 3GPP LTE: Introducing Single- Carrier FDMA Ensuring Reliable Operation and Performance in Converged IP Networks Testing Storage Area Networks and Devices at 8.5-Gb/s Fibre Channel Rates Choosing an Appropriate Calibration Method for Vector Network Analysis Making Traceable EVM Measurements with Digital Oscilloscopes Exploring Terahertz Measurement, Imaging and Spectroscopy: The Electromagnetic Spectrum’s Final Frontier Interpreting Quoted Specifications when Selecting Digitizers

Agilent Measurement Journal - Issue 4 - 2008

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