Agilent Measurement Journal - Issue 4 - 2008 - (Page 42)

Utilizing ECal modules To ensure temperature stability, ECal modules use a set of solidstate impedance standards on a heated plate. During network analyzer calibration, these solid-state impedance standards are measured with correction off. These raw (uncorrected) measurements are then compared with the expected values of the impedance standards stored on ﬂash memory inside the ECal module. The network analyzer reads the impedance state values and compares them to the measured values. The difference is used to calculate the calibration factors (or error terms). ECal is an excellent option for minimizing human errors during calibration. To use ECal, the module is connected to the network analyzer via USB. Once the ECal module warms up, it is connected to the network analyzer test ports and ECal is selected in the calibration menu. The module automatically senses the port connections and begins its calibration process. The process, which typically takes less than 30 seconds, is highly repeatable and, when properly performed, provides accuracy that rivals many manual calibration techniques.3 Unlike other methods, ECal modules are ﬂexible and can be recharacterized with different connectors by performing the user characterization function available on some VNAs. ECal modules are available with coaxial connectors. After connecting the coaxial-to-waveguide adapters, a user characterization can be performed, enabling the ECal module to behave as a waveguide module (Figure 4).4 Calibration techniques and tradeoffs When deciding which calibration technique to apply, users often consider accuracy versus ease-of-use. Ideally, it is most useful to apply the method that requires the highest accuracy and the lowest possible skill level. Unfortunately, there is a tradeoff between these two factors. Table 2 leverages the results of Agilent research and summarizes the calibration techniques discussed here, comparing required skill level, repeatability, cost and accuracy. The different categories are rated as low (L), medium (M) and high (H). The derivation of these accuracies is beyond the scope of this article; however, details are available from the references listed at the end. In general, the SOLT values provided in Table 2 are for twoport calibrations because this is the most common application. In contrast, one-port SOL calibrations do not require the thru so will typically be slightly more accurate. Subsequently, any uncertainty due to the thru standard would not be included as part of the calibration. Only the data for two-port TRL and its derivatives are provided because TRL calibrations are not applicable to one-port measurements. Examining accuracy considerations Accuracy is the ability of an instrument to measure the actual value within a stated error speciﬁcation. A typical network analyzer is capable of measuring magnitude within hundredths of a decibel and phase within a few millidegrees. This level of accuracy is obtainable only if proper calibration and measurement practices are followed. In other words, the question, “Which calibration technique is the most accurate?” is meaningless if poor calibration or measurement techniques are employed. Indeed, proper connector care, torque, instrument operation and other best practices can have a greater impact on the measurement accuracy of a network analyzer than the selected calibration method. Poor calibration practices can lead to inaccuracies much worse than those shown in Table 2. Figure 4. After adding waveguide adapters, an ECal module can be recharacterized and used as a waveguide module. Network analyzers, broadly speaking, have no inherent guaranteed accuracy. These instruments rely on the measurement of known calibration standards during the calibration process 42 Agilent Measurement Journal

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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