Agilent Measurement Journal - Issue 4 - 2008 - (Page 41)

EX Port 1 a1 b1 ED ES ERF S11 S22 A A S12 A EL S21 A Port 2 ETF b2 Other calibration techniques based on SOLT include offsetting one of the calibration standards. This “offset SOLT” method is effective for waveguide and other high frequency applications. For instance, an offset load can be considered a compound standard consisting of two known offsets (transmission lines) of different lengths and a load element.2 “Quick SOLT” or QSOLT is used in multiport applications where the number of measurement ports on the network analyzer is greater than two. This is sometimes referred to as an N-port solution, where N represents the number of ports. The number of calibration steps is proportional to the number of ports. QSOLT uses mathematical algorithms requiring fewer calibration steps to solve for the full N-port error model with a minimum set of connections. Most SOLT calibrations are performed manually and are relatively easy to implement. Agilent network analyzers provide guided (step-by-step) calibration, which reduces human error and improves repeatability. As with most measurements, however, proper calibration techniques must be practiced to ensure the maximum performance SOLT calibration is capable of providing. Figure 2. The systematic errors shown in Figure 1 are described here by the 12-term error model ﬂow graph. There is one ﬂow graph for the forward direction (six terms shown here) and another for the reverse direction (six more terms). Sliding load Actual directivity Understanding TRL calibration Measured directivity Figure 3. Using a sliding load can improve calibration accuracy of directivity measurements at high frequencies. SOL with reciprocal thru is better known as “unknown thru.” This method allows the use of devices such as cables, circuit board traces or ECal modules as the thru during calibration, as long as a few basic guidelines are followed.1 This method is especially useful when dealing with noninsertable devices — those with same-sex or incompatible connectors that require an adapter to complete the thru connection during calibration. This adapter represents an error in the calibration. Unknown thru is useful for eliminating the need for a precision or calibrated adapter and minimizing cable movement during calibration. It is generally more convenient and accurate than other adapterremoval techniques. TRL calibration is extremely accurate — in most cases more accurate than SOLT calibration. However, few calibration kits contain TRL standards. TRL calibration is most often performed in situations requiring a high level of accuracy when the available calibration standards do not share the same connection type as the DUT. This usually is the case when using test ﬁxtures or making on-wafer measurements with probes. Therefore, in some cases it becomes necessary to construct and characterize standards in the same media type as the DUT conﬁguration. It is easier to manufacture and characterize three TRL standards than the four SOLT standards. TRL calibration has another important advantage: The standards need not be deﬁ ned as completely or accurately as SOLT standards. While SOLT standards are completely characterized and stored as the standard deﬁ nition, TRL standards are modeled rather than being completely characterized. However, TRL calibration accuracy is directly proportional to the quality and repeatability of the TRL standards. Physical discontinuities such as bends in the transmission lines and beads in coaxial structures will degrade TRL calibration. The interface must be clean and allow repeatable connections. Agilent Measurement Journal 41

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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