Effective calibration is paramount to accurate vector network analyzer (VNA) measurements. Since the introduction of the VNA around 1970, a number of different calibration routines have been created for VNAs, based on the types of circuit standards used, such as short-openload- reflect (SOLR) to augment the classic short-open-load-thru (SOLT) calibrations. Both of these types of calibrations have a high dependence on the load model and this report introduces a flexible VNA calibration program called VersiCal that enables the use of improved calibration load models for SOLR and SOLT calibrations using loads with complex electrical behavior. The software provides an improved thru model for SOLT calibrations with non-zero-length thru standards, and helps deliver excellent broadband calibration accuracy comparable to a multiline thru-reflect-line (TRL) VNA calibration at higher frequencies, without the low-frequency inaccuracies often associated with TRL calibrations. In addition, filebased descriptions of calibration standards are facilitated so that the test engineer can use eiher measured descriptions, EM analyses, or alternative models for the calibration standard to be used.

Calibration algorithms such as TRL and line-reflect-match (LRM) approaches have been widely adopted for RF/microwave VNAs,^1,2 relegating traditional SOLT calibrations to lower-frequency measurements.³ The main reason for this is the lack of accurate models to characterize the behavior of the lumped short, open and load calibration standards at higher frequencies, especially the load and in some cases the thru standard. Despite the acuracy of TRL and LRM calibrations, TRL will always lose accuracy at some low frequency or at frequencies where delay-line phase shift becomes too close to a multiple of 180 deg. LRM calibrations also generally assume idealized load models.

The behavior of current load and thru standards can differ significantly from the ideal at higher frequencies, degrading the accuracy of a calibration. However, a more exhaustive modeling of the load and thru standards will provide improved calibration results that are comparable to those achieved with multiline TRL calibrations using an algorithm called a complex openshort- load-thru (cSOLT) calibration.⁴ As an extension to this approach, a complex short-open-load-reciprocal (cSOLR) algorithm was developed5 based on an extension of the original SOLR method.6 These new cSOLT and cSOLR solutions are embodied in the VersiCal software, allowing users to achieve excellent calibration integrity across a broadband frequency range. The software uses LabView software from National Instruments and supports most RF/microwave VNAs.

A SOLT algorithm extracts the error terms used for calibration from uncorrected measurements of the four standards and computes the error terms needed to correct the raw data. It requires three reflection standards (usually a short, open, and matching load) to obtain the reflection error terms and a fourth (a direct thru connection between the reference planes) to determine transmission tracking and load match errors. While open and short standards are usually well behaved and can be easily modeled, it is sometimes difficult to find a load for a given testfixture environment that will provide a good impedance match over a wide frequency range. An example of a difficult setup would be using a surfacemount or on-wafer load for calibrations with a hybrid microstrip fixture. Another example is found in commercial coaxial calibration kits whereby the use of a sliding load is available to overcome the difficulty in fabricating a sufficiently low reflection broadband (e.g. > 4 GHz) coaxial load. Unfortunately, the sliding-load approach requires about five additional measurements, adding to the calibration time and complexity. It also requires a precision air-line-based sliding load standard that is not available for onboard or on-wafer calibrations and is too fragile to meet the needs of many calibration environments. As implemented in most VNAs, the load is assumed to be nonreflecting or modeled with a simple series resistive-inductive (RL) network, and the thru standard is modeled with an ideal transmission line model, typically with zero length or as a pure time delay.

A complex load model (Fig. 1) can overcome limitations in traditional SOLT calibrations and yield improved broadband accuracy. ⁴ It may also reduce cost. With the new model, any surface-mount resistor may be used as the load standard, avoiding the laser-trimmed loads of more expensive calibration boards.

Improved VNA calibrations may in some cases also result from use of an enhanced model for the thru standard, when it must be of finite length. In cases where, for example, a probe-tip calibration or a shift in the reference place is necessary, a user must account for the length of the thru standard and some of its physical parameters as part of the calibration. VersiCal provides a model for an imperfect thru standard that will account for conductor losses, does not assume a real and constant characteristic impedance, and can use either the loss tangent or the dielectric conductivity to model dielectric losses along with a magnetic tangent and the conductor self inductance.⁷ This model is very flexible since it does not require any of the physical characteristics of the line. VersiCal also uses the advanced thru model to account for possible offset lengths (with losses) in the reflection standards.

Even with this detail in modeling the thru standard, there will be cases where the behavior of a given thru standard does not fit a model. This can happen when a measurement setup does not allow for a straight connection between reference planes, as with a 90-deg. orientation between input and output connectors. Versi- Cal’s cSOLR algorithm combines the original SOLR method⁶ with more accurate modeling of the load standard. In addition, the SOLR as originally implemented requires an estimate of the electrical length of the device used as reciprocal standard. VersiCal implements an automatic root selection algorithm, which is a variation of that proposed earlier that enables the code to automatically select the right solution for the calibration without any estimation of the length.⁸

VersiCal supports the following calibration algorithms:

• SOLT,³ which is the classical shortopen- load-thru algorithm based on a system of three equations obtained over three measurements of three well-known standards. To obtain transmission error terms, a fourth measurement of a known through standard is required. Typically, the load is assumed to be nonreflecting or modeled with a simple series RL network, and the thru standard is modeled with an ideal transmission line model (typically zero length).
• SOLR,⁶ which was developed by Ferrero and Pisani² and follows the same procedure as SOLT but allows any reciprocal device as a thru standard for calibration purposes.
• cSOLT,⁴ which was developed at USF and provides a more accurate and complex model for the load and thru standards.
• cSOLR,⁵ which implements the SOLR algorithm combined with more accurate/complex models for the load standards to mak cSOLT.
• mSOLT,⁹ which allows the use of standard definitions of any of the models from a measurement (or simulated) file. This allows a user to skip the modeling stage and use any .s2p file as a standard definition.

The VersiCal 3.2 calibration software supports a wide range of VNAs, including the 37xxxD (“Lightning”) and MS446XX (“Scorpion”) series from Anritsu Co., the 87XX and PNA analyzers from Agilent Technologies, the ZVA, ZVB, and ZBT series of VNAs from Rohde & Schwarz, and the older 8510 VNA from Agilent and 360B from Wiltron Co. (now Anritsu). VersiCal is capable of measuring the switch error in order to be able to perform cSOLR. From the previous list only those VNAs with four samplers will support the cSOLR option.

The various algorithms listed earlier can be combined in any form to achieve a VNA calibration. VersiCal supports all the classical frequency-dependent capacitance and inductance models used for open and short standards, respectively, and offset lengths (which can account for possible losses). The software features an intuitive graphical user interface (GUI) for control and data entry (Fig. 2).

VersiCal is structured on three main columns corresponding to the three basic functions of the program: acquire raw data, specify a model, and compute and send error coefficients to the VNA. The first column contains the controls that allow a user to obtain raw data corresponding to standard calibrations from a VNA. Once measured, this data is stored in text files in a disk-drive directory specified by a user. An online VNA is not needed for this step if raw data is already available as an .sp2 file; a user simply provides the name for the .sp2 file.

Continue on Page 2