Specifying Calibration Standards and Kits for Keysight Vector Network Analyzers

Introduction 

Measurement errors in network analysis can be separated into two categories: random and systematic errors. Both random and systematic errors are vector quantities. Random errors are non-repeatable measurement variations and are usually unpredictable. Systematic errors are repeatable measurement variations in the test setup. 

Measurement errors 

Systematic errors include impedance mismatch, system frequency response, and leakage signals in the test setup.  In most microwave measurements, systematic errors are the most significant source of measurement uncertainty. 

The source of these errors can be attributed to the signal separation scheme used. Numerous publications are available on vector network analyzer (VNA) calibration techniques.

Keysight Technologies, Inc. application notes: 1287-1, 1287- 2, and 1287-3 also provide insights on VNAs and VNA error correction. It is recommended that a user be familiar with these calibration techniques and terminologies to get the maximum understanding from this application note.

Measurement calibration

A measurement calibration is a process that mathematically derives the systematic error model for the VNA. This error model is an array of vector error coefficients used to establish a fixed reference plane of zero phase shift, zero reflection magnitude, lossless transmission magnitude, and known impedance. The array of coefficients is computed by measuring a set of “known” devices or calibration standards connected at a fixed measurement plane.

Different calibration techniques are used to solve different error models. The definition of calibration standards and types are set up differently for the applicable calibration techniques. Solving the full 2-port twelve-term error model using the short/open/ load/ thru (SOLT) calibration method is an example of only one of the many measurement calibrations available.

The type of measurement calibration selected by the user depends on the device to be measured (i.e., 1-port or 2-port device) the calibration standards available, and the extent of accuracy enhancement desired. A combination of calibrations can be used in the measurement of a particular device, such as adapter removal calibration for non-insertable devices.

The accuracy of subsequent device measurements depends on the accuracy and stability of the test equipment, the accuracy of the calibration standard model, and the calibration method used in conjunction with the error correction model.

This application note covers calibration standard definitions, calibration kit content, and its structure requirements for Keysight’s vector network analyzers. It also provides some examples of how to set up a new calibration kit and how to modify an existing calibration kit definition file.

Calibration Kits

A mechanical calibration kit consists of a set of physical devices called standards, as shown in Figure 1. Each standard has a precisely known magnitude and phase response as a function of frequency. In order for the VNA to use the standards of a calibration kit, each standard must be “assigned” or organized into standard classes which correspond to the calibration method used by the VNA.

Keysight currently supplies mechanical calibration kits with 1.0-mm, 1.85-mm, 2.4-mm, 3.5-mm, 7-mm, and Type-N 50 ohm, Type-N 75 ohm, Type-FD 75 ohm, and 7-16 coaxial connectors. Rectangular waveguide calibration kits include X, P, K, R, Q, U, V, and W bands.

Calibration for microstrip and other non-coaxial media is described in Product Note 8510-8A: Keysight Network Analysis Applying the 8510 TRL Calibration for Non-Coaxial Measurements, literature number 5091-3645E. A calibration kit may support many calibration methods.

Connector definitions

In addition to calibration standard definitions and standard class assignments, calibration kits also provide definitions of connectors. Keysight’s PNA network analyzer family uses the connector definition to define connector:

• Frequency range 
• Gender (male, female, no gender)
• Impedance 
• Media (coax, waveguide, etc) 
• Cutoff frequency (waveguide
• Height/width ratio (waveguide)

A calibration kit may be defined with multiple connectors. Each 1-port calibration standard must be associated with a defined connector. Two-port standards, such as thrus and adapters, may be associated with two different defined connectors. 

Calibration standards definition

The S-parameters of VNA calibration standards must be defined sufficiently and accurately to satisfy the requirements of the calibration methods for which they will be used. Calibration standards may be defined in various ways. Keysight’s VNAs support two types of calibration standard definitions: calibration coefficient model and data-based model. 

Calibration coefficient model 

The majority of VNAs define calibration standards by using a transmission line model for 2-port standards and a terminated transmission line model for 1-port standards.

The transmission line and termination may be represented by a signal flow graph as illustrated in Figure 4. 

Transmission line characteristic impedance and propagation constants can be derived from the line’s physical properties [9], [10]. 

Keysight’s VNA uses offset delay, offset loss and offset Z0 instead of ZC, and γl to model the transmission line. 

With these “offset” definitions, the VNA can compute the transmission line’s characteristic impedance and, propagation phase and loss constants of the calibration standard without defining the dielectric constant of the calibration standard’s transmission medium which may be different from that of the device under test. This assumes that the offset loss and offset delay values were derived using the same dielectric constant.