VNA calibration methods and standards

Importance of VNA calibration

Measurement calibration is the process of removing systematic errors from a measurement system. By connecting specially designed calibration standards at the reference plane – the point where the device under test (DUT) will be attached – VNAs quantify the influence of the test setup and correct subsequent measurements.

Errors are inherent to any measurement system, and VNAs are no exception. These errors fall into three main categories:

• Drift errors: Caused by environmental changes, particularly temperature variations, after calibration.
• Random errors: Resulting from test setup variables such as noise, inconsistent cable connections or user practices.
• Systematic errors: Predictable and consistent errors due to imperfections in the VNA or test setup components, such as cable losses or impedance mismatches.

While drift and random errors can only be minimized through environmental control and good practices, systematic errors can be almost entirely removed through calibration.

It is important to note that measurement calibration is not the same thing as instrument calibration, which verifies that an instrument is functioning within its specifications. Instrument calibration is performed by periodically by a service center, whereas measurement calibration is carried out by the user each time measurements are made.

VNA calibration standards

VNA calibration relies on calibration standards, which are terminators or couplers with precisely known magnitude and phase responses. They are used during the calibration process to quantify and correct the errors introduced by the VNA and the test setup.

These standards are typically delivered as part of a calibration kit, and the data for each standard is stored in calibration kit definition files, which are often preloaded on the VNA or can be imported.

There are four common standards:

• Through (T): Establishes a direct, ideal connection between two ports, providing a baseline for transmission measurements.
• Open (O): Represents an open circuit at the reference plane. However, at higher frequencies, this standard develops capacitance, and this behavior must be account for in the calibration process.
• Short (S): Represents a short circuit and is another standard for reflection measurements. Like the open, its non-idealities must be defined for accuracy.
• Match (M), sometimes called “load”: Provides a termination matched to the characteristic impedance (e.g., 50 ohms), minimizing reflections at the reference plane.

Calibrating with standards can be divided into manual calibration and automatic calibration.

In manual calibration, each standard is manually connected and disconnected at the reference plane in the correct sequence. This method is accurate but time-consuming and prone to operator errors.

In automatic calibration or autocal, standards are built into an autocal unit, which is controlled by the VNA. The unit automatically switches standards at the appropriate points in the routine, significantly speeding up the process and reducing human errors. This is particularly advantageous for multi-port systems, where manual calibration can be labor-intensive.

VNA calibration types

Calibration types in vector network analysis determine the specific standards used and the process of connecting them during the calibration routine.

One-port calibration is used for reflection measurements and can be categorized into two main types:

• Full one-port calibration: Requires sequentially connecting an open, short and match standard at the reference plane. This is the most accurate one-port calibration method but is also time-consuming due to multiple standard connections.
• Reflection normalization: Uses a single standard, either an open or a short standard, to normalize the reflection response. This is faster than full one-port calibration, but it is also less accurate.

Two-port calibration is used for transmission measurements and involves more complex procedures to account for error terms affecting both ports. There are three main types of two-port calibration:

• Transmission normalization: Requires only a single through standard to measure transmission. Calibration can be performed in one or both directions. It is fast but limited in terms of accuracy because it does not address reflection errors at the ports.
• One-path two-port calibration: Combines full one-port calibration on one port with transmission normalization between two ports. It is a hybrid calibration method that strikes a balance between speed and accuracy.
• Full two-port calibration: Comprehensive calibration method that provides the most accurate results for two-port measurements, correcting all reflection and transmission errors for both ports.

Full two-port calibration can again be divided into two types:

• Through-open-short-match (TOSM) method
• Unknown-open-short-match (UOSM) method

TOSM is the standard and most widely used method for full two-port calibration. The process involves performing one-port calibration (open, short and match) on both ports and then connecting a through standard between the two ports and measuring in both directions – a total of eight sweeps are required. This method enables accurate, precise measurements of all S-parameters; however, it can be labor- and time-intensive due to the connection of multiple standards.

UOSM is a variation of TOSM where the known through standard is replaced with an unknown coupler, which must have symmetrical characteristics in both directions. It is particularly useful when the DUT has different connector types (e.g., SMA on one end and N-type on the other) and provides a practical alternative in situations where a standard through is unavailable.