Vector Network Analyzer (VNA) Tutorial: Basics and Calibration

This tutorial covers the fundamentals of Vector Network Analyzers (VNAs), including their basic operation, transmitter and receiver block diagrams, measurement capabilities, and calibration techniques. We will also explore the differences between Scalar Network Analyzers (SNAs) and VNAs.

Network Analyzer Basics

As we know, a network analyzer is primarily used to measure impedance. At low frequencies, impedance can be easily calculated by dividing voltage by current using instruments like voltmeters, current meters, and sine wave generators. However, this method is not practical at high frequencies.

At higher frequencies, a different technique is employed which measures incident, reflected, and transmitted waves. These measurements are then used to derive various S-parameters. For example, at high frequency, the ratio of reflected signal to incident signal gives us $S_{11}$, also known as G.

Network analyzers come in two main types:

• Scalar Network Analyzer (SNA): Measures only the magnitude of the signal.
• Vector Network Analyzer (VNA): Measures both the magnitude and phase of the signal.

The VNA provides a more complete characterization of the device under test (DUT).

The concept of a VNA can be illustrated using the analogy of light rays incident on a lens. Some of the incident light is reflected by the lens, while the rest is transmitted through it. If there are no obstacles after the lens, the light passes through unimpeded. However, if there is an obstacle, part of the light is reflected back ($A_2$) towards the lens. This reflected light then splits into two parts: one merges with the original reflection ($B_1$), and the other merges with the transmitted light ($B_2$). As a result:

• Reflected wave before the lens = original beam reflected by lens + part of the reflection from the obstacle
• Transmitted wave = original beam transmitted through lens + partial reflected beam from obstacle

This analogy can be applied to the 2-port S-parameter model of an RF device. The figure above depicts a 2-port DUT with S-parameters and four wave types. The waves leaving the device ($b_1, b_2$) are a linear combination of the waves entering the DUT ($a_1, a_2$).

VNA Transmitter and Receiver Block Diagram

VNAs typically measure incident and reflected waves using a series of bridges or couplers, often referred to as signal separation or directional devices. Imperfections in VNA measurements are primarily due to the coupling factor and directivity of these directional devices.

The figure above shows a simplified block diagram of a VNA, highlighting the transmitter and receiver sections. A key component is the source, which provides the stimulus signal used to characterize the response of the DUT. This source is tunable in both frequency and power and can be configured for single-tone or multi-tone operation to support various types of measurements.

The source sweeps across a range of frequencies to collect the frequency response of the DUT. It can also sweep across a range of power values to obtain gain compression results at a fixed continuous wave frequency.

The VNA receiver section (as depicted in the diagram) measures the phase and magnitude of the traveling waves. This is achieved by converting the analog signals into digital form. The diagram shows a single-stage down-converter, which is used to convert the RF signal to an intermediate frequency (IF) before processing the incident and reflected waves.

There are two main architectures for VNAs:

• T/R Set (Transmission/Reflection test set):
• Full S-Parameter Test Set:

VNA Measurements and VNA Calibration

A Vector Network Analyzer (VNA) is used to measure S-parameters of active and passive microwave components. Active components include transistors, amplifiers etc. While passive components include attenuators, cables, isolators, couplers, filters, circulators, bridges, antennas, transformers, duplexers, diplexers, triplexers, switches etc.

VNAs are essential for measuring parameters such as impedance, insertion loss, return loss, and group delay.

VNA calibration is crucial for achieving accurate measurements and minimizing errors. For a 2-port full S-parameter VNA, the following calibration types are commonly used:

• One path, 2 port calibration
• Frequency response calibration
• Full S-parameter calibration

This VNA tutorial provides a foundation for understanding the basics of SNAs and VNAs, and highlights the key differences between scalar and vector network analyzers.