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How Does a Vector Network Analyzer Work?

September 6, 2023

What Does a Vector Network Analyzer Do and How Does a Vector Network Analyzer Work?

A VNA produces an RF stimulus signal which can be in the range of 9 kHz to 330 GHz. That signal can be applied to a Device Under Test (DUT) and the output of the DUT can be applied to a second port for analysis. In addition, any reflections from the input of the DUT which travel back to the source port can be analyzed as well. This is what differentiates the device from other test equipment – it is capable of separating signals traveling in two different directions on a transmission line and measuring them.
how does a vector network analyzer work
The Port 1 and Port 2 bridges shown are the components which separate the forward and reverse traveling RF waves. Receivers R1 and R2 pass only a sample of the wave exiting Ports 1 or 2. Receivers A and B pass only a sample of waves entering Ports 1 or 2. Note that only three receivers are active at any time. Receiver R1 is active when the stimulus is switched to Port 1, and R2 is active when it is switched to Port 2.
When the stimulus signal is switched to Port 1, the magnitudes and phases of the signals on reflection receivers A and B are normalized to stimulus receiver R1. In this way, the absolute output power of the stimulus is irrelevant. Similarly, when the stimulus is switched to Port 2, measurements A and B are normalized to receiver R2.
The performance of a linear DUT can be predicted under any source and load impedance with knowledge of transmitted and reflected signals (S-parameters) from both sides.

What is a Vector Network Analyzer Used For?

Vector Network Analyzers can be used for a variety of applications. For example, a VNA might be used to evaluate an amplifier used in an RF system. RF amplifiers may be characterized for gain, return loss, P1dB, output match, and stability. These characteristics are important to verify when designing an amplifier into a system.
A VNA can be used to evaluate the properties of an RF filter, as filters are an important part of most RF systems. A receiver designed to operate over a set bandwidth will invariably have a filter on the front end to limit the input to those frequencies. If the signal is mixed up or down to another frequency, a filter is required before the mixer to prevent ambiguous mixing at the image frequency.
A VNA can measure the passband insertion loss of a filter, S21, and the return loss at the input, S11. It can also measure the stopbands and VNAs from CMT have exceptional dynamic range which allows them to measure both passband and deep stopbands in one measurement.
A VNA can be used to evaluate an antenna. An antenna should convert a signal on its feedline into radiated RF energy if the frequency is within its operating bandwidth. A reflection measurement is sufficient to evaluate the suitability and health of an antenna. If there is high reflection, then clearly there will be little energy radiated. Conversely, if the reflection back from the antenna is low, it can be assumed that energy is being radiated away. Return loss (S11) or VSWR are the usual measured parameters.
A VNA can be used to measure cables and waveguides. It is very common to use a VNA in the production of RF cables. Handheld portable 1-Port VNAs from Copper Mountain Technologies are conveniently used to verify proper cable performance while it is still on the production machine. Cable feedlines to antennas may be evaluated using the Time Domain mode to verify characteristic impedance over distance to check for damage or moisture ingress.
Waveguide transmission lines can be measured with a suitable coaxial to waveguide adapter. As an example, the long waveguide run from the radar to the transmitting antenna on a naval vessel may be evaluated using the Time Domain feature much like a cable feedline. Dents, dings, or rusted waveguide section bolts can be quickly located.
The dielectric properties of materials may be measured with a VNA. Millimeter wave VNAs may be used to perform material measurements. A sheet of a material to be measured is held in a frame between two antennas connected to a VNA. Two lenses focus the beam to transform the circular wavefronts into plane waves. The dielectric properties of a sheet material may be measured in this way.
This might be important to evaluate a material used for a radome. The complex dielectric constant of the material can be calculated from over-the-air S-parameter measurements. 
Learn more about the uses for vector network analyzers in this article