RF Scalar and Vector Network Analyzers

The RF network analyzer is an active test instrument which generates a signal that is applied to the device under test, and then it measures the response.

Functions of a RF Network Analyzer

Signal Separation

The signal separation element is often called the test set. It provides two functions:

• Measures a portion of incident signal for rationing. This may be accomplished using a splitter or directional coupler.
• Separate the incident (formed) and reflected (reverse) travelling waves at the input.

Receiver and Signal Detection

When signal is passed through the device under test and separated from the source signal, it is processed in the RF network analyzer so that the results can be gained. The first stage consists of a radio receiver with a demodulator or detector. Typically, a tuned/radio receiver is used to provide the best sensitivity, dynamic range as well as harmonic/spurious signal rejection.

Signal Processing and Display

The processed RF signals from the receiver and detector section are displayed in a format that can be interpreted.

Types of RF Network Analyzers

Network analyzers can be classified as:

• Scalar network analyzer
• Vector network analyzer

Scalar Network Analyzer (SNA)

The scalar network analyzer (SNA) is used to measure the frequency response of any device or a system. It consists of two components:

• Sweep generator
• Spectrum generator

The sweep generator generates constant amplitude sinusoid with varying frequency starting from low to high. The lowest and highest value of the frequency and the time to increase from the minimum to maximum frequency are user configurable.

The spectrum analyzer plots the frequency contents of the input signal. If the output of the sweep generator is connected to the input of the spectrum analyzer (i.e. without connecting to a device under test) then the spectrum analyzer will show a constant horizontal line on the screen. As the amplitude of all the harmonics of the sweep generator output signal are the same, the line will shift vertically, if the amplitude of the sweep generator output signal is increased. Now if, a device under test whose frequency responses are to be obtained is placed in between the sweep generator and the spectrum analyzer, meaning the output of a sweep generator is connected to the input of the device and output of the device is connected to the input of the spectrum analyzer, the spectrum analyzer will directly display the frequency response of the device.

You can also read: Radio Frequency Spectrum Analyser

Vector Network Analyzer (VNA)

A Vector Network Analyzer (VNA) works in almost the same way as the scalar network analyzer, however the difference between the two, is that the former can measure amplitude as well as phase of an RF signal. That is why it is called the vector network analyzer.

The sweep frequency range can be adjusted by the user. The VNA measures the frequency response of the device under test over the adjusted range of sweep frequency. First, the VNA is used to measure the incident test signal from the sweep generator (which is a constant line on the screen), then the reflected signal from the test device, by using a bidirectional coupler and after that the transmitted signal from the RF device under test. Then the VNA automatically reverses the connections to measure the same quantities looking into the device from the opposite direction. After completion of the process, the VNA displays these measured quantities as a function of frequency.

The Vector Network Analyzer (VNA) is often required in RF design applications. This instrument is often used to characterize RF device performance in terms of network scattering parameters or S parameters.