Vector Network Analysis: A Quick Rundown On The Basics

A review of the fundamentals of vector network analysis could provide a necessary understanding of how the electrical response of complex and high frequency devices are measured.

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With the increased complexity and higher-frequency operation of devices, circuits, and systems, there is a critical need to accurately and efficiently characterize these advanced devices at every stage from research and development (R&D) through production.  Measurements of magnitude and phase data—known as vector measurements—allow for a much more detailed analysis of devices, circuits, and systems. Understanding the magnitude and phase responses of a device under test (DUT) over a range of frequencies is a necessary step to do the following: fully characterize linear networks; effectively design impedance-matching networks; measure complex impedance; provide accurate models for computer-aided-engineering (CAE) circuit-simulation programs; perform inverse Fourier transforms; and apply error correction to enhance the accuracy of the measurements. The basic ins-and-outs of vector network analysis are explained in a 15-page application note by Agilent Technologies, “Understanding the Fundamental Principles of Vector Network Analysis.”

Understanding the basics of power transfer through devices is necessary in generating and utilizing data of a DUT’s response. Tools like impedance maps (i.e., Smith Charts) are helpful in visualizing a DUT’s electrical behavior. The flagship device for making these measurements is a vector network analyzer (VNA). Using power waves at various frequencies, the VNA measures the incident power (power sent into the DUT), reflected power, and transmitted power from each external connection of the device (port). 

With impedance-matched connections at each port, the VNA is able to make the vector measurements accurately and compute the characterizing parameters known as scattering parameters (S-parameters).  S-parameters fully describe the input and output characteristics of the DUT based upon power measurements. Other measurement parameters exist, such as H, Y, and Z parameters. But these parameters add more complications to effectively measure the electrical response of a DUT. These parameters can easily be mathematically derived from the S-parameters when needed.

Several key measurements derived from S-parameters are return loss (power lost in reflection), voltage standing wave ratio (VSWR is the normalized power of the RF response over the frequency range), the transmission coefficient (the ratio of transmitted voltage to the incident voltage), and gain/attenuation (the ratio of signal power either gained or lost during transmission through the DUT). Often, the phase response of the DUT contains an additional negative slope (insertion phase) due to the electrical length of the DUT. This effect can be removed by mathematically using the electrical delay feature of the VNA so that the nonlinear phase response of the DUT can be analyzed. Measuring the amount of time the signal travels through the DUT versus frequency (a measurement known as group delay) can provide additional information on the phase distortion of the DUT. 

Signal distortion is a corruption of the frequency response of a communication signal caused by linear and nonlinear effects, which modify the spectral components of the signal. Linear distortion effects include magnitude and phase changes, while nonlinear effects include changes in a signal’s frequency components, such as spurious signal responses. Signal distortion must be considered while designing modern communication systems, as the primary goal is to effectively relay signals containing data. 

Agilent Technologies, 5301 Stevens Creek Blvd., Santa Clara, CA 95051; (408) 345-8886.

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