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What are the Basics of a VNA?

August 29, 2023

How does a VNA Work ?


A block diagram is an excellent starting point to understanding vector network analyzer basics. The image below depicts a simplified block diagram for a USB vector network analyzer. 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. A vector network analyzer uses only three active receivers 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.

Typical USB vector network analyzer Block Diagram

Figure 1: Typical VNA Block Diagram Image

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 are the applications of Vector Network Analyzers?

CMT VNAs are being utilized by engineers in many different industries including defense, automotive, materials measurement, medical, broadcasting, and telecommunications. There are many applications for VNAs across these industries. One example is the characterization of RF amplifiers for gain, return loss, P1dB, output match, and stability. All of which are vital to the design and performance of an amplifier.

VNAs are also used to evaluate the properties of an
RF filter. VNAs can be used for s21 parameter insertion loss, or S11 return loss measurements. Antennas are also often evaluated with a VNA. Reflection measurements are most suitable for evaluating antenna performance.

VNAs are commonly used in the production of RF cables
and waveguides as well. 1-Port VNAs are a convenient tool for verifying proper cable performance. Time Domain mode is specifically used to verify characteristic impedance over distance which enables cable manufacturers to check for damage or moisture ingress. Waveguide transmission lines can also be measured with a suitable coaxial to waveguide adapter by using Time Domain.

VNAs are also adept at measuring the dielectric
properties of materials. Millimeter wave VNAs are the best option for this type of measurement. A sheet of a material to be measured is held in a frame between two antennas connected to a VNA as in the image below. 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.

What is the difference between SNA and VNA?

There is often confusion around the functionality of a vector network analyzer vs spectrum analyzer. A USB vector network analyzer (VNA) is very different from a spectrum analyzer (SNA). VNAs are used to measure the transmission and reflection characteristics of a Device Under Test (DUT), while spectrum analyzers are suitable for measuring the amplitudes of frequencies applied to its input. Where an oscilloscope might display the time domain response of a number of input signals, a spectrum analyzer displays the Fourier transform of the signals, showing the distinct amplitude of each frequency component.

A spectrum analyzer is intended to visualize the spectral components of input signals. Its architecture is
optimized for clean visualization of signals without visible side-lobes or spurious signals. A vector network analyzer is intended to only measure its own stimulus signal, and its architecture is optimized for fast measurement speed. A VNA can be used as a rudimentary spectrum analyzer, but it isn’t optimized for the task. To do so would require a much more complicated architecture and would significantly increase the product cost.

Why are VNAs so Expensive?

Many factors contribute to overall network analyzer cost. Vector network analyzers require programmable microwave sources and use high-speed 14-bit ADCs to digitize the IF and an FPGA to implement the DSP requirements to digitally mix the IF down to zero and apply the IF bandwidth filter using a digital FIR filter. These three component types are not inexpensive. The broadband microwave amplifiers needed to boost the sources to a high enough level to serve as Local Oscillator (LO) and Stimulus source add to this cost. If the maximum stimulus level output needs to be +10 or +15 dBm, the actual output of the amplifier needs to be perhaps 3 or 4 dB more than this to account for loss through the measurement bridge. A broadband 9 or 20 GHz amplifier with this non-saturated output power is pricey.

It is the combination of broadband high frequency components like these, plus the FPGA, which overwhelmingly
contribute to the cost of goods for the VNA.

However, if the VNA is developed with integrated microwave VCO/PLL sources rather than a
multiloop phase locked system with separate VCOs and a Direct Digital Synthesizer (DDS) to achieve the small frequency steps, it can be implemented at a lower cost. The economical Compact models from CMT are designed in this way. The Cobalt models use the more sophisticated signal synthesis methods and achieve higher dynamic range as a result.
To read more about an introduction to vector network analyzers, read this article by CMT on the Microwaves and RF website