VNA FAQ: An Introduction
Engineering is hard. RF engineering is even harder. Complicated concepts, tons of terminology—it’s information overload. Even the simplest of questions have convoluted answers, making it difficult to know how to even get started learning.
My solution? Introducing the “VNA FAQ” blog series! In each blog of this series, I’ll provide bite-sized answers to a handful of related, common VNA questions. So, let’s start at the beginning.
What is electrical network analysis?
In engineering, we define electrical networks as systems of interconnected electrical components. Through electrical network analysis, engineers examine an electrical component’s frequency domain response and impedance characteristics at a particular signal frequency and power.
Fundamentally, vector network analysis measures the incident, reflected, and transmitted waves that travel along transmission lines. A good analogy to think about for this is light striking a clear lens. Of the entire light beam striking the glass, which we call the incident wave, some of the light is reflected from the lens surface. We call the reflected light energy the reflected wave. However, most of the light energy continues through the lens. We call the light that makes it through the lens the transmitted wave. (Figure 1).
Figure 1: Lightwave analogy to high-frequency device characterization.
But if the lens had a mirrored surface, most of the light would reflect, with little or none passing through it. While the wavelengths for radio frequency (RF) and microwave signals differ from that of our example light beam, the principle remains the same. Engineers use vector network analyzers (VNAs) to test electrical network parameters such as signal reflection and transmission.
Essential for RF and microwave component analysis, VNAs critically impact the design and production processes of various passive and active devices including filters, antennas, and power amplifiers. Vector network analyzers accurately measure the incident, reflected, and transmitted energy injected into your device under test (DUT), the energy reflected back down the DUT towards the signal source, and the energy successfully transmitted through the DUT’s terminating port.
How does a VNA work?
A network analyzer conducts component characterization tests by providing calibrated stimulus signals to the RF network, then measuring the vector response over the selected frequency using phase and amplitude information. Network analyzers include both signal sources and receivers. The receivers detect changes to the output signal from a device network and compare that to the source signal input into that device.
To assess the device’s effect on the current and voltage, a VNA measures both the amplitude and phase responses caused by the device. The resulting transmission and reflection measurements, impedance, and s-parameters enable engineers to characterize their DUT.
Why does the DUT’s impact on the input signal matter?
Input signals stimulate device behavior. If the components within the electrical network degrade the integrity of the input signals, that hinders the intended operation of devices further down the signal chain. In communication systems, the signals traveling through the electrical network often carry information content. Engineers not only need to ensure the successful delivery of these information signals but also must maximize the efficiency of their transmission while minimizing signal distortion. In order to successfully operate the electrical system, maximize transmission efficiency, and minimize signal distortion, engineers characterize each of the network component’s effects on the amplitude and phase of swept-frequency and swept-power test signals. Swept signal measurements stimulate the DUT with a series of frequency or power levels instead of just one. Doing so allows engineers to better understand their DUT’s performance across an entire operational range, rather than just a single frequency or power instance.
What’s the most important network analysis measurement?
Here’s one of those simple questions with a complicated answer. In short, measurement “importance” depends on who you ask, and what device or system you ask them about. So, while “the most important network analysis measurement” depends on the particular technical situation, scattering parameters are the go-to, foundational measurement for RF networks. S-parameters tell you how well your DUT transmits an RF signal at a given frequency. S-parameters relate to familiar measurements such as gain, loss, and reflection coefficient. Engineers often use the measured S-parameters of multiple different devices in a cascade to predict overall system performance.
Typically, when you measure a device’s S-parameters, you need to take multiple measurements. The number of S-parameters for a given device equals the square of the number of device ports. For example, a two-port device has four S-parameters. The numbering convention for S-parameters is:
- The first number following the "S" identifies the port at which energy emerges.
- The second number indicates the port at which energy enters.
So, S21 measures the power emerging from port 2 as a result of applying an RF stimulus to port 1. S-parameters with the same number, like S11 or S22, indicate a signal reflection measurement while S-parameters with different numbers, like S21 or S12, indicate signal transmission measurements.
Figure 2: Two-port DUT S-parameter diagram.
Engineers refer to S-parameters measured traveling from port 1 to port 2 as forward S-parameters. To capture forward S-parameters, engineers measure the magnitude and phase of the incident, reflected, and transmitted signals while terminating the output with a load that precisely equals the characteristic impedance of the test system. In the case of a simple two-port network, S11 represents the input complex reflection coefficient or impedance of the DUT, while S21 represents the forward complex transmission coefficient. By placing the source at the output port of the DUT and terminating the input port in a perfect load, it is possible to measure the other two S-parameters, S22 and S12.
Figure 3: Ratio measurement calculations that result in S-parameters.
2-port device S-parameter Summary (Figure 4):
- S11: The S11 reflection coefficient shows the return loss of your amplifier. It tells you the amount of power that reflects from port 1. Reflected power at port 1 necessarily reduces the total power available for transmission (either in the forward or reverse direction) through your device. The reflection coefficient also maps directly to port impedances. When looking at S11 on a Smith Chart, engineers try to create an input-matching network that facilitates the greatest power transfer.
- S12: The S12 transmission coefficient captures the reverse isolation of your amplifier. It reports how much reverse power attenuation occurs in the amplifier. This lets engineers know how much RF energy is traveling the "wrong way" in their system.
- S21: The S21 transmission coefficient indicates the insertion loss or gain of your amplifier. It tells you the transmission ratio (output magnitude/input magnitude) in the forward direction.
- S22: The S22 reflection coefficient shows the signal power reflected at your amplifier’s output. It lets you know how much power transfers through the output port of your amplifier. Engineers use S22 to create an output match network that facilitates the greatest power output from the amplifier.
Figure 4: Scattering parameter measurement on the network analyzer.
Altogether, scattering parameters capture the linear behavior of your DUT at a given frequency. The S-parameters should remain constant regardless of input power.
What advancements are being made in network analysis technology?
While classical VNAs focused on only performing S-parameters, modern VNAs perform an entire instrument rack’s worth of tests! Today, network analyzers perform time domain reflectometry, spectrum analysis, complex distortion analysis, noise figure measurements, and more all from just one box.
Today, we cover some introductory information into the wonderful world of network analysis. We use VNAs to analyze the performance characteristics of electrical networks. While there are many measurements VNAs do, all of which are important in their own way, the foundational network analysis test is scattering parameters.
There’s a lot to learn in the field of RF engineering. Sometimes it seems like too much. Instead of getting overwhelmed and throwing in the towel, check out Keysight University for free educational courses and boot camps. Also, stay tuned for the next “VNA FAQ” post!