Vector Network Analyzer Basics
August 31, 2023Vector Network Analyzer Working Principle
A USB vector network analyzer produces an RF stimulus signal which ranges from 9 kHz to 330 GHz with Copper Mountain Technologies VNAs. The signal is then applied to a Device Under Test (DUT) and the output of the DUT can be applied to a second port for analysis. Reflections from the input of the device under test which are sent back to the source port can also be analyzed. The ability to separate signals traveling in opposite directions on a transmission line and produce a measurement is one of the main differences between a VNA and other electronic test devices such as spectrum analyzers or signal analyzers.
For a full 2-port measurement, the stimulus switches back and forth between Ports 1 and 2. When at Port 1, Receiver A, R1, and B are measured. The value A/R1 will produce a raw vna s11 measurement and B/R1 will produce a raw vna s21 measurement. The stimulus then switches to Port 2 where B, R2, and A are measured. B/R2 will be raw S22 and A/R2 will be raw S12. The normalization occurs in an FPGA after the three receiver measurements are made in each direction. Within the FPGA, the IF Bandwidth filtering is applied using DSP and the data is then heavily decimated.
The now low data-rate decimated raw data, S11, S12, S21, and S22 are sent via USB connection to the Copper Mountain Technologies (CMT) VNA software running on the host computer so that calibration can be applied, and the results are displayed in the desired format.
What are the Components of a VNA?
To produce a VNA with outstanding specifications, many individual elements need to be chosen wisely and optimized. Below, we’ll look at some of the most critical components.
1. Directional coupler
The coupler is responsible for creating a reference signal proportional to the output incident signal, so that the measurement result can be accurately shown as a ratio of power transmitted or reflected (S-parameter). A well-built directional coupler can lower the VNA’s noise floor, which results in increased dynamic range. A stable coupler can also maintain its characteristics over temperature, allowing users to perform calibration less frequently while maintaining accuracy of the test result.
2. Mixer
The mixer is another very important component inside the VNA. Modern VNAs normally use a mixer instead of a sampler, which older VNAs used because of its simpler design and lower cost. A good mixer contributes to a low noise floor and minimizes both unwanted spurious responses and trace noise. To make a good mixer, in addition to selecting high quality components, excellent shielding is necessary to minimize crosstalk and enable production of a high dynamic-range instrument. Providing a common and coherent LO to all the mixers is necessary to improve measurement trace noise as well as to reduce LO phase noise.
3. Source
The VNA source is not only an essential module, but also a major contributor to the total instrument cost. In principle, the source can be either external or integrated. The advantage of using an external source is improved purity of the signal because the source can be more completely shielded and isolated from other modules. The advantages of using an integrated source include high sweeping speed, enabling a more compact measurement solution, reduction of cost, and shorter and simpler interconnections between the source and other components. Through proper design of the source and its shielding, a sufficiently clean internal source can be obtained without the drawbacks of using an
4. Attenuator
Most modern VNAs also incorporate a step attenuator between the reference coupler and the test coupler, so that a greater power output range can be achieved compared to purely ALC-based circuits. Adding a step attenuator not only widens the output power range, but also provides a good match to the test port. The attenuator will reduce the difference between the power source match and ratio source match, thereby improving the output port match. Another appealing improvement the step attenuator brings relates to the noise level of signals. The attenuator allows for a large signal in the reference channel even when a small signal is needed at the test port, which will contribute to generation of a low noise signal at the test port.
5. Digital Processing
After the RF components and modules have done their job, signals arrive at the digital section of the VNA for sampling and processing. Because of the high degree of integration and synchronization among the various RF components of the VNA, a dedicated digital processing section is critical for optimization of the system’s performance.
The speed and precision of a VNAs digital processors is vital to overall VNA performance, influencing such specifications as noise floor, maximum measurement speed, and measurement latency. Modern VNAs incorporate advanced FPGAs, high speed DSP chips, or both to accomplish the digital signal processing needed to produce the raw measurement data at a high speed.
The raw measurement data must also be transmitted expeditiously to the application processor, be it an internal processor or external processor in the case of a modular VNA. For example, high speed low latency interfaces such as Ethernet and USB are often used in modern VNAs to shuttle raw results to the application layer.
6. Software & Interfaces
A modern VNA will have user and programming interfaces with the post-processing feature set needed for analysis of results and automation of tests. The graphical user interface typically will be a stand-alone application running in a modern operating system, providing advantages to the user of a stable platform, ease of data transfer to other applications on the same machine, and built-in automation interfaces to other machines and networks.
Network Analyzer Block Diagram
A typical Network analyzer block diagram is shown in Figure 2. The stimulus signal is routed to either Port 1 or Port 2 and passes through a directional bridge on each side. The bridges are capable of separating signals by direction of travel, so the two output ports of the bridges are a sample of the amount of signal leaving the port and the amount entering the port. A mixer on each port of each of the bridges reduces the high stimulus frequency to a much lower IF frequency, usually below 20 MHz. The constant frequency IF is digitized, and a low latency DSP filter is used to apply the IF Bandwidth (IFBW). Only a single conversion is done, not dual as in a spectrum analyzer.
A block diagram such as the one shown above is susceptible to mixer image ambiguity. For instance, if the LO is 10 MHz above the stimulus to create a 10 MHz IF, then the desired response is caused by a signal 10 MHz below the LO, but a signal 10 MHz above will also create the very same IF signal. However, that is not how a VNA operates.
There is only one signal— the stimulus— and it is completely controlled by the USB vector network analyzer and is always at LO-IF in frequency. The image is not an issue for normal VNA measurement, so a single conversion (homodyne) architecture is acceptable.
The receivers in the VNA are also tracking receivers as in an SA, and you can use a VNA as a rudimentary SA by turning off the stimulus signal and observing the signal power measured by one of the receivers— A or B — which respond to signals entering VNA ports one or two respectively.
What Are the Different Type of Vector Network Analyzers?
Network Analyzers can be separated into two categories, based on their form factor. Traditional VNAs, which feature a built-in PC and USB VNAs, which are used with an external PC. Copper Mountain Technologies pioneered Metrology Grade USB VNAs to offer an alternative to the traditional instrumentation. Subsequently, USB VNAs have gradually been adopted by the RF industry and are now produced by various test equipment providers.
USB VNAs separate the measurement module from the processing module, bringing the measurement results to any external PC using the VNA software. The user can take advantage of the latest OS processing power, bigger display, and more reliable performance of an external PC while simplifying the maintenance of the analyzer. USB VNAs are flexible. They can be easily adapted to multiple users and are well-suited for lab, production, field, and secure testing environments.
The biggest advantage of a USB VNA is that it doesn’t lock the user into a built-in computer that is already outdated. Unlike the conventional VNA, with USB instruments, the user can easily upgrade the external PC as needed and a USB VNA has vastly fewer potential points of failure. The most commonly failing part of a conventional VNA is the built-in processing module (on-board computer) and its peripherals – display, control knobs, and buttons. This problem is eliminated by outsourcing signal processing to an external PC, which can be easily and inexpensively replaced by the users according to their needs.
A defining characteristic of a USB VNA is external data storage. The measurement module can be easily and independently shared between multiple users. Because the VNAs generally weigh less than 20 pounds they can be easily moved between environments. USB instruments are much better suited to customization than conventional instruments as they make it easier to change connector type or position, dimensions, and proportions of the housing to meet application-specific needs. With the portable form factor, USB VNAs can also be built into larger test systems.
What Can You Do with a VNA?
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. VNAs are often used to evaluate the properties of an RF filter such as insertion loss and return loss which are important aspects of most RF systems.
Antennas may be evaluated with a VNA too. 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.
It is very common to use a VNA to measure cables and waveguides which is useful 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.
The dielectric properties of materials may also be measured with a VNA. Millimeter wave VNAs are frequently used to perform material measurements. A sheet of material being measured is held in a frame between two antennas connected to a VNA as shown above. 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.
How Expensive is a VNA?
The network analyzer cost will vary greatly depending on several factors. The number of ports on the instrument and the broadband frequency range of the VNA are two of the most important factors when it comes to cost. Another important factor in the price of a network analyzer is the software capabilities. Many software features such as time domain and gating conversion, frequency offset mode, and fixture simulation (embedding/de-embedding), require additional software licensing which can dramatically increase the cost of the system. Fortunately, those features, and many other software features, are included at no additional cost when purchasing a CMT VNA. Finally. because USB VNAs are used with an external PC and do not have a built-in computer, they are a low cost network analyzer.
More on Vector Network Analyzer Basics
VNAs are complex pieces of test equipment, but there are a number of resources available from CMT to try to learn more about them. You can access a couple of webinars on VNA basics here and here. There is another webinar on some tips and tricks for the VNA software available here.