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Vector Network Analyzer Basics

August 31, 2023

Vector 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 DUT 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 (uncorrected) 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 to R1 or R2 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 vector network analyzer 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 Bridge

The Bridge is responsible for separating the outgoing signal from any incoming signal, possibly a reflection or a direct signal originating from the other port. These incoming signals, A and B, are ratioed to the outgoing signal R1 or R2. A well-built stable Bridge can also maintain its characteristics over temperature, allowing users to perform calibration less frequently while maintaining accuracy of the calibrated result.

  1. Mixer

The mixer is another very important component inside the VNA. The microwave signals from A, B, and R1 or R2 are mixed down to a constant low IF frequency by a Local Oscillator (LO) which tracks the stimulus signal offset by the IF frequency. A good mixer with low conversion loss contributes to a low noise floor and minimizes both unwanted spurious responses and trace noise.

  1. Stimulus and LO Sources

The VNA sources are not only essential modules, but also major contributors to the total instrument cost. The source can be created from a integrated circuit with internal Voltage Controlled Oscillators (VCOs) which are locked to the internal 10 MHz time-base with a fractional-N Phase Locked Loop (PLL). This approach has the lowest material cost but suffers from spurious responses which must be mitigated if they fall within the IF bandwidth of the measurement. The speed to switch from one frequency to the next will depend on the choice of components and higher speed components often cost more. A Direct Digital Synthesis (DDS) circuit can change frequencies very quickly and might be used to improve overall source switching speed. Clearly, there will be tradeoffs between cost and source switching. At high IF bandwidths, the speed of the two sources directly affect the measurement time per point—and the consequent sweep speed—as the frequencies must switch to a new frequency and then settle before a measurement can be made. At lower IF bandwidths the latency of the DSP filter dominates the measurement speed at each frequency.

  1. Attenuators

The stimulus output level of the VNA must have a broad range to be able to measure both amplifiers and low-loss DUTs. It is common to turn the stimulus power down to -30 dBm when measuring a 30 dB amplifier. Depending on the VNA model, signals above 0 dBm into the second port may compress at the mixer, resulting in measurement error. Internal attenuators are employed to reduce the stimulus output level to -50 or -60 dBm minimum. The acceptable operating input level for the VNA will be given in its data sheet.

  1. Digital Processing

The IF signals from the Bridges are digitized by and Analog to Digital Converter (ADC) and the resulting digitized signal is applied to the DSP circuit. Higher IF Bandwidths allow for faster measurement speed—up to the settling time of the Stimulus and LO—but higher IF Bandwidths require a higher IF frequency which requires an ADC which can be clocked faster and a DSP circuit which can handle the higher data rate. So faster measurement speed results in higher component cost.

The DSP takes in the digitized IF signal and mixes it down to a zero frequency IF using Numerically Controlled Oscillators (NCOs). The IF, now centered at zero, is applied to a DSP filter set to the IF Bandwidth of the measurement. There is a delay or latency through the digital filter which is approximately 1.3/IF Bandwidth. For example, a 1 kHz IF Bandwidth might have a 1.3 mS latency. The VNA measurement at each frequency can happen no faster than this. This limit is set by the physics of filtering. After the IF Bandwidth filter has been applied, it is now possible to “decimate” the data. If the original data rate into the DSP filter was 10 Mbits, then after filtering to 1 kHz, only 1 of every 500 points might be kept and the data rate greatly reduced for transport of USB to the host computer for application of calibration and formatting for display.

After decimation, the data rate shipped over the USB bus is not a challenge and presents no bottleneck for overall measurement speed.

  1. Software & Interfaces

Legacy VNAs contained embedded Windows operating systems to post process the measured raw data and a built-in screen to display the results. This design made sense fifteen or so years ago and was a natural evolution from the dedicated microprocessor design which preceded it. But today’s operating systems become obsolete within a year or two and become vulnerable to exploits by hackers. At best the obsolete OS is a challenge for the IT group and at worst, in some defense related industries it may be required to be upgraded to the latest version which generally breaks the linkage to the VNA measurement software, necessitating time-consuming and expensive changes to low-level operating system programming.

With the advent of high-speed data transfer over USB, the natural evolution of the VNA is to move the control, post-processing and display functionality out of the VNA and into custom software running on a host PC. This evolution over the old legacy designs allows the user to operate the VNA with a laptop or desktop computer which may be upgraded as needed. Today’s personal computers have all the horsepower needed for the job. Additionally, if security is a concern, after power down, the VNA contains no setup or measurement information. All sensitive information resides on the user’s PC which is already subject to security protocol. The VNA itself may be safely moved from one secure lab to another.

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 two or more as in a Spectrum Analyzer (SA).

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 Types 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.

Focused Beam Material Measurement System from Compass Technology Group

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 at the sample position. 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, the measurement speed and the broadband frequency range of the VNA are three 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, cost is lower.

What are the Vector Network Analyzer Basics?

VNAs are complex pieces of test equipment, but there are a number of resources available from CMT available 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.