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Considerations In Choosing A Network Analyzer



February 17, 2016

The Network Analyzer

Essential in the development of RF circuits and systems, the network analyzer is a very specialized piece of test equipment. Combining a precise signal source and sensitive receivers, the network analyzer displays the amount of energy that is transmitted or reflected by a network. The term “network” refers to any linear circuit or system that signals travel through. This can include antenna arrays of all types also. Just because there is not a physical connection between the source and receiver doesn’t mean it is not a system! Network analyzers are useful for electrical engineering, electronics engineering, and communication systems design, and especially for microwave engineering. They are also known as a VNA, or vector network analyzer. It should also be pointed out that this article is not referring to computer networks, only electrical networks comprised of devices and components.

 

What Do Network Analyzers Test?

The types of devices that a network analyzer can test range from passive devices like filters to active modules like amplifiers. Network analyzers can characterize practically anything that conducts or radiates. A partial list of passive and active examples is shown below. Both frequency and power can be swept.

 

Types Of Test Articles:

Passive

Active

• Duplexers

• Transceivers

• Delay lines

• LNAs

• Filters

• Converters

• Isolators

• Amplifiers

• Adapters

• Antennas

• Cables

• Receivers

• Waveguide

• Tuners

• Transmission lines

• Modulators

• Bridges

• Transistors

  

 

Network Analyzer Types & Technology

The two basic types of network analyzers are:

• Scalar network analyzer (SNA) - measures amplitude properties only

• Vector network analyzer (VNA) - measures both amplitude and phase properties

An SNA is functionally equivalent to a spectrum analyzer and tracking generator. Network analyzers also come in several types that are differentiated by their frequency range and number of ports. Some can have as many as 16 ports, but the most common number of ports are two and four. A four-port network analyzer has the ability to analyze differential networks, which is important for low noise systems and cellphones. All network analyzers will have 50-ohm real impedance but can mathematically normalize the displayed result to any impedance you wish. Since network analyzers have their own signal source they will typically have at a minimum two test connections labeled “ports”. They use a notation known as scattering parameters, aka “S-parameters”, to represent signals sent and received, also referred to as incident and reflected waves. Network analysis deals with the accurate measurement of the ratios of these signals. Scattering parameters or S-parameters (the elements of a scattering matrix or S-matrix) describe the electrical behavior of linear electrical networks when undergoing various steady state stimuli by electrical signals. S-parameters are inherently complex, (z = x +/- jy, or z = rejQ) linear values, generally expressed in log magnitude format. Since the circuit under test is treated as a black box, linear matrix algebra is used to mathematically manipulate the measurements.

 

Nomenclature

In a two-port analyzer you will have four possible signals to display. They are known as S11, S12, S21, and S22. They represent the amount of signal sent from port 1 reflected back to port 1, or S11. Accordingly S21 is the amount of signal received, or incident, at port 1 from port 2. S12 is the reciprocal of S21 in that it represents the amount of signal received at port 2 sent from port 1. And lastly S22 is the amount of signal reflected back to port 2 that originated from port 2.

 

Network Analyzer Measurement Types

The types of measurements that can be displayed are return loss, transmission, phase, impedance, and real and imaginary amplitude. Values can be displayed in dBm, degrees, ohms, linear, VSWR, and log magnitude.

 

Network Analyzer Applications

There are several measurements that only a network analyzer can make. Perhaps the most interesting is impedance vs. frequency using a Smith chart display.[1] This will show how test article impedance changes with frequency. This is useful for matching amplifier input and output impedances for maximum power transfer or minimum noise performance. Another useful application is using the S parameters of S11, S12, S21, and S22 to measure the linear performance of an amplifier.

 

Some of the linear parameters you can measure are:

• Gain

• Gain flatness

• Group delay

• Linear phase response

• Reverse isolation

 

Some of the non-linear measurements that can be made are:

• 1 dB compression

• Amplitude modulation to phase modulation, (AM/PM)

• Amplitude modulation to amplitude modulation conversion, (AM/AM)

• Swept harmonic distortion, very useful

• Intermodulation distortion, third order intercept (TOI)

 

This can be applied to anything from a simple single transistor buffer to a complete amplifier module. VNAs are used to characterize RF mixer also. The parameters that are important in the performance of mixers that VNAs measure are:

• Mixer conversion phase

• Mixer conversion loss

• Mixer LO feedthru

• IF frequency flatness

 

Perhaps the most valuable measurement capability that a VNA has is the ability to measure and characterize differential circuits. Because of differential circuits ability to reject noise they are essential in the design of cell phones. VNAs can measure important parameters such as common mode rejection, which causes noise and signal degradation, critical in the design of receiver front ends. With a four port VNA, all of the important parameters can be derived and displayed at one time. This ability to display many results simultaneously greatly enhances design cycle time by enabling engineers to see end-to-end performance on one screen.

 

The Importance Of Network Analyzer Calibration

Because network analyzers require phase information in order to be accurate, calibration must be performed to take out all of the error terms. This is a critical step in order to ensure you are measuring only the device under test (DUT) and not the test fixture. One of the most common calibration techniques is called SOLT, which stands for short, open, load, and through. By connecting these terminations in turn at the interface of the DUT, you will systematically correct the errors that are induced by the cabling and adapters to the VNA.

 

Synopsis

In choosing which type network analyzer you need, you need to determine what the upper and lower frequency range is of the system you will be measuring. You also need to understand what type of measurement you need to make.[2] Network analyzers, or VNAs, which include a phase matched reference source, makes them an incredibly powerful measuring instrument. They are essential tools for many production and especially R&D environments.

Since 2005, Axiom Test Equipment has been renting and selling a variety of network analyzers. Additionally, we offer repair on most brands and types of Network Analyzers. Major manufacturers we carry include Anritsu, Rhode & Schwarz, and Keysight (formerly Agilent). Axiom Test Equipment offers a complete range of network analyzers for your needs, with frequencies ranging from 100 kHz up to more than 50 GHz.

 

View these online at http://www.axiomtest.com/Spectrum-Analyzer

 

 

[1]"A Collection of Smith Chart Resources." A Collection of Smith Chart Resources. Analog Instruments Company, 9 July 2014. Web. 27 Jan. 2016. <http://www.sss-mag.com/smith.html>.

[2] Please see http://www.keysight.com/upload/cmc_upload/All/BTB_Network_2005-1.pdf for a complete treatment of network analyzer capabilities. Network Analyzer Basics. N.p.: Keysight, n.d. PDF.



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