The MSA as a Vector Network Analyzer

Sam Wetterlin



Scotty’s Modular Spectrum Analyzer (MSA) can operate as a spectrum analyzer, a spectrum analyzer with tracking generator, or a vector network analyzer (VNA). This document provides an overview of the MSA as VNA, with reference to other documents that provide more detail on certain topics.


A spectrum analyzer analyzes signals generated externally. A VNA analyzes the response of devices when stimulated by a test signal generated by the VNA. This same test signal is used as a tracking generator for the spectrum analyzer, and is therefore sometimes referred to as the TG signal.


The MSA/VNA is operated in either Transmission or Reflection mode. The mode is selected by menu Mode-->VNA Transmission or Mode-->VNA Reflection. In either mode, the response of the MSA itself is first calibrated, using menu Operating Cal-->Perform Cal.


For Transmission mode, the calibration involves establishing a “through” connection from the TG output to the MSA input. The calibration essentially measures the strength and phase of that signal. Then the device under test (DUT) is inserted in place of the through connection, and the strength and phase of its transmitted signal is measured, relative to the calibration signal.


In Reflection mode, the calibration involves either a simple Reference calibration, much like that for Transmission mode, or more extensive Open-Short-Load (OSL) calibration. The DUT is then attached to the test fixture, and the MSA measures its reflection coefficient, which can be converted to other quantities such as impedance or equivalent resistance-inductance-capacitance (RLC) circuits. More detail is available on OSL calibration, and the standards used in that calibration.


The general procedure for testing in Transmission and Reflection modes is described here. That document provides a good overview of test fixtures and procedures.


Traditionally, a VNA makes use of a reflection bridge to measure reflection and impedance. There are many designs for such bridges. Several documents describe bridges, in the context of using them for “manual” return loss measurements, meaning measurements without full VNA capabilities. These same bridges can be used with the MSA, where calibration can greatly improve their performance. There is a Three-Bead Balun Bridge and a Baluns-Plus-Beads Bridge, both of which are fairly simple. A much fancier Parallel-Line Bridge and an Active Bridge are described here. The latter are very high quality bridges. The Active Bridge is especially nice for use from 100 kHz to 150 MHz, because it has sufficient performance to be used with the simpler Reference calibration, and because it maintains a relatively strong signal to the MSA, which is especially helpful at lower frequencies.


It is also possible in Reflection mode to use simple test fixtures that are not actually bridges. The Series Fixture and Shunt Fixtures can be used in two ways. By telling the MSA in the calibration dialog what type of fixture is being used, the MSA can make the necessary calculations to use them with simple Reference calibration. Or, OSL calibration can be used and the MSA does not even need to know what kind of fixture it is.


Reflection coefficients and transmission coefficients are part of a set of parameters known as S-Parameters, for which we have provided an overview. Reflection coefficients can be graphed on a Smith chart, which accompanies the regular graph in Reflection mode.


It is not always possible to calibrate exactly where you want the calibration plane to be, so the MSA provides the ability to do plane extension. The calibration plane is mathematically extended along a transmission line of specified electrical length, stated as the time delay experienced along that line.


The Active Bridge mentioned above consists of two op amps, and can be used simply as a buffer amplifier rather than as a bridge. As a buffer amplifier, it can provide a solid 50-ohm interface to the DUT, and provide other benefits. Its use in this way is described here. For use simply as a buffer amplifier, the Buffer Op Amp provides dual 9 dB amplifiers with excellent input and output return loss; intended for use to 150 MHz, its bandwidth is actually 350 MHz.


Some of the documents cited above describe the measurement of impedance. Other uses of the MSA/VNA include antenna testing and evaluation of transmission lines. [Determining Characteristic Impedance, Coax Stubs, Coax Loss, and Transmission Line Math] The MSA can also perform Filter Analysis and Crystal Analysis, which use Transmission mode but do not require phase information, and therefore can also be done in SA/TG mode for units that do not measure phase. The MSA can operate as a component meter, as described in the overview cited above. Test results with relatively crude fixtures show very good performance measuring resistors, capacitors, and inductors, with accuracy of 1-2% over a broad range. A more precise fixture is also presented in that last document.


The basic operation of the MSA covers a range from approximately 100 kHz to 1 GHz, but this is actually only one of three possible frequency modes. That range is called 1G mode. The range can be shifted to 1-2 GHz (2G mode) or 2-3 GHz (3G mode), as described here.


Everyone wants their graphs to be pretty. Information on customizing the appearance of MSA graphs is provided here.