FEATURE TEST & MEASUREMENT
measurements, but they can compromise measurement accuracy, and the use of specific co-efficient data is preferable when high accuracy is called for.
GOOD PRACTICE IN TEST PREPARATION As described earlier, it is common to assume that there is a fault in the VNA when test measurements show a noticeable variance from the expected values. Before assuming such a fault and calling out a service engineer, it is prudent for the user to check that the fault does not lie elsewhere. The user should perform the vector
error correction calibration a second time to verify that the VNA and calibration cables are working effectively. Dirty calibration standards affect
calibration results, especially at high frequencies. As such, they must be cleaned with swabs soaked with de-natured alcohol. Users must also check that the correct
THE LINK BETWEEN CALIBRATION AND ACCURACY The physical characteristics of a measurement set-up using a VNA inherently give rise to imperfections in the raw data output. An ideal calibration would cancel out the effect of these. The imperfections can be found in
three parameters: match, directivity and frequency response. Because it is a broadband instrument,
the VNA offers a raw match which can lead to errors of more than 1dB. Correcting mismatch will greatly reduce the size of this error. In terms of directivity, the directional
coupler on a VNA allows the separation of the incident signal to the DUT from the reflected signal from the DUT. Even though VNA manufacturers use high quality couplers, there will always be some residue of unintended coupling between the signal paths. This can have an effect on tests which are intended to measure reflected signals of very low amplitude. Lastly, when calibration cables are used for measurement purposes, they can have an effect on the overall frequency response. Again, these imperfections must be calibrated out. The purpose of calibrating a test set-
up is to nullify the effect of directivity, match and frequency response errors. In fact, this kind of calibration would better be termed a ‘vector error correction’: it is quite different from the periodic manufacturer’s calibration required to maintain the quality rating of a VNA. In turn, the manufacturer’s calibration is actually a verification. The procedure
34 FEBRUARY 2017 | INSTRUMENTATION
A dirty calibration standard can invalidate test results
requires the VNA to be calibrated using known calibration standards. Ideally, these should be dedicated calibration standards for the VNA in question, as supplied in the manufacturer’s calibration kits. The calibration will also use customer-owned calibration cables, if they are available. The VNA and cables are then tested to specified verification standards, and S-parameter measurements are taken. If the results meet the desired
specification, then the test has verified that the VNA, calibration standards and calibration cables are effective.
IMPORTANCE OF CO-EFFICIENT DATA Assuming the use of a verified VNA, the engineer’s first step in implementing a precise test set-up will be to define an accurate reference plane from which to measure. This is only possible if the co-efficient data, supplied with most calibration kits, is entered into the VNA prior to performing a calibration. Separate co-efficient data sets are supplied for coaxial, waveguide and microstrip cabling calibrations, and their use provides the most accurate measurement results. During the calibration process, the
analyser mathematically removes the offset data to give a zero or reference point at the connector measurement reference plane. The measurement reference plane of a connector is regarded as the contact point of the outer conductors. If no calibration standards are available, average default definitions are stored in the VNA. These might suffice for some
calibration standards were used and that they were used in the correct order. When performing a vector error correction, it is helpful to use the manufacturer’s calibration kit for the connector type in question. The kit will contain the correct standards, which need to be inserted into the VNA at the start of the calibration. These standards provide the VNA with precise information so that the reference plane can be accurately determined during the calibration procedure. In addition, when using manual calibration kits, there is a risk of human error invalidating the calibration results, since manual calibrations can take a long time. In calibration, patience is a virtue: it is worth accepting that the process cannot be rushed, and adopting a methodical approach. The avoidance of errors in the first attempt at a calibration will save the time that would be wasted on a repeat performance of the process.
AT FAULT: MAN OR MACHINE? With VNAs, as with almost any field in which technology is applied, the most common source of error is the human user rather than the machine. Fortunately, the careful observation of
the practices described in this article, combined with the use of high-quality and undamaged components such as cables and connectors, will eliminate all of the common sources of user- generated error, and will lead in almost all cases to many years of trouble-free and accurate measurement with a VNA.
Anritsu
www.anritsu.com
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