Like fine-tuning a race car: the role of calibration and maintenance in optimising VNA performance
VNAs are therefore capable of supporting engineers in the analysis and characterisation of extremely sophisticated and high-performance devices with impressive accuracy and resolution (see Figure 1).
Engineering units which have invested in a VNA will naturally want their advanced instrument to be working at peak performance whenever needed. Perhaps surprisingly, the measures necessary to ensure that the VNA operates reliably and produces accurate results are relatively simple to implement, and cost little (in comparison to the purchase price of the instrument).
And as device operating speeds have grown ever faster, the impact of even minute non-linearities or distortions on the accuracy of a VNA’s measurements has grown. So calibration and connection practices which might in years gone by have been acceptable now risk invalidating the results produced by an expensively acquired VNA.
This article shows how following best practice in calibration and maintenance – a course that calls for time and care more than money – can produce a huge reward in improved reliability and accuracy of device measurements.
Figure 1: basic architecture of a VNA.
Common causes of VNA failures
When a VNA owner performs regular calibration and maintenance operations, they will enjoy two main benefits:
- more reliable operation and less frequent instances of failure;
- more accurate measurements.
To regular users of VNAs, this might appear obvious: nevertheless, experienced users often attribute problems with a VNA’s measurements, or even a complete breakdown, to a fundamental design flaw in the instrument, when in fact the cause is a failure on the user’s part to follow the manufacturer’s guidelines for regular maintenance.
In practice, the total failure of a VNA is rare. When it does happen, it is often because of a failure of the instrument’s power supply. And this most commonly occurs for one of two reasons – and both are preventable:
- Dirty air filters causing overheating. An air filter covered in dust and dirt will prevent the fan from drawing in sufficient air to cool power components. When these exceed their rated maximum temperature, they are prone to failure. Regular cleaning of the air filters prevents such failure;
- A poor working environment leads to the same result: if the VNA is used in a very dirty, dusty or hot location, power components will again tend to become overheated.
Another common cause of reported VNA malfunctions is phase lock error. Any of the following symptoms suggests that the VNA is suffering from a phase lock error condition:
- The VNA will not sweep correctly;
- There is no RF output;
- An IF signal is missing;
- A fully reversing two-port VNA sweeps in one direction only;
- There is a sampler problem at the receive.
Again, phase stability problems are not evidence of a fundamental fault in the VNA; most often, they arise from the use of inappropriate or low-quality cabling components. So economising in the specification of these relatively low-cost elements of a system can render worthless the much larger investment in the VNA itself.
Figure 2: splayed centre pins in a co-axial connector will impair signal quality.
High-quality cables are fragile parts, and require careful handling and storage. When not in use, they should always be removed from the VNA’s ports to avoid the risk of accidental damage or wear.
It is also important that VNA users should have a good understanding of the various cable and connector types that they use with the VNA. In particular, coaxial cable and coaxial connectors can suffer from failure or poor performance because of any of the following:
- Over-tightened adaptors to Universal connectors;
- Damaged threads;
- Splayed centre pins (see Figure 2).
Users who have a proper understanding of connectors will know how to avoid damaging them when mating and unmating. A general understanding of connector care will also help to prolong the life of adaptors, standards, connectors and cables.
In relation to the VNA itself, users can do much to ensure high reliability and a long operating life by taking the following precautions:
- Know the RF specifications of the instrument, and avoid subjecting it to excessive RF power;
- Follow normal ESD safe practices;
- Store and use the VNA in a clean working area with low levels of dust and other particles, and keep both temperature and humidity stable.
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 imperfections. The imperfections can be found in these parameters:
- Match: because it is a broadband instrument, the VNA offers a raw match which can lead to errors of more than 1-dB. Correcting mismatch will greatly reduce the size of this error;
- 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;
- Frequency response: 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, then, is to nullify the effect of match, directivity 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 (normally annual) manufacturer’s calibration required to maintain the quality rating of a VNA.
In turn, the manufacturer’s calibration is actually a verification. The procedure 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 good.
Importance of co-efficient data
Assuming the use of a verified VNA, the engineer’s first step in implementing an accurate 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, are entered into the VNA prior to performing a calibration. Separate co-efficient data sets are supplied for coaxial, waveguide and microstrip calibrations, and their use provides for 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 measurements, but inevitably they 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 above, it is common to assume that there is a fault in the VNA when test measurements show a marked 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.
- Perform the vector error correction calibration a second time. This verifies that the VNA and its calibration cables are working as they should;
- Dirty calibration standards affect calibration results, especially at high frequencies. (see Figure 3). They must be cleaned with swabs soaked with de-natured alcohol;
- Check that the correct calibration standards were used, and in the correct order.
Figure 3: a dirty calibration standard can invalidate test results.
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.
It should also be noted that, 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 above, 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.
The author, Adam Purkiss holds a BSc and is a Specialist VNA Support Engineer at Anritsu (UK) — www.anritsu.com
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