TEST & MEASUREMENT FEATURE
prevents such failures. Similarly, if the VNA is used in a very dirty, dusty or hot location, power components may become overheated. Another common cause of reported
Like fine-tuning a race car
Adam Purkiss of Anritsu shows how following best practice in calibration and maintenance can produce a huge reward in improved reliability and accuracy of device measurements
T
he modern Vector Network Analyser (VNA) is an extraordinary feat of
engineering, enabling the precise measurement and analysis of signals across a broad frequency spectrum, in some cases up to 110GHz. VNAs are capable of supporting engineers in the characterisation and analysis of highly sophisticated devices with impressive accuracy and resolution. Engineering units investing in a VNA
will naturally want their instrument to be working at peak performance whenever needed. The measures necessary to ensure that the VNA operates reliably and produces accurate results are relatively simple to implement. 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 have been acceptable in the past now risk invalidating the results produced by an expensively acquired VNA.
COMMON CAUSES OF VNA FAILURES When a VNA owner performs regular calibration operations, they will benefit from more accurate measurements, more reliable operation and less frequent instances of failure. To regular VNA users, this might appear obvious, however, even experienced users
INSTRUMENTATION | FEBRUARY 2017 33
often attribute problems with a VNA’s measurements, or even a complete breakdown, to a design flaw in the instrument, when in fact the cause is a failure on the user’s part to follow the manufacturer’s maintenance guidelines. 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. This most commonly occurs for one of two reasons – and both are preventable. Dirty air filters can cause overheating as dust and dirt will prevent the fan from drawing in sufficient air to cool power components. When these components exceed their rated maximum temperature, they are prone to failure. Regular cleaning of the air filters
VNA malfunctions is phase lock error. Symptoms that the VNA may be suffering from a phase lock error include: the VNA will not sweep correctly; there is no RF output; an IF signal is missing; the fully reversing two Port VNA sweeps in one direction only; or there is a sampler problem at the receiver. Again, phase stability problems are not evidence of a fundamental fault in the VNA. They most often 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 the much larger investment in the VNA itself worthless. 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 over-tightened adaptors to universal connectors, damaged threads or splayed centre pins. 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
The basic architecture of a VNA
can do much to ensure high reliability and a long operating life by avoiding subjecting the instrument to excessive RF power, following normal ESD safe practices and storing and using the VNA in a clean working area with stable temperature and humidity.
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