Optimising VNA performance through calibration

The modern vector network analyser (VNA) is an astonishing feat of engineering, enabling the precise measurement and analysis of signals across a broad frequency spectrum, in some cases up to 110 GHz. VNAs are therefore capable of supporting engineers in the analysis and characterisation of extremely sophisticated and high-performance devices with impressive accuracy and resolution (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.

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.