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.