Applying Limits and Pass/Fail Conditions to Your Vector Network Analyzer Test Setup

During development, you may be using your VNA in a methodical, controlled manner so that your device can be fine-tuned to meet performance specifications. This same setup often becomes the foundation for a test that you wish have executed in in production to ensure a batch of this type of device meets quality and performance expectations. However, a production environment does not afford the operator with the luxury of time and it is of the utmost importance to know whether a device passes or fails as quickly as possible to isolate problem parts and to possibly identify a trend across multiple devices to understand if something in the production process needs to be adjusted. 

To accommodate prompt test status conditions, you can configure your test setup to use the Limit Test feature. Limit testing allows you to identify the minimum or maximum measurement levels over a range of frequencies or at a single point. Based on how the limit criteria are defined, the test status is presented to the operator on the VNA display where it can immediately be identified. Additionally, to ensure status visibility, the status can be made visible in large print in the center of the trace viewing area. 

Let us suppose your goal is to test a large quantity of antennas that are said to be matched between 824 and 894 MHz, and your operator is to ensure that the return loss is 10 dB or more across this range. Additionally, you want the operator to check that what you have identified as the point with greatest return loss - say at 841 MHz - is between 30 and 40 dB. Your setup with the device connected may appear like what is shown in the following image. 

You can access the limit capabilities by navigating to Analysis->Limit Test. To establish limit conditions for the 824 to 894 MHz you would first click on the Edit Limit Line function button, then click on the Add function button.

The Limit Table is exposed at the bottom of the VNA software display area. You would then set the type to Max, the 'Stimulus Start' to 824 MHz, the 'Stimulus Stop' to 894 MHz, and the 'Response Start' and 'Stop' to -10 dB.

Navigating up one level, you can enable the both the 'Limit Test' and the 'Limit Line' by clicking on the function buttons which read this, toggling their state to ON.

This enables the Limit Test and you can see the status displayed in the upper right corner as well as the limit line positioned in the trace area adjacent to the trace and covering the same frequency band. 

To ensure the test status stands out to the operator, toggle the function button for 'Fail Sign' to read ON.

This will place a very obvious status in the center of the trace display for the operator to see. 

To add the additional check at the 841 MHz point, you would return to the Limit Line Table and add another row, this time setting the type to Single, the 'Stimulus Start' and 'Stop' to 841 MHz,  the 'Response Start' to -30 dB, and the 'Response Stop' to -40 dB.

You will notice that limit line indicators are added to the display to effectively mark these new limit test boundaries. When both limit conditions are met, 'Pass' is presented to the operator. 

If either limit condition is violated, 'Fail' is presented to the operator, which is the case below where the single point test has a measured value that reads outside the specified response limits. 

For information on Bird VNAs, see https://birdrf.com/Products/Analyzers/VNA.