Using Limit Lines

Limit lines allow you to compare measurement data to performance constraints that you define.

• Overview

• Create and Edit Limit Lines

• Display and Test with Limit Lines

• Testing with Sufficient Data Points

Other Analyze Data topics

Overview

Limit lines are visual representations on the VNA screen of the specified limits for a measurement. You can use limit lines to do the following:

• Give the operator visual guides when tuning devices.

• Provide standard criteria for meeting device specification.

• Show the comparison of data versus specifications.

Limit testing compares the measured data with defined limits, and provides optional Pass or Fail information for each measured data point.

You can have up to 100 discrete lines for each measurement trace allowing you to test all aspects of your DUT response.

Limit lines and limit testing are NOT available with Smith Chart or Polar display format. If limit lines are ON and you change to Smith Chart or Polar format, the analyzer will automatically disable the limit lines and limit testing.

By default, limit lines are drawn in the same color as the trace on which they are created. However, all limit lines can be drawn in Red by setting a preference. Learn more.

Create and Edit Limit Lines

You can create limit lines for all measurement traces. The limit lines are the same color as the measurement trace.

Limit lines are made up of discrete lines with four coordinates:

• BEGIN and END stimulus - X-axis values.

• BEGIN and END response - Y-axis values.

Limit Table

Note: To ADD a limit line to the table, change the last limit line to either MAX or MIN

1. In the Type area of the Limit Table, select MIN or MAX for Limit Line 1.

   • The MIN value will fail measurements BELOW this limit.

   • The MAX value will fail measurements ABOVE this limit.

2. Click BEGIN STIMULUS for Limit Segment 1. Enter the desired value.

3. Click END STIMULUS for Limit Segment 1. Enter the desired value.

4. Click BEGIN RESPONSE for Limit Segment 1. Enter the desired value.

5. Click END RESPONSE for Limit Segment 1. Enter the desired value.

6. Repeat Steps 1-5 for each desired limit line.

Displaying and Testing with Limit Lines

After creating limit lines, you can then choose to display or hide them for each trace. The specified limits remain valid even if limit lines are not displayed.

Limit testing cannot be performed on memory traces.

You can choose to provide a visual and / or audible PASS / FAIL indication.

With limit testing turned ON:

• Any portion of the measurement trace that fails is displayed in red.

• Any portion of the measurement trace that does NOT fail remains unchanged and silent.

Display failed trace points or trace segments

You can display the data points that fail limit line testing as red dots or as a red trace segment. The default behavior (red trace) can be changed with a Preference setting. Learn how.

PASS is the default mode of Pass / Fail testing.

A data point will FAIL only if a measured point falls outside of the limits.

• If the limit line is set to OFF, the entire trace will PASS.

• If there is no measured data point at a limit line stimulus setting, that point will PASS.

Testing with Sufficient Data Points

Limits are checked only at the actual measured data points. Therefore, It is possible for a device to be out of specification without a limit test failure indication if the data point density is insufficient.

The following image is a data trace of an actual filter using 11 data points (approximately one every vertical graticule). The filter is being tested with a minimum limit line (any data point under the limit line fails).

Although the data trace is clearly below the limit line on both sides of the filter skirts, there is a PASS indication because there is no data point being measured at these frequencies.

The following image shows the exact same conditions, except the number of data points is increased to 1601. The filter now fails the minimum limit test indicated by the red data trace.

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