When utilizing a limit point test on my Keysight VNA the marker value, at that point, is beyond the limit line. However, the limit line Pass / Fail indicates a Pass condition. Why?
Figure 1 below outlines a typical scenario. The limit is set to a single point at 800MHz with a maximum limit value of -14dB. As displayed in the image the point limit indication is "Tr3: Pass" (i.e. a PASS condition). However, review of the display and the Marker 1 indication at 800MHz reveals:
>1 800.0000000 MHz -13.879 dB
The marker value at 800MHz indicates -13.879dB which is greater than the limit point maximum pass / fail criteria.
The apparent issue arises due to the following factors. The first factor is the marker mode setting. Keysight vector network analyzers support two marker modes, "continuous" and "discrete". In "continuous" mode the marker may be positioned to any location on the trace. If the marker location is set such that it is not on an actual sampled data point then the displayed value is linearly interpolated between the adjacent data samples. For this test the analyzer was configured to:
Start Frequency = 300KHz;
Stop Frequency = 1GHz;
Number of Points = 21;
Sweep Type Linear;
Based upon this setup the discrete (actual sampled) measurement points are:
300000, 50285000, 100270000, 150255000, 200240000, 250225000, 300210000, 350195000, 400180000, 450165000, 500150000, 550135000, 600120000, 650105000, 700090000, 750075000, 800060000, 850045000, 900030000, 950015000, 1000000000
Additional details surrounding the interaction of analyzer settings such as start frequency, stop frequency, number of points, and marker modes with limit testing are available via the following resources:
- Keysight Technologies 8712ES and 8714ES RF Network Analyzers User's Guide, Chapter 4, refer to the section labeled 'Additional Notes on Limit Testing'.
- Keysight E5070B/E5071B ENA Series RF Network Analyzers User's Guide, Chapter 11, refer to the 'Note' section.
- Keysight E5071C ENA Network Analyzer Web Help, Measurement > Data Analysis > Limit Test, refer to the 'Note' section.
When the limit test is a single point test configuration, the test frequency of interest must be equal to a measurement point. If the limit point does not coincide to an actual measurement point the correct limit test will not be performed.
The following are provided as the correct configuration settings to utilize limit point testing. Set the marker mode to discrete in order to determine the nearest actual measurement point (800.06MHz) for the limit test and set the limit point stimulus frequency (or frequencies) to this discrete value. Refer to Figure 2, noting the limit stimulus setting and 'Tr1: Fail' indication. Alternatively modify the limit point mode to a limit line and set the limit line start/stop frequencies to encompass the desired measurement point. See Figure 3 noting the limit stimulus settings and 'Tr1: Fail' indication.