Setting the Acquisition start point for Delta-L3 measurements
Application Note AP9019 

Setting the Acquisition start point for Delta-L3 measurements on Atlas insertion loss measurement system

When performing Delta-L testing, the location of the acquired waveforms in the Atlas display window is critical for getting good results from a measurement.  If the waveforms are positioned incorrectly parts of the waveform will not be acquired and processed, leaving vital data out of the ensuing calculations. Although every opportunity has been taken to automate this process there are still factors outside of our control which can cause the waveforms to be displaced and this must be checked for and corrected for optimum results.

This note deals with Delta-L3 measurements but the principles apply to all Atlas measurements .

The picture below shows a typical display of the acquired Delta-L3 waveforms

In this case the beginning of the short line trace has been cut off as the start point is too far to the right. 

The Start point settings are seen in the lower left of the test set up window and are shown below:

Setting the value of “Start Aquire at end of Probe” offset is set at zero is reflected in the set up editor:

The resulting effect on the acquisition is shown below:

This has reversed the situation and it is the Long line waveform that is restricted from reaching full amplitude as it is cut short. The Short line however is starting well into the display and nothing is lost. It could start further to the left without loss of data.

Adjusting the start point again, in this example to +150ps:

The resulting waveform set is now optimised in the window:

Optimising the waveform display

Whenever a new test set up is created or the probing arrangements for an existing test are changed then the fit of the waveforms within the window should be checked and any adjustments made to optimise the location of the waveform in the window.

As a quick check on settings, it is possible to select and execute the short line test and then watch the oscilloscope display to see where the waveform appears.  The test may then be edited and this operation repeated until the ideal set-up is obtained. The picture below shows the oscilloscope display using the +150 ps offset as the optimised example shown above .