PTR vs. TO and How to Adjust

PTR vs. TO and How to Adjust

Enabling PTR in Smaart

Both Smaart Suite and RT v9.4+ feature adjustable Plot Time Reference (PTR) and Time Offset (TO). In Smaart, however, this is considered an advanced feature. As such, it must first be enabled from the "Advanced" tab of the Preferences dialog, accessible by navigating to Preferences > Advanced or by using the hotkey [Alt/Opt] + [J]. As an advanced feature, it is excluded from Smaart LE.

Once enabled, the bottom-left corner of the Phase plot displays a trace's Plot Time Reference (or PTR). When timing a measurement delay in Smaart's Real Time mode, the delay finder identifies the moment of peak energy arrival and sets the Measurement Time Reference (MTR) to this value. Upon capture, a trace's PTR is equal to its MTR, and remains so until adjusted.

PTR is also the time relative to which phase is calculated and displayed: 

• Energy arriving ahead of the PTR will appear with upward-sloping "leading" phase.
• Energy arriving behind the PTR will appear with downward-sloping "lagging" phase. 

Adjusting Plot Time Reference

The PTR is displayed on the Phase plot alongside (-) and (+) buttons. These buttons can be used to alter the PTR in increments of 0.01 ms. Alternatively, clicking on the displayed PTR will change it into an editable field where you can manually enter a new value. 

When you change a trace's PTR from its original MTR value, the difference will be displayed in the beneath the Plot Legend (in top-right corner) as a positive or negative value. This value is the trace's Time Offset (or TO). This placement mimics the dB offset shown on the magnitude plot when dragging a trace to change its level. Changing a trace's PTR will not affect its original MTR, however, which is still viewable by right-clicking it in the Data Bar and selecting "Info".

The relationship between Plot Time Reference, Time Offset, and Measurement Time Reference can be illustrated using this equation:

PTR = MTR +/- TO

PTR Behavior in Data Modeler

Data Modeler's PTR Behavior is similar to Smaart Suite and RT, except it does not need to be enabled. In Data Modeler's Modeler and Sum modes, a trace's Plot Time Reference is set equal to its Measurement Time Reference (or MTR) by default, and can be adjusted from there.

In Sum Mode, however, PTR can't be changed manually. The phase plot's PTR is instead selected by DM to be shared by all displayed phase traces. The trace with the longest MTR is selected as a time reference for all traces, so the user must apply DSP Delay to visually align traces in the Phase plot. Doing so will not affect the plot's Time Offset.

Sum Mode's Relative Phase plot shows the phase of all sum contributors relative to a single PTR. When a Sum is created from traces with different MTRs, DM elects the larger value as the Time Reference (Tref). It then automatically adds Time Offset to the trace with the shorter MTR such that MTR + TO = Tref. This process is repeated per sum, such that each has its own Tref and corresponding PTR. As a consequence, only one Sum Result can be displayed at once. In cases where both Sum contributors have the same Measurement Delay, one is arbitrarily chosen as Tref and the other receives a Time Offset of 0 ms.

In Modeler Mode: When creating a sum with mixed measurement delays, your default view of phase has a different PTR for each trace, but the "absolute" phase from each data set is still used to calculate the Sum Result. In this situation, two traces that match in the Phase plot may not sum coherently. One potential solution is to add Time Offset to the trace with the lowest Measurement Delay to match its PTR to that of the later-arriving trace. With both traces possessing the same PTR, the Phase plot will accurately reflect the summation behavior.