Skew And Deskewing; Methods To Adjust Skew; Using A Phase Adjuster; Channel Deskew And Channel Delay - Tektronix DSA8300 Quick Start User Manual

Skew and deskewing

The propagation delay inherent in connecting cables and probes can result in inaccurate amplitude and time-correlated
measurements. This is caused by the difference between two or more delays, known as skew. Skew can be present in multi-
channel applications, and is of particular concern in differential systems. To obtain the best measurement and analysis results
from your instrument, you will need to remove skew, a process known as deskewing.
In differential (non-TDR) applications, skew can occur in acquired signals as they propagate from their source to the mainframe.
In TDR applications, skew can occur in signals that propagate from the instrument to the device or system under test (DUT),
specifically the TDR stimulus pulses, as well as from acquired signals.
The following procedures and examples explain how to minimize and measure skew in non-TDR and TDR systems.
Quick tips
■ Minimize skew by using balanced cable pairs.
■ Use the Differential Channel Alignment application to align input channels and TDR steps on supported electrical modules.
See Align acquisition (and TDR steps) for two-channel modules
■ Measure and match your cables. TDR modules such as 80E10B, 80E08B, and 80E04 enable you to measure cable delays
with very high precision.
■ In differential systems, use differential probes or differential SMA to single-ended active convertors, such as the Tektronix
P7380SMA.

Methods to adjust skew

Using a phase adjuster

Skew can be adjusted with the use of a variable delay line (phase adjuster). Tektronix part number 015-0708-00 is a phase
adjuster with a 25 ps range and VSWR of 1.3:1 at 18 GHz. The advantage of a phase adjuster is that it is functionally invisible.
The disadvantage is that signal fidelity is slightly impacted.

Channel deskew and channel delay

The DSA8300 provides two different methods for input signal deskewing (correcting signal timing differences that are external to
the instrument): channel deskew and channel delay. While both channel deskew and channel delay let you compensate
individual channels for external signal path timing differences, they have different impacts on acquisition performance.
Channel deskew (available for all 8000 series sampling modules) adds a specified delay to the trigger-initiated acquisition strobe
signal that is sent to all module channels simultaneously. Using channel deskew requires separate acquisitions (on sequential
triggers) to acquire data for each channel with a different channel deskew value.
Channel deskew works best for repetitive signals. Data is captured over sequential triggers, with the instrument changing the
channel deskew value for each channel per acquisition. The number of acquisitions needed to display the waveforms increases
linearly as a function of the number of different channel deskew settings for actively acquired channels. To get the best
performance when using channel deskew, keep the number of unique deskew values to a minimum.
DSA8300 Quick Start User Manual
on page 28.
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