When should/shouldn’t I use phase shifters for skew?
First of all, PCB skew is not phase shifter skew, which often has bandwidth issues. Phase shifters work well for simple narrow band carrier based signaling systems, but not broad-band digital systems.
PCB skew, such as provided by our SKEW-112 platform, has following advantages over phase shifters:
- The skew is calibrated using NIST traceable methodology and recorded to <100femtoseconds or 0.1psec accuracy, there is no calibration. Skew calibration is really challenging, we do all that work for you, and provide program table. We are creating skew matched calibrated cables every day, so we have special measurement techniques using both VNA and TDR. SKEW-112 is easy to use, WRT team provides apps support.
- The SKEW-112 has channel performance like breakout regions or common transmission paths for Serial Link systems, which means SCD21 is reasonable (diff to common mode), and relates to the skew. Backplanes do not have simple skew, the have coupled systems, and they do not have phase shifters. Simple phase shifters offer no coupling, so it only mimics TX skew, not channel skew.
- Signal integrity is exact same for all lengths, so doing Design of Experiments works well since channel performance does not change like it does with phase shifters.
- 1.5 VSWR is 14dB, SKEW-112 maintains over -20dB return loss through 2X Nyquist. For PAM4 systems return loss or S/N ratio through 1X Nyquist is very important.
- How is customer going to calibrate skew between set ups? SKEW-112 will work in multiple locations using simple Excel spreadsheet.
WRT applications staff will also calibrate skew of your cables (WRT will do this complimentary right now, later this will be optional) and have our own 2psec, 1psec, or 0.375psec max skew matched cables for purchase. We also provide the measured S-parameters of cables.
