I have encountered a strange result while attempting to tune a differential pair of traces. (I am tuning the Tx+/Tx- and Rx+/Rx- pairs of Ethernet transmit receive traces) One would think that you would use “Tune Differential Pair Length” to make both traces the same length but that choice produces bizarre results in that no matter which trace of any of the two pairs of traces I select it tells me that the trace is too short and in every case it tells me that the trace should be 3.9370 inches long. This is plainly wrong since BOTH traces cannot be too short with respect to the other trace and in addition I have no idea where 3.9370 inches comes from since it does not seem to relate to anything. I cannot make sense of this result.
When I use instead “Tune Differential Skew/Phase” this choice produces sane results and eventually tells me that the pair of differential traces is tuned. I am not sure what that means but whatever it means, they are “tuned”.
It must be that “Tune Differential Pair Length” means something other than make both traces the same length but I can’t figure out what it might mean. Internet searches have not helped.
Any guidance would be appreciated.
“tune skew” means tune the difference between the + and - traces in the pair out.
“tune length” means tune the length of the overall differential pair (i.e. change both + and - at the same time)
The latter is normally used when you have multiple diff pairs that need to be the same delay (length)
To set the target length for length tuning, right-click and open Length Tuning Settings. The default target length is 100mm.
Hi Kend,
Tune differential pair skew/phase
Skew tuning is performed in the case of more than one line. Every line in your circuit will have a certain delay. Differential pair traces carry the same signal but in opposite polarity and are synchronized with respect to the time.
Skew does not look at the overall target length. It just considers the routed length from point A to B. Check the skew function ‘Tune differential pair skew/phase’ in the given screenshot. It says that once routing is completed, make sure that the lengths of both the lines are within the tolerance limit.
It is showing that D+ is longer than D- by 0.0265in. And it requires length matching.
We will move our cursor (see the bump in the below screenshot) to perform the length matching. This is how we will increase the length of D-.
Tune differential pair length
- It will not consider one line at a time but differential pair as a whole.
- For length tuning, right-click and go to length tuning settings.
- You can change/set the target length as per your requirement.
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