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About matching length between pairs involving capacitors

mulfycrowh , 01-05-2023, 03:06 PM
Hi everyone,

AC-coupling capacitors are important for HDMI differential pairs.
Meaning that we have one capacitor on each polarity of each pair.
Giving 8 capacitors.

Since it is better to locate the capacitor near the source, sometimes we don't have room to apply accordions between the source and the capacitor.
The attached screenshot shows the issue on C_DDI3_1HPi.
But we have room to apply accordions between the capacitor and the load.

Is there a syntax to allow that?

WhoKnewKnows , 01-05-2023, 03:40 PM
Perhaps use Altium's X-signals trace tuning tool? It should let you manage a signal's entire length through a variety of components. Perhaps using a capacitor network instead of individual capacitors could help promote more usable space for trace tuning elements? 🤷
mulfycrowh , 01-06-2023, 01:58 AM
@WhoKnewKnowsI chose the option to create xSignals for all the nets involved in C_DDI3_1HP.
I've got 8 xSignals.
I create a xSignals class.
They are not tuned in length at present time
I set up a rule for length matching on this class.
I don't understand, I don't get any error after running Design Rule Check.

mulfycrowh , 01-06-2023, 08:48 AM
I have to check "Group Matched Lengths" and not "Within Differential Pair Length".
WhoKnewKnows , 01-06-2023, 10:07 AM
@mulfycrowh 🤷‍♂️

@robertferanec has at least one video in which he demonstrates how to use X signals in the 3 usual uses, including in the final case where the differential signal has some intermediate component such as a resistor or capacitor between a sending and receiving IC. I'm not certain about this since it's been quite a while since I've set up X signals (thankfully, lately, I haven't had to delete and reinitiate X signals on anything as I have had to do in the past, so I'm out of practice 😉) but if too few or too many rules are set up on various nets that you're also trying to use X signals on, it could complicate things, perhaps?
WhoKnewKnows, 01-06-2023, 10:08 AM
Oops, I meant to suggest searching up that video and reviewing. While you're there, be sure to follow Robert's channel if you haven't already. He's approaching 100k subscribers. !!!!
robertferanec , 01-11-2023, 06:09 AM
Try to play with different rules. The "within diff pair" rule is a rule which is used between P and N signals. For this your rule, work with the xSignals as a single ended signals and make a rule to make them similar length.

This video may help: https://youtu.be/jpk-hfsFaqA
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