[Arm-netbook] HDMI High-Frequency Layout: Recommendations

Richard Wilbur richard.wilbur at gmail.com
Thu Oct 12 19:15:30 BST 2017

On Oct 11, 2017, at 18:04, Luke Kenneth Casson Leighton <lkcl at lkcl.net> wrote:
> On Wed, Oct 11, 2017 at 5:58 PM, Richard Wilbur
> <richard.wilbur at gmail.com> wrote:
>> What does PADS currently give as the impedance of the HDMI
>> differential lines?
> each line is 90 ohms.
>> Does it give an impedance for a particular trace?
> yes.  90 ohms.  or... 89.
>> (That would likely be the single-ended impedance.)

Those are surprisingly high for single-ended impedance on that board stack and geometry.  I would have expected something less than or equal to about 60 Ohm.  89 or 90 Ohm sounds more like what I might expect for our differential impedance (less than twice the single-ended impedance).

>> Does it offer a
>> differential impedance value for a pair of traces?
> no.  from what i gather you're expected to read up on diff-pair rules.

What version of PADS Layout are you using?  Mentor responded to a question about version 9.2 saying it calculates a general impedance value taking into account:

"PADS uses Diff Pair gap value and the two nearest planes (Cam or Split/Mixed) to calculate trace impedance; if the trace goes between two planes (Stripline), or just one trace to the nearest plane  (Micro Stripline) in other case. Cut outs are ignored for both cases."[*]

[*]  https://communities.mentor.com/thread/5743

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