George,
GZ: However, I think we agree that the issue of PoDl designs isn’t really germane (no pun intended) to the baseline wander problem (PoDL diplexers may be one source of a low frequency cutoff), because the PoDL diplexer is on the PHY side of the MDI, and hence outside the link segment.
CD: This resolves the basis for my comment.
GZ: I would submit that a start at 2.5 Mhz or lower would be sufficient for PHY concerns (as that is the scaled frequency relative to 10GBASE-T1’s use of 1 MHz).
CD: This brings the debate back to link segment and testing; I'll defer PHY concerns to others.
Regards, Chris
-----Original Message-----
From: George Zimmerman <george@xxxxxxxxxxxxxxxxxxxx>
To: Christopher T. Diminico <cdimi80749@xxxxxxx>; STDS-802-3-B10GAUTO@xxxxxxxxxxxxxxxxx <STDS-802-3-B10GAUTO@xxxxxxxxxxxxxxxxx>
Sent: Tue, Jan 18, 2022 10:02 am
Subject: RE: [802.3_B10GAUTO] AC coupling capacitors information
OK – so now we are on the same page. German has suggested (in his preso) that a 2 MHz lower limit would not impact PoDL designs and that they could be implemented (I tend to agree, after some discussions with relevant experts that 2 Mhz
or 2.5 MHz is a safe lower edge). However, I think we agree that the issue of PoDl designs isn’t really germane (no pun intended) to the baseline wander problem (PoDL diplexers may be one source of a low frequency cutoff), because the PoDL diplexer is on
the PHY side of the MDI, and hence outside the link segment. I would submit that a start at 2.5 Mhz or lower would be sufficient for PHY concerns (as that is the scaled frequency relative to 10GBASE-T1’s use of 1 MHz)
We have heard some discussion of test concerns, that I think we deal with by an note saying that the frequencies under 10 MHz may be tested separately, or met by design.
From: Christopher T. Diminico <cdimi80749@xxxxxxx>
Sent: Tuesday, January 18, 2022 6:54 AM
To: George Zimmerman <george@xxxxxxxxxxxxxxxxxxxx>; STDS-802-3-B10GAUTO@xxxxxxxxxxxxxxxxx
Subject: Re: [802.3_B10GAUTO] AC coupling capacitors information
George,
I'm asking to what lower frequency should we specify for these additional components. I believe German was suggesting lower limit based on PoDL designs.
Chris – I’ve seen this on link segments in general. They tend to relate to additional components. We agree that the transmission path requirements of the link segment
still apply if you put in midspans or other components – but that is the issue – if they are unspecified at the relevant frequencies, there is no constraint.
GZ: I think that German’s point is quite valid, and have seen examples where assuming that the link segment would be well behaved (rather than specifying it) causes
problems for PHYs, such as those German describes.
CD: Please share your examples of automotive link segments that are not well behaved below 10 MHz so we can specify and not assume.
CD: If entities such as midspans are located within a link segment the transmission path requirements (link segment) still apply.
Chris – First, remember that link segment does not equal cable. A link segment is the transmission path between the two MDIs, which may contain connectors, filters,
and other passive devices.
While it might be tempting to add a qualitative statement as below, adding such a paragraph is technically meaningless. It provides nothing that can be evaluated.
For example – is that only specifying a zero-frequency null? To what depth? After what frequency should the exponential be held? Under a constraint like that, you could have a band-stop filter starting at 10 MHz (say, if you wanted a mid-span Power injector).
That is why the lower frequency bound is important – it provides technical clarity on the performance that a PHY implementer can expect of the link segment. I think that German’s point is quite valid, and have seen examples where assuming that the link segment
would be well behaved (rather than specifying it) causes problems for PHYs, such as those German describes.
The link segment is not AC-coupled. We can explicitly preclude AC-coupling in the link segment by adding a sentence for those concerned of
165.7 Link segment characteristics -
Add sentence end of paragraph
AC-coupling shall not be implemented within
the link segment (between TP1 and TP4).
1. AC couple capacitor value range and ESR characteristics.
2. PHY termination circuit values
This will help put together a simulation profile for the front end circuit.
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