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Hi
Vicente, all: I
share your concern. I don't think that the draft standard correctly deals
with the topology that you describe. I
commented on this in an earlier draft of the standard. I think we need to say what is meant by AC
coupling by providing one or more specifications that place some bounds on
it. Regards, Steve
A. From: Mellitz, Richard
[mailto:richard.mellitz@intel.com] Hi Vicente, 803.3ap really doesn’t say the caps are on the board. The cap is
after TP4 and is the domain of the Rx. Yes it's a challenge for chip folks as
they will need to tell there customers how to deal with this issue. Regards, Rich Mellitz, Intel -----Original Message----- Hello colleagues, I would like to understand the perspective of the group with regards to external DC block capacitors in 10Gbps channels such as the one being designed by this group. My past experience indicates it will be difficult to find DC block capacitors that are specified for operation at 5GHz and beyond - at least capacitors that are priced reasonably
such that their cost is a small proportion of the link cost. In the farily recent past I have had difficulty finding capacitors with low enough ESRs even at 4Gbps operation. Even if such a capacitor is found the transmission line discontinuities (vias, pads etc) associated with such capacitors present a significant degradation that would be nice to eliminate. I will ask a secondary question. Has it been considered to require the DC block capacitor to be
internal? Many SERDES vendors have DC blocks integrated within their receivers
and located downstream from the internal 50 ohm termination but upstream from their receiver bias network (which has much larger impedance than 50 ohms). Such placement allows the capacitor to be much smaller in value and thus integrateable and still allows the receiver to be biased independently of the transmitter which I believe is the main purpose of the DC block. Another benefit of the internal capacitor is that it will not have the associated discontinuties that an external capacitor will have. Thanks in anticipation of your reply. Vicente Cavanna HP ProCurve Networking |