Re: [BP] DC block capacitors in 10G NRZ channels
Hi Charles,
While putting the AC coupling capacitor on chip is 'not really that
easy', we want to make sure we do not preclude it. Especially since we
are not limited to just passive components to realize these functions.
In that vein, the best choice for putting the AC coupling capacitor on
chip may involve putting the cap AFTER the termination (the
configuration Vincente describes).
So the standard needs to be clear that putting the AC coupling capacitor
after the termination is allowed and that for system designers, there
may be DC current flowing through the termination resistors. Up until
know, the description of AC coupling appears to be incomplete.
Brian
-----Original Message-----
From: Charles Moore [mailto:charles.moore@avagotech.com]
Sent: Friday, June 02, 2006 12:07 PM
To: STDS-802-3-BLADE@listserv.ieee.org
Subject: Re: [BP] DC block capacitors in 10G NRZ channels
While i recognize that putting the AC coupling capacitor on chip would
be desirable it is not really that easy.
The 100nF off chip coupling capacitor leads to a coupling time
constant of 10 us. Such a long time constant is needed to limit
baseline wander with scrambled data, as independent analysis by Rick
Walker, Steve Anderson, and me indicate.
Generating such a long time constant is going to be hard in today's
deep sub-micron processes. One could use, for instance, a 100pF
capacitor and a 100 kOhm resistor, but size of both components would
be a problem, parasitics in the capacitor will make meeting return
loss specs hard and leakage current through the 100 kOhm resistor may
casue offset problem.
charles
|--------------------------------------------------------------------|
| Charles Moore
| Avago Technologies
| ISD
| charles.moore@avagotech.com
| (970) 288-4561
|--------------------------------------------------------------------|
Jia Gongxian wrote:
> Hi Vicente,Steve,All,
>
> I have same concern about the AC coupling, according to the draft,
AC
> compactors does belong to RX, but doesn't clearly say, whether its in
> linecard or in IC chip, I do think these will cause problems while we
> design a system. if in linecard, the pad and vias will surely cause
the
> impedance discontinuity, different processing in AC coupling area
> of PCB will have different effect on the channel performance. How well
> we will process the AC compactors area ? As a system designer, I don't
> know, because the spec of backplane channel doesn't include this
part.
>
> In order to reduce the risk, I do agree Vicente that it's a better
> choice that place the AC coupling into the IC package.
>
> Regards,
>
> Jia Gongxian
>
> Huawei Technologies Co.,Ltd.
>
> ----- Original Message -----
> *From:* Steve Anderson <mailto:steve.anderson@XILINX.COM>
> *To:* STDS-802-3-BLADE@listserv.ieee.org
> <mailto:STDS-802-3-BLADE@listserv.ieee.org>
> *Sent:* Wednesday, May 31, 2006 11:02 PM
> *Subject:* Re: [BP] DC block capacitors in 10G NRZ channels
>
>
>
> 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]
> *Sent:* Tuesday, May 30, 2006 8:53 PM
> *To:* STDS-802-3-BLADE@listserv.ieee.org
> *Subject:* Re: [BP] DC block capacitors in 10G NRZ channels
>
>
>
> 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-----
> From: Cavanna, Vicente Vaca (Sr. ; ProCurve ASICs)
> [mailto:vicente.cavanna@HP.COM]
> Sent: Tuesday, May 30, 2006 6:45 PM
> To: STDS-802-3-BLADE@listserv.ieee.org
> Subject: [BP] DC block capacitors in 10G NRZ channels
>
>
>
> 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
>