Re: [BP] DC block capacitors in 10G NRZ channels
All,
Be aware that the draft standard for p802.3ap not only does not
mandate the value of an A.C. coupling capacitor but it also does not
mandate the type of A.C. coupling to be implemented. How this "D.C.
block" is implemented is solely up to the implementor of the link. Yes
there are some test setup where A.C. coupling caps of certain values are
specified, this does not mean that is how one has to implement /
construct a link.
Howard Baumer
Cavanna, Vicente Vaca (Sr. ; ProCurve ASICs) wrote:
>Hi Charles,
>
>I agree with Brian that the spec should not preclude internal DC blocks
>(if indeed it does).
>
>It does not seem unreasonable (in regards to the concerns you raised
>about size and offset voltage) to integrate a 10pF capacitor and a 5K
>bias resistor to obtain a 50ns time constant. Such a time constant would
>be adequate for an 8B/10B code (even at 1 Gbps) or for other codes
>whose disparity is better bounded than for the 64B/66B code.
>
>Such a SERDES with integrated DC block capacitor, unless it had some
>means to bypass the DC block or to comensate for baseline wander, would
>not be suitable for 64B/66B codes, but would be very attractive for many
>other applications.
>
>
>Vicente
>
>-----Original Message-----
>From: Brian Brunn [mailto:brian.brunn@XILINX.COM]
>Sent: Monday, June 05, 2006 8:38 AM
>To: STDS-802-3-BLADE@listserv.ieee.org
>Subject: 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
>>
>>
>>
>
>
>
>