Re: [BP] DC block capacitors in 10G NRZ channels
I agree with Pat that the spec is incomplete wrt AC coupling. I
commented on this about 3 or 4 meetings ago. I'm glad to see that it
is getting attention.
From the point of view of the system designer, if there are no
external capacitors, this is direct coupling. We should probably
start calling it what it is (like OIF does) and acknowledge that it has
some very desirable features. But it requires a more complete
specification. If we want the standard to work with present and next-
generation ICs, then the instantaneous voltage applied to a receiver pin
probably cannot go outside of the range of -0.3 volt to 1.3 volt.
Regards,
Steve A.
-----Original Message-----
From: Pat Thaler [mailto:pthaler@broadcom.com]
Sent: Tuesday, June 06, 2006 11:19 AM
To: STDS-802-3-BLADE@listserv.ieee.org
Subject: Re: [BP] DC block capacitors in 10G NRZ channels
But the transmitter spec (72.7.1) allows common mode voltage between 0
and 1.9 Volts - the trasmitter doesn't have to be offset to 0 volt
common mode (that would be right on the edge of the spec). Positive
swing can go up to 300 mV above 1.9 Volts or a bit more since the 1.2 V
p-p differential max is into the transmitter test load so the output
might be larger into the receiver impedance.
There does seem to be a whole in our specs (in 70, 71 and 72) in that
the specification for AC coupling doesn't specify the common mode range
over which the coupling is expected to function. That is also lacking in
Clauses 39 and 47. When we specified isolation in twisted pair
standards, we put it in, but not when we specified AC coupling.
Since AC-coupling specs (e.g. 72.7.2.3) say the "AC-coupling is
considered to be part of the receiver for purposes of this specification
unless explicitly stated otherwise," it would make sense to add common
mode tolerance to the input signal amplitude specification. Or it could
be added to the AC-coupling subclauses.
Regards,
Pat
-----Original Message-----
From: Charles Moore [mailto:charles.moore@AVAGOTECH.COM]
Sent: Tuesday, June 06, 2006 8:24 AM
To: STDS-802-3-BLADE@listserv.ieee.org
Subject: Re: [BP] DC block capacitors in 10G NRZ channels
all,
A few random points having to do with AC coupling:
1. Putting a blocking capacitor between the receiver and the
transmitter guarantees that no DC current will flow between
them. If the Rx termination is directly exposed it will
matter what voltage it is terminated to. The most logical
termination voltage is ground but that is likely pull the
Tx output lower which could be a problem for some Tx's.
2. Everyone will implement the Rx differently and cost, including
return loss, will be implementation dependent.
3. Our maximum swing is 1.2V p-p differential. This is 300 mV
peak single ended, which is not likely to turn on clamping
diodes, although point 2 still holds.
charles
|--------------------------------------------------------------------|
| Charles Moore
| Avago Technologies
| ISD
| charles.moore@avagotech.com
| (970) 288-4561
|--------------------------------------------------------------------|
Steve Waldstein wrote:
> Vincente,
>
> Just as a follow up I wanted to let you know we've got direct
> experience with your second point and found that this Rx architecture
> for the termination to ground, followed by a DC blocking capacitor,
> followed by an additional bias network to optimize the common mode did
> in fact challenge the return loss significantly. The additional
> capacitance observed with this configuration was significant even
> after the capacitance of the clamping diodes were reduced from a
> previous generation significantly. That is why I highlighted my
> concern. Just an FYI.
>
> Steve
>
>
>
>
> -----Original Message-----
> From: Cavanna, Vicente Vaca (Sr. ; ProCurve ASICs)
> [mailto:vicente.cavanna@hp.com]
> Sent: Monday, June 05, 2006 7:35 PM
> To: Steve Waldstein; STDS-802-3-BLADE@listserv.ieee.org
> Subject: RE: [BP] DC block capacitors in 10G NRZ channels
>
> Hi Steve,
>
> Regarding your first point, yes, if the DC block is located after the
> 50 ohm termination and the clamp diodes, then the clamp voltage cannot
> be set independently of the TX DC bias. In particular the clamp
> voltage can no longer easily be set such as to allow equal excursions
> of the signal in both directions from its bias point. This is
> definitely a limitation to be considered. The RX can of course still
> be biased in its sweet spot, independently of the TX bias.
>
> Regarding your second point, I understand your concern about having
> two resistors where you only had one, and that each resistor (and for
> that matter also the DC block capacitor) will have some capacitance to
> GND and potentially negatively affect the return loss. My belief is
> that the additional capacitance can be small (compared to main
> contributors such as clamp diode capacitance) but I am not a SERDES
> designer and would prefer to hear from one. However I would claim
> that any such degradation is much less than that due to having to
> introduce an external DC block in the signal path.
>
> Vicente
>
> -----Original Message-----
> From: Steve Waldstein [mailto:Steve.Waldstein@TUNDRA.COM]
> Sent: Monday, June 05, 2006 2:05 PM
> To: STDS-802-3-BLADE@listserv.ieee.org
> Subject: Re: [BP] DC block capacitors in 10G NRZ channels
>
> Vincente and Brian,
>
> I am new to this reflector so please excuse me if this has been
> brought up before.
>
> I support allowing the implementer to determine where the DC clock
> capacitor is but isn't it important to ensure that the implementation
> of the termination does not limit the signal swing at the receiver? If
> the capacitor comes after the termination then the common mode point
> of the termination effects the allowed swing. If the termination is
> on-die and common mode point is ground, like in PCIexpress, then the
> single ended swing is limited by the clamping diodes of the
> technology. The implementer will have to ensure that the clamp can not
> turn on if the maximum swing arrives at the receiver. Will this be a
> problem in this configuration?
>
> Also, this kind of configuration, with the capacitors after the
> termination, typically increase the receiver capacitance since you
> have the parasitic capacitance of the termination and then the
> parasitic capacitance of the network setting the common mode point of
> the receiver after the DC blocking capacitor. This creates another
> type of it challenge on meeting any reasonable return loss
requirements.
>
> Steve Waldstein
> Tundra Semiconductor Corporation
> 39 Darling Avenue
> South Portland, ME 04106
> Voice: (207) 773-2662 x3047
> FAX: (207) 773-1550
> E-mail: steve.waldstein@tundra.com <mailto:steve.waldstein@tundra.com>
>
>
> -----Original Message-----
> From: Cavanna, Vicente Vaca (Sr. ; ProCurve ASICs)
> [mailto:vicente.cavanna@hp.com]
> Sent: Monday, June 05, 2006 3:09 PM
> To: STDS-802-3-BLADE@listserv.ieee.org
> Subject: Re: [BP] DC block capacitors in 10G NRZ channels
>
> 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
>>