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Hi
Joel,
I have
two questions:
(1)
If, for the sake of the channel masks, the cap is placed between TP1 and
TP4 (i.e. "the channel owns the cap"), why does this force a channel
designer into placing a cap on their channel?
(2) (pardon the nieve question) Do we know that
IC vendors are planning to exit out of their packages on balls other than
the outside rows?
Thanks,
Brian
Brunn
XIlinx
-----Original Message----- Richard,From: owner-stds-802-3-blade@listserv.ieee.org [mailto:owner-stds-802-3-blade@listserv.ieee.org]On Behalf Of Joel Goergen Sent: Friday, August 20, 2004 7:59 AM To: STDS-802-3-BLADE@listserv.ieee.org Subject: Re: [BP] what constitutes the channel I read through your presentation and there are a few things to discuss. I wasn't as clear as I should have been on the clean launch. The data I was refereing to clearly shows a single ended launch ... so 8 to 9 ohms single ended. I have not looked at the differential response, but would guess it should fall within the same 15% passing through a connector thru-hole, press-fit. I'm pretty much shocked to hear that chip vendors don't have that much data on the BGA bad. You will never get a clean launch through the BGA pad because we just can't carve out the ground or play with the geometry like you can on an fxp layout. Lets not forget the necking required to get out of the package. I can go along with providing a clean launch if all the chip people agree the packaging will be 4mm pitch BGA ... yeah ... 4! Some type of isolation has been used by IEEE since way back ... either a blocking cap or a magnetic. This helps eliminate the common mode problem that occurs on large back planes, helps control Differential noise for EMI, and makes hot-swap much easier in terms of power supply control ... as you well know, most chip vendors now require multiple voltages, as well as power tracking or syncronization on the ramp up or down. I have no issue specifying some type of requirement on the blocking cap pad or type, but once you approach the chip package, I can't accept any kind of control like that. This whole conversation scares me ... it makes me think that all this time every chip I have ever used doesn't meet the Sxx11/Sxx22 requirements that it should meet because know one understands the pad or can de-embed it for modeling and test. The real channel goes from TP1 to TP4, with the cap on the rx side. It always has been this way ... and for good reason. We can not dictate to the channel that a cap has to be used should a system designer decide not to use it. Placing it in the rx side allows the chip model to either embarce it or not. There is nothing stoping the rx side from NOT using a cap. But place it in between TP1 and TP4 and everyone will have to have it. The material and channel models become very difficult. We scratch all our work and all our design efforts. -joel Mellitz, Richard wrote: I think many Si vendor will take issue with 1 below. So let me see if I can compromise on 2 to make it more palatable to all. The track I was on was to define the AC cap and trace to pin of the Rx as a "clean launch". I think I demonstrated in the presentation that with close attention to detail and real good "cherry picked" caps, this can be pretty much done. The rub is the "real" product channel. I propose that if the AC cap to Rx is not clean then it is to be included into the channel. I think this addresses everyone's issues. Performance: This insures the highest possible performance is supported by the standard. Superior product design will meet the 40" and more. Lower cost designs need to make tradeoffs an probably won't reach as far. Measurement: This is enabled with the "clean AC launch". Now the issue we need to address is what is "clean" ... SD11, SD21, etc. Maybe this is another ad hoc sidebar but I think the measurement experts can plant a good stake in the ground here. Silicon Vendors: A clean launch pretty much gets you to the BGA pad as I demonstrated in the presentation. Since this launch for testing is relatively transparent, Si vendors should not have serious issues of including it the Rx. Board/system designers: Since we now have the "bar" set for performance (channel spec), board/system designers can trade off connectors, material, stack-up, etc. We now include the AC cap to Rx as part of the channel tradeoffs if it's not as "clean" as we would like. What do y'all think? Rich Mellitz Intel Corporation 803-216-2160 -----Original Message----- From: owner-stds-802-3-blade@listserv.ieee.org [mailto:owner-stds-802-3-blade@listserv.ieee.org] On Behalf Of Charles Moore Sent: Thursday, August 19, 2004 8:04 PM To: STDS-802-3-BLADE@listserv.ieee.org Subject: [BP] what constitutes the channel To me it is clear that the "channel" goes all the way from the Transmitter (the transistors and resistors) to the receiver (the load and input buffer) but: it is also clear to me that the traces on the chip, the chip pads, the connection from the chip to package and the IC package are inaccessible to anyone but the IC supplier and as such it makes no sense to include them in the measured channel. They are best considered part of the Transmitter or Receiver. It is less clear to me that the connection between the package and the PC board is part of the Rx or Tx but i am willing to go along with this. It also clear to me that not all system builders will be willing to have IC suppliers dictate the design of their PC boards. If the IC supplier cannot define the thickness of boards, the number of layers, the choice of layers used to connect to capacitors, the capacitors to use, and the distance to the capacitors, it will be impossible for the IC supplier to guarantee proper operation of the receiver. I can see two ways out of the problem: 1. Have the system designer take responsibility for receiver performance by characterizing the path from capacitor to IC and requiring extra performance from the IC to cover losses in this path. This solution has two difficulties: a. It requires more performance from the ICs which are likely to be hard pressed to work without additional losses. b. It will mean that each system design will require different IC designs, negating the advantages of having a standard. 2. Put a normative specification on the path from the capacitor to the IC. This could be done in either of two ways. a. Establish specifications on the path from the capacitor to IC. This would involve having another ad-hoc committee or another project for the channel ad-hoc, another element to be specified, and another element to be added to simulation, with uncertainties due to unknown S11 and S22 phase. b. Lump the path from the capacitor to the IC into the "channel." If there are nasty uncertainties having to do with vias to the capacitors etc., i think that it will be best to have them out in the open where we can deal with them in standards rather than hiding away as implementation details. Note: when i use the term: "path from the Capacitor to IC" i intend it to include the capacitor, and any vias and pads necessary to connect to it but not the connection between the board and the IC. Low frequency losses due to AC coupling through the capacitors will be negligible in the measured range: with 10nF capacitors the loss at 50MHz will be less than 5E-5 dB. charles -- |--------------------------------------------------------------------| | Charles Moore | Agilent Technologies | ASIC Products Division | charles_moore@agilent.com | (970) 288-4561 |--------------------------------------------------------------------| |