How we know the exact HPF location to guarantee the signal is well passed through. A running 8b10b stream may slightly lose its DC balance with a short period. I just have no idea the exact amount of imbalance and the period length. Then have no idea to chose the pole with high confidence. Regards, Neo --- On Thu, 5/14/09, T.K. Jeon <tkjeon@xxxxxxxxxxx> wrote: From: T.K. Jeon <tkjeon@xxxxxxxxxxx> Subject: [SI-LIST] Re: AC coupling of SATA and SAS I/O To: "Ward, Richard" <richard.ward@xxxxxx>, "Alief" <saifj_m@xxxxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx> Date: Thursday, May 14, 2009, 6:30 PM Hi Alief, You can avoid the baseline wander by choosing the capacitor such that your cut off frequency is much lower than the lowest frequency component from your data stream. Even if there is baseline wander due to ac coupling, many receivers in serial interface employ DFE these days, so it will restore the baseline wander at the receiver. Regards, TK ________________________________ From: Ward, Richard [mailto:richard.ward@xxxxxx] Sent: Tuesday, May 12, 2009 8:27 PM To: Alief; si-list@xxxxxxxxxxxxx; T.K. Jeon Subject: RE: [SI-LIST] Re: AC coupling of SATA and SAS I/O Hi Alief, Yes, but it's data dependent jitter and therefore bounded + the standards stated below tend to use parity balanced, run-length limited coding. Given the limited run-length and the translation from that voltage droop to edge jitter - this isn't much of a problem in real life (well, there are usually many other problems that float to the top of the swamp before this one). Regards, Richard ________________________________ From: Alief [saifj_m@xxxxxxxxx] Sent: Tuesday, May 12, 2009 7:30 PM To: si-list@xxxxxxxxxxxxx; T.K. Jeon; Ward, Richard Subject: Re: [SI-LIST] Re: AC coupling of SATA and SAS I/O Dear all, There apparently is one impairment that AC coupling does introduce : some increase in jitter. The AC coupling causes the signal to droop : the percentage droop is approx (tw/RC)*100 where tw is the time between data transitions; and, the added jitter is equal to (tr*tw)/(2*RC) where tr is the transition time of the signal. AFN, Alief --- On Sat, 5/9/09, Alief <saifj_m@xxxxxxxxx> wrote: From: Alief <saifj_m@xxxxxxxxx> Subject: [SI-LIST] Re: AC coupling of SATA and SAS I/O To: "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>, "T.K. Jeon" <tkjeon@xxxxxxxxxxx>, "RichardWard" <richard.ward@xxxxxx> Date: Saturday, May 9, 2009, 4:37 AM Hi Richard, Thanks v much for your response and sorry for being a little tardy in getting back - I was out of town all day and have just got back. I understand now where you were coming from in terms of your comments regarding to 0V bias. Also, I appreciate your comments about hot swap involving uncommon risk - mainly due to the "human intervention" factor. I too like using AC coupling and agree that the AC caps can be your friends in preventing transients, failures, etc. The only thing I would like to add is that having worked with different types of caps and having been burned by cap failures I wonder about the reliability of the coupling caps : is it comparable to the reliability of the silicon? Since these caps are in the signal path and are not redundant (like bypass caps etc can be); does anyone know of reliability info for these caps ? Thanks, Alief --- On Fri, 5/8/09, Ward, Richard <richard.ward@xxxxxx<http://us.mc435.mail.yahoo.com/mc/compose?to=richard.ward@xxxxxx>> wrote: From: Ward, Richard <richard.ward@xxxxxx<http://us.mc435.mail.yahoo.com/mc/compose?to=richard.ward@xxxxxx>> Subject: [SI-LIST] Re: AC coupling of SATA and SAS I/O To: "Alief" <saifj_m@xxxxxxxxx<http://us.mc435.mail.yahoo.com/mc/compose?to=saifj_m@xxxxxxxxx>>, "si-list@xxxxxxxxxxxxx<http://us.mc435.mail.yahoo.com/mc/compose?to=si-list@xxxxxxxxxxxxx>" <si-list@xxxxxxxxxxxxxx<http://us.mc435.mail.yahoo.com/mc/compose?to=si-list@xxxxxxxxxxxxxx>>, "T.K. Jeon" <tkjeon@xxxxxxxxxxx<http://us.mc435.mail.yahoo.com/mc/compose?to=tkjeon@xxxxxxxxxxx>> Date: Friday, May 8, 2009, 7:24 AM Hi Alief, My initial statement was directed at one of the comments below: "If your transmitter has 0V DC bias, then I guess you don't need to implement AC coupling". I read this as a statement of 0V common-mode from the Tx. A large portion of the Serdes out there and used for these standards are VTT referenced or GND referenced swings and given the supply ranges for the type of systems these go into (often somewhere between 0-0.8V to 0-5V), I don't believe the statement above (or I misunderstand it and someone will correct me). We're agreed on the function of the cap - but the prevous thread seemed to contradict that (to me anyway). The point about the differently referenced systems was for interoperability yes, but that fact that it allows a wider range of Si to work with those standards if ac caps are there is a benefit to all - and therefore usually a good thing for a standard to have. For hot-swap, one of the protections the circuit designer needs to put in is for "things to go wrong" and short. This is a failure mode. It may be a transient, but it may be someone, somehow, managed to stuff the wrong connector in the slot - my children are experts at that. We could argue this isn't strictly hot-swap, but it is a precaution many circuit designers take and label under the hot-swap banner. The diff pairs, being the most sensitive signals are usually the most difficult to protect. For EM breakdown limits, time is obviously one of the major parameters. So if it's a short on a high-speed signal that's ac coupled...we've blocked the major dc component and therefore reduced the time and the EM risk. That was the only point I was making there - caps can be your friends in a failure mode. Regards, Richard ________________________________ From: Alief [saifj_m@xxxxxxxxx<http://us.mc435.mail.yahoo.com/mc/compose?to=saifj_m@xxxxxxxxx>] Sent: Thursday, May 07, 2009 11:36 PM To: si-list@xxxxxxxxxxxxx<http://us.mc435.mail.yahoo.com/mc/compose?to=si-list@xxxxxxxxxxxxx>; T.K. Jeon; Ward, Richard Subject: Re: [SI-LIST] Re: AC coupling of SATA and SAS I/O Hi Richard, Thanks a bunch for your response. I'm really not sure why you found it necessary to state that "AC caps aren't used to set the Rx bias". Of course, the AC cap does not set the Rx bias. In fact, how can one expect an AC cap to set a bias ? What the AC cap does is to remove the common mode level not set it; and then, of course, that permits the Rx to set the common mode wherever it chooses to. You have made good points about how ac coupling helps differently referenced systems work together - I would classify that under interoperability.. Those are all good points worth noting. However, I am afraid I am not aware of any long term effects of hot swap. Doesn't hot swap only cause transients ? Assuming there are no application mismatches or design marginalities/errors how could hot swap result in long term effects ? Would greatly appreciate your expounding on that. Regards, Alief --- On Thu, 5/7/09, Ward, Richard <richard.ward@xxxxxx<UrlBlockedError.aspx>> wrote: From: Ward, Richard <richard.ward@xxxxxx<http://us.mc435.mail.yahoo.com/mc/compose?to=richard.ward@xxxxxx>> Subject: [SI-LIST] Re: AC coupling of SATA and SAS I/O To: "Alief" <saifj_m@xxxxxxxxx<http://us.mc435.mail.yahoo.com/mc/compose?to=saifj_m@xxxxxxxxx>>, "si-list@xxxxxxxxxxxxx<http://us.mc435.mail.yahoo.com/mc/compose?to=si-list@xxxxxxxxxxxxx>" <si-list@xxxxxxxxxxxxxx<http://us.mc435.mail.yahoo.com/mc/compose?to=si-list@xxxxxxxxxxxxxx>>, "T.K. Jeon" <tkjeon@xxxxxxxxxxx<http://us.mc435.mail.yahoo.com/mc/compose?to=tkjeon@xxxxxxxxxxx>> Date: Thursday, May 7, 2009, 9:48 PM Hi Alief, My view is a little different from that below. AC caps aren't used to set the Rx bias, but to allow the Rx to set it's own bias point. This is especially useful for rack-to-rack systems where the common mode cannot be well controlled. It also has the advantages of: - allowing VTT referenced and GND referenced systems to work together - allowing a single Serdes to cover multiple standards - allowing newer (restricted supply) devices to work with legacy devices - ... (I'm sure someone on this list has written a book or two on this...) The hot-swap ability doesn't really demand dc blocking caps, but they give advantages. "hot-swap" is a bit of a nebulous term too, often. One of the risks of hot-swap is shorting the connections (signal-signal or signal-supply). Some devices will be designed to withstand the short-term current only (not long-term lifetime degradation). Having the caps means a long-term effect, actually becomes a short-term one, therefore EM considerations are less restrictive on the circuit design side. I believe PCIE defined them at the Tx for this (shorting) reason (whereas a majority of non-pluggable applications put them at the Rx side, why? well it depends). Regards, Richard -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx<http://us.mc435.mail.yahoo.com/mc/compose?to=si-list-bounce@xxxxxxxxxxxxx><http://us.mc435.mail.yahoo.com/mc/compose?to=si-list-bounce@xxxxxxxxxxxxx> [mailto:si-list-bounce@xxxxxxxxxxxxx<http://us.mc435.mail.yahoo.com/mc/compose?to=si-list-bounce@xxxxxxxxxxxxx><http://us.mc435.mail.yahoo.com/mc/compose?to=si-list-bounce@xxxxxxxxxxxxx<UrlBlockedError.aspx>>] On Behalf Of Alief Sent: 07 May 2009 12:41 To: si-list@xxxxxxxxxxxxx<UrlBlockedError.aspx><http://us.mc435.mail.yahoo.com/mc/compose?to=si-list@xxxxxxxxxxxxx>; T.K. Jeon Subject: [SI-LIST] Re: AC coupling of SATA and SAS I/O Thank you for your response T.K. Yes, a 0V DC bias would establish an automatic reference (in theory); thus it would be akin to DC coupling. However, I am hesitant to make my common mode 0V; because of issues with ground loops, ground path parasitics, etc and also because the receiver would see negative voltages. I realize that this is to first order a biasing issue and allows for interoperability. However, are there other considerations ? I have heard/read that it might permit hot swappability. Is this true. If I had a known common mode level doesn't that make it easier for hot swapping ? Also, are there other issues which AC coupling helps with ? Thanks a bunch in advance, Alief --- On Thu, 5/7/09, T.K. Jeon <tkjeon@xxxxxxxxxxx<http://us.mc435.mail.yahoo.com/mc/compose?to=tkjeon@xxxxxxxxxxx><http://us.mc435.mail.yahoo.com/mc/compose?to=tkjeon@xxxxxxxxxxx>> wrote: From: T.K. Jeon <tkjeon@xxxxxxxxxxx<http://us.mc435.mail.yahoo.com/mc/compose?to=tkjeon@xxxxxxxxxxx><http://us.mc435.mail.yahoo.com/mc/compose?to=tkjeon@xxxxxxxxxxx<UrlBlockedError.aspx>>> Subject: RE: [SI-LIST] AC coupling of SATA and SAS I/O To: "Alief" <saifj_m@xxxxxxxxx<http://us.mc435.mail.yahoo.com/mc/compose?to=saifj_m@xxxxxxxxx><http://us.mc435.mail.yahoo.com/mc/compose?to=saifj_m@xxxxxxxxx>>, "si-list@xxxxxxxxxxxxx<http://us.mc435.mail.yahoo.com/mc/compose?to=si-list@xxxxxxxxxxxxx><http://us.mc435.mail.yahoo.com/mc/compose?to=si-list@xxxxxxxxxxxxx>" <si-list@xxxxxxxxxxxxxx<http://us.mc435.mail.yahoo.com/mc/compose?to=si-list@xxxxxxxxxxxxxx><http://us.mc435.mail.yahoo.com/mc/compose?to=si-list@xxxxxxxxxxxxxx>> Date: Thursday, May 7, 2009, 6:55 PM Alief, AC coupling is needed to maintain the correct DC bias for receivers. If your transmitter has 0V DC bias, then I guess you don't have to implement AC coupling. TK -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx<http://us.mc435.mail.yahoo.com/mc/compose?to=si-list-bounce@xxxxxxxxxxxxx><http://us.mc435.mail.yahoo.com/mc/compose?to=si-list-bounce@xxxxxxxxxxxxx<UrlBlockedError.aspx>> [mailto:si-list-bounce@xxxxxxxxxxxxx<http://us.mc435.mail.yahoo.com/mc/compose?to=si-list-bounce@xxxxxxxxxxxxx><http://us.mc435.mail.yahoo.com/mc/compose?to=si-list-bounce@xxxxxxxxxxxxx<UrlBlockedError.aspx>>] On Behalf Of Alief Sent: Thursday, May 07, 2009 10:46 AM To: si-list@xxxxxxxxxxxxx<http://us.mc435.mail.yahoo.com/mc/compose?to=si-list@xxxxxxxxxxxxx><http://us.mc435.mail.yahoo.com/mc/compose?to=si-list@xxxxxxxxxxxxx> Subject: [SI-LIST] AC coupling of SATA and SAS I/O Greetings everyone, I am currently looking at SATA and SAS SerDes designs. It looks like the data transmission is AC coupled. Is this a standards requirement ? Could anyone enlighten me on why SATA/SAS specifies only AC coupled links [if that is in fact the case]; i.e.. what was the thinking/justification for restricting transmission to ac coupled only. Also, do the other standards : Gbit Ethernet, PCI Express, RapidIO, etc. have similar restrictions/requirements ? Thanks v much in advance, Alief ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx<http://us.mc435.mail.yahoo.com/mc/compose?to=si-list-request@xxxxxxxxxxxxx><http://us.mc435.mail.yahoo.com/mc/compose?to=si-list-request@xxxxxxxxxxxxx> with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxxx<http://us.mc435.mail.yahoo.com/mc/compose?to=si-list-request@xxxxxxxxxxxxxx><http://us.mc435.mail.yahoo.com/mc/compose?to=si-list-request@xxxxxxxxxxxxx> with 'help' in the Subject field List technical documents are available at: http://www.si-list..net<UrlBlockedError.aspx><http://www.si-list.net/<UrlBlockedError.aspx>> List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are 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