[SI-LIST] Re: AC Coupling bandwidth Consideration for 8b/10b
- From: wolfgang.maichen@xxxxxxxxxxxx
- To: Mike Harwood <Mike@xxxxxxxxxxxxxx>
- Date: Thu, 14 May 2009 10:36:49 -0700
One other thing to keep in mind is that you don't want to go overboard
with the time constant either. Two reasons are
- larger time constant means longer time for the levels to settle to
steady values (the waveform drifts at the beginning of the pattern and
needs a certain number of warmup cycles, given by the time constant of
your R-C filter; note that effective resistance R=100 Ohm for a 50 Ohm
line because the capacitor "sees" the two line segments in series).
- larger capacitors tend to have larger parasitics (inductance and
resistance), which degrades your signal fidelity.
So for the given bit rate a capacoitor value between 10nF anf 100nF
(assuming 50 Ohm line impedance) is the sweet spot (and thus the common
choice). Those values are easily available in small, low-inductance
packages.
Wolfgang
Mike Harwood <Mike@xxxxxxxxxxxxxx>
Sent by: si-list-bounce@xxxxxxxxxxxxx
05/14/2009 01:46 AM
To
si-list@xxxxxxxxxxxxx
cc
Subject
[SI-LIST] Re: AC Coupling bandwidth Consideration for 8b/10b
As well as the longest run length you need to pay attention to the
disparity in the number of 1's and 0's. For a 8b10b pattern of at least
20 bits I believe that the disparity is never greater than 2 and the
greatest number of consecutive bits is 5. Generally 8b10b patterns are
reasonably well behaved in this regard and that's why people put up with
the 20% coding overhead.
Rearranging V=V0 * e(-t/CR) for a 1% droop in 5 bits (5ns) gives
CR=497ns and equates to about 10nF in a 50 Ohm environment. I'd say that
75nF gives a good degree of marging.
Another interesting concept in choosing the DC coupling cap value is the
type of receiver used and the channel between TX & RX. Signals
tranmitted through long (say >18") of FR4 can take several tens of bits
to settle fully at 1Gb/s. This is a low pass effect easily determined by
examining the pulse response. If this effect is not corrected in the
receiver equalizer then some level of droop in the high pass filter is
desirable. I've seen performance improvements in higher speed SerDes
with this correction.
Mike.
It's just a single pole HPF. Calculate C based on the longest run
interval, the acceptable percentage droop and the line impedance.
Steve.
Neo wrote:
> > Hi,
> >
> > How we calculate the necessary HPF bandwidth for a say 1Gb/s 8b10b
coded NRZ bit stream?
> > I think in some spec it is defined as 75nF and 50ohm. What's the
theory behind this number?
> >
> > Thanks,
> > Neoflash
> >
> >
> >
> >
> > ------------------------------------------------------------------
> > To unsubscribe from si-list:
> > 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@xxxxxxxxxxxxx with 'help' in the Subject field
> >
> >
> > List technical documents are available at:
> > http://www.si-list.net
> >
> > 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 viewable at:
> > http://www.qsl.net/wb6tpu
> >
> >
> >
> >
>
-- Steve Weir Teraspeed Consulting Group LLC 121 North River Drive
Narragansett, RI 02882 California office (866) 675-4630 Business (707)
780-1951 Fax Main office (401) 284-1827 Business (401) 284-1840 Fax
Oregon office (503) 430-1065 Business (503) 430-1285 Fax
http://www.teraspeed.com This e-mail contains proprietary and
confidential intellectual property of Teraspeed Consulting Group LLC
------------------------------------------------------------------------------------------------------
Teraspeed(R) is the registered service mark of Teraspeed Consulting
Group LLC
------------------------------------------------------------------ To
unsubscribe from si-list: 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@xxxxxxxxxxxxx with 'help' in the Subject field List
technical documents are available at: http://www.si-list.net 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 viewable at:
http://www.qsl.net/wb6tpu
--
Harwood & Szczepanek Consulting:
Web-site: http://www.Harwood.org.uk
Contact: Mike Harwood + 44 7761 530963
Contact: Andre Szczepanek +44 7894 278285
------------------------------------------------------------------
To unsubscribe from si-list:
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@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.net
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 viewable at:
http://www.qsl.net/wb6tpu
------------------------------------------------------------------
To unsubscribe from si-list:
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@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.net
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 viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
