[SI-LIST] Re: Maximum frequency consideration for high-speed digital analysis of differential signals
- From: "Grossman, Brett" <brett.grossman@xxxxxxxxx>
- To: <rula.bakleh@xxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Fri, 6 Jan 2006 10:14:24 -0800
Wow, if I had a dime for every time this question came up...
I had attended a seminar at DesignCon a few years back when I was
struggling with this same question on the 40Gb/s interconnects we were
designing. The instructor (Henri Merkelo) asked a question of the
audience as to the necessary bandwidth for a 10Gb/s. From an audience
of folks with collectively dozens of products already shipping in the
10Gb/s space, there were plenty of different answers. Here is a
sampling.
1. BW=3D0.35/tr(10-90)
2. BW=3D0.22/tr(20-80)
3. f0=3D0.5*datarate, then BW=3D3*f0
4. or BW=3D5*f0
5. or BW=3D10*f0 (i.e. odd harmonics of the fundamental)
6. f0=3D0.5*datarate or 1.0*datarate depending on the coding scheme (for
example RZ vs. NRZ)=20
7. BW =3D datarate / 2*log2(number of encoded symbols)
8. Enough bandwidth such that when passed through a brick-wall (or
n-pole) filter the residual power spectral density of enough bits of a
PRBS, is small enough.
There are lots more, but I'll save the rest (along with the truth about
the steroid abuse and wild parties of SI engineers) till I right my
'tell all' book of the signal integrity world ;^) =20
In reality, I think you will be hard pressed to find a rule of thumb
that satisfies all your needs. All those that have been mentioned by
myself and others are a good start to help better understand the 'big
knobs' in your system. The cost of trying these different approaches
and evaluating the sensitivity of your system to the different choices
can be pretty reasonable in a simulation environment. BTW, my response
is intended to be very general and not specific to the Rocket I/O
interface. For that interface some industry standard or customer
reference designs may exist, but I am sorry that I haven't worked with
it before. Maybe some of the Xilinx folks will have more input.
My $0.02,
-Brett
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of art_porter@xxxxxxxxxxx
Sent: Friday, January 06, 2006 8:18 AM
To: Cortex.Chen@xxxxxxxxxxxx; tom@xxxxxxxxxxxxx; rula.bakleh@xxxxxxxxxx;
si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Maximum frequency consideration for high-speed
digital analysis of differential signals
Well it all depends on the shape of the filter response. Is it =3D
single-pole RC? 3rd-order Chebyshev? Gaussian? The ratios listed below =
=3D
are true for Gaussian but not necessarily for all responses. For =3D
instance, for today's more brick-wall-like scope responses, the 10-90 =
=3D
numerator is closer to 0.42. =3D20
Art Porter
Agilent=3D20
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
=3D
On Behalf Of Cortex.Chen@xxxxxxxxxxxx
Sent: Thursday, January 05, 2006 7:26 PM
To: tom@xxxxxxxxxxxxx; rula.bakleh@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Maximum frequency consideration for high-speed =
=3D
digital analysis of differential signals
0.35/Tr ---- 10% to 90%
0.22/Tr ---- 20% to 80%
=3D20
Regards,
=3D20
Cortex
________________________________
From: si-list-bounce@xxxxxxxxxxxxx =3DA5N=3DB2z Tom Dagostino
Sent: 2006/1/6 [=3DACP=3DB4=3DC1=3DA4=3DAD] =3DA4W=3DA4=3DC8 10:03
To: rula.bakleh@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Maximum frequency consideration for high-speed =
=3D
digital analysis of differential signals
I'm not sure how you are computing your bandwidth. If you have a 25 =3D
psec
risetime (assuming 10-90%) your -3dB bandwidth is 0.35/25psec or 14 GHz.
When analyzing bandwidth requirements for systems like this most people
=3D
talk
about passing the 3rd or 5th harmonic of the clock. In your case that =
=3D
is
7.5 or 12.5 GHz. Some analysis of your system requirements is in order.
Tom Dagostino
Teraspeed(R) Labs
13610 SW Harness Lane
Beaverton, OR 97008
503-430-1065
tom@xxxxxxxxxxxxx
www.teraspeed.com
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
401-284-1827
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of rula.bakleh@xxxxxxxxxx
Sent: Thursday, January 05, 2006 2:19 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Maximum frequency consideration for high-speed
digital analysis of differential signals
Hi Everyone,
I'm designing a differential signal for the Rocket I/O interface. My =3D
signal
speed is 5 Gbit/sec, operating frequency is 2.5 GHz, and fastest edge =
=3D
rate
is 25 psec or 40 GHz.
In regards to performing channel analysis, what is the highest frequency
content to consider when designing this particular differential signal
serial link? If I am measuring or simulating S parameters for various
portions of this link what is the highest frequency of interest that I =
=3D
need
to consider as a rule of thumb and why?
Is it the operating frequency only, multiples/harmonics of the operating
freq, edge rate frequency (f=3D3D1/trise), multiples/harmonics of the =
edge
=3D
rate
freq, or some fraction of the edge rate? I'm finding conflicting
information: some people say that it's the full edge rate, others say =
=3D
0.35
or 0.5 of the edge rate, and yet others say multiples of the edge =3D
frequency
should be taken into account for analysis and design. I'm hoping that
somebody could shed some light on this topic for me.
Thank You,
Rula Bakleh
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