[SI-LIST] Re: Maximum frequency consideration for high-speed digital analysis of differential signals
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: brett.grossman@xxxxxxxxx
- Date: Fri, 06 Jan 2006 14:33:55 -0500
Just to throw something out there ....
I contend that BW is not the most interesting specification to be
concerned about for serial links. BW only tells part of the story. The
other half is told by the phase response (or group delay). I generally
like to perform simulation and modeling at a minimum of 3X the Nyquist
frequency for the data stream, in order to accurately capture phase
information. For 10G NRZ links, that would require a 15 GHz bandwidth
in all modeling and simulation.
regards,
Scott
Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax
http://www.teraspeed.com
Teraspeed® is the registered service mark of
Teraspeed Consulting Group LLC
Grossman, Brett wrote:
>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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