[SI-LIST] Re: Maximum frequency consideration for high-speed digital analysis of differential signals

• From: steve weir <weirsi@xxxxxxxxxx>
• To: Cortex.Chen@xxxxxxxxxxxx, <rula.bakleh@xxxxxxxxxx>
• Date: Sun, 08 Jan 2006 22:08:48 -0800

```Cortex, the context of Fknee in HSDD is for a trapezoidal, binary NRZ=20
signal.  The idea is that a channel that is flat out to Fknee will pass=20
such a signal with little distortion.  Fknee is significant in that it=20
encompasses both:  most of the signal energy, and bandwidth to the point=20
that spectral content falls rapidly, typically at a two pole rate or=20
faster.  That's a great metric for moderate speed logic signals.  The=20
typical SERDES channel will not have the luxury of such a wide=20
bandwidth.  Nor will it need it.  Gain, preemphasis, and equalization=20
restore amplitude and flatten the diminished and distorted signal seen at=20

Better references for this sort of work would be either, Dr. Johnson's more=
=20
recent book "High Speed Signal Propagation", or Eric Bogatin's "Signal=20
Integrity Simplified".  Both expend significant effort explaining frequency=
=20
issues of both signals and materials including application to high speed=20
SERDES.  They also apply to low speed protocols running long distances.  In=
=20
either case the channel is not flat through the signal bandwidth.

Steve.

At 12:16 PM 1/9/2006 +0800, Cortex.Chen@xxxxxxxxxxxx wrote:
>
>Rula,
>
>Maybe you could focus on knee frequency (0.5/Tr, Fknee) instead of BW/f3dB.
>It means that the behavior of a circuit at frequencies above Fknee hardly=
=20
>affects digital performance.
>
>In detail, refer to the section 1.1 of High-Speed Digital Design - A=20
>Handbook of Black Magic (Dr. Howard Johnson's Book@93).
>
>I also have a question for this equation, 0.5/Tr.
>It seems to be extracted from the 10% to 90% of Tr.
>Who can tell me the knee frequency for 20% to 80% of Tr?
>
>Regards,
>
>Cortex
>
>-----Original Message-----
>From: Ed Sayre III [mailto:esayre3@xxxxxxxx]
>Sent: Friday, January 06, 2006 11:48 PM
>To: Cortex Chen (=B3=AF=A5=C3=AAN)
>Cc: tom@xxxxxxxxxxxxx; rula.bakleh@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
>Subject: Re: [SI-LIST] Re: Maximum frequency consideration for high-speed=
=20
>digital analysis of differential signals
>
>Cortex,
>
>     The value of 0.35 in your calculation is base on the pole location of
>the 1st order system.  How are you deriving the 0.22 value?.  I am very
>curious to understand the thinking behind this.
>
>Regards
>-Ed
>
>
>
>At 10:25 AM 1/6/2006 +0800, Cortex.Chen@xxxxxxxxxxxx wrote:
> >0.35/Tr ---- 10% to 90%
> >0.22/Tr ---- 20% to 80%
> >
> >Regards,
> >
> >Cortex
> >________________________________
> >
> >From: si-list-bounce@xxxxxxxxxxxxx =A2DN2z Tom Dagostino
> >Sent: 2006/1/6 [?P=A1=A6A?-] ?W?E 10:03
> >To: rula.bakleh@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
> >Subject: [SI-LIST] Re: Maximum frequency consideration for high-speed
> >digital analysis of differential signals
> >
> >
> >
> >I'm not sure how you are computing your bandwidth.  If you have a 25 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=
talk
> >about passing the 3rd or 5th harmonic of the clock.  In your case that 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=
signal
> >speed is 5 Gbit/sec, operating frequency is 2.5 GHz, and fastest edge=
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=
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=3D1/trise), multiples/harmonics of the edge=
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=
0.35
> >or 0.5 of the edge rate, and yet others say multiples of the edge=
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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