RE: [SI-LIST] : 5V PECL vs 3.3V PECL - VTT???

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From: Peterson, James F (FL51) (james.f.peterson@honeywell.com)
Date: Wed May 31 2000 - 04:16:44 PDT


Steeve
 
You mention setting Vtt around the middle point between VOH and VOL. You
also say that you set 5V PECL Vtt at 3V, which is correct, but is not the
middle point - it's more like VOL.
Anyway, regarding your question : I believe everything slides down 1:1 for
3.3V PECL (but check the values of VOL and VOH).
BTW : the great thing about using a Vtt that is different than Vee is to
make the value of Rtt close to the t-line impedance.
 
Jim

-----Original Message-----
From: Steeve Gaudreault [mailto:sgaud@nortelnetworks.com]
Sent: Friday, May 26, 2000 7:26 AM
To: 'si-list@silab.eng.sun.com'
Subject: [SI-LIST] : 5V PECL vs 3.3V PECL - VTT???

Hi all,

Something confused me lately and I don't find any information that explains
this....I need some help.

For a 5V PECL driver, its VTT is set at 3V (Vcc - 2V is the rule). Does the
same rule applies for 3.3V PECL Drivers ie VTT = (3.3V - 2V) = 1.3V ????

I tend to set VTT around the middle point between VOH and VOL, but for 3.3V
PECL VTT its quite different when I do the math.

any help would be appreciated.

thanks,

Steeve Gaudreault
Nortel Networks

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