From: Mike Saunders ([email protected])
Date: Wed Jun 06 2001 - 11:15:48 PDT
Actually, I mis-stated the termination voltage for the PECL pair, and also
got sidetracked by your mentioning AC coupling in the first place. For
starters, you apparently are NOT interfacing two different signaling
levels, so the AC coupling is not needed. This is the purpose of using the
xceiver in the first place. A parallel termination can be used from VCC
through a resistor to your signal line, then again from the signal through
a resistor to VCC - 2V. The thev. equivalent should be 50ohms and should
be terminated to ~1.98V (which is VCC - 1.32V as D.C. stated). The VCC -
2V, or VTT, is an alternative to using VEE in parallel termination schemes
and saves power, but it is not the terminating voltage.
>Since you are interfacing two different signaling levels, place the DC
bias resistors and AC coupling caps as close to the fo
>transceiver input as possible. Place the thev. termination for the PECL
driver immediately before the AC caps (the ECL
>outputs need a DC path to VEE, so the termination must occur BEFORE the AC
coupling). You are correct to set the DC bias to
>1.96V for your inputs, but the thev. termination for the PECL load should
be terminated to 1.3V through the equivalent of
>50ohms (you could also use a "Y" or "middle stub" termination on your PECL
diff. pair). The AC coupling caps in the middle
>will take up the slack, but you'll get some RC delay and edge rate
degradation using them. Make sure you simulate at speed
>for your application.
>Another alternative for level-shifting would be to use a resistor tree.
Both level-shifting and termination techniques for
>PECL are detailed in some good app notes by On-Semi, which can be found at:
I am using a differential pecl to ttl and ttl to differential
pecl translator in my design in which the differential pecl o/p is using to
drive one fo transceivers(hfbr 5903).As the fo transceiver i/p are in a
different level I used capacitor coupling and biased the differential o\p
at the fo i/p side.what I did is, biased the lines to middle of fo i/p
(1.96v) with a thevenin termination which provides a 50 ohm impedance.
But what I read is that the termination voltage should be vcc-2,i.e.
1.3v(3.3-2).How can I satisfy both of this condition or I am doing
something false?The fo I/p levels are as follows
vil min-1.49v vil max-1.825 vih min-2.135 vihmax-2.42
Thanks for replying
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