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threshold level of the receiver’s differential amplifier. Once the Receiver recognizes the
state change, variations in the falling edge are not significant.
To avoid edge rate variations due to driver turn-off, we must equate the voltage to which
the driver is taken at turn-off with a point in the logic swing which will guarantee that the
Receiver changes state. Since PECL logic swings are 800 mV, we may safely choose a
500 mV change at the driver (100 mV past the midpoint) as a guaranteed state change
at the Receiver. If the driver turns off instantly, we require the voltage divider formed by
RE and Z0 to produce a 500 mV change from VOH. We can write:
V
OH RE/(RE + Z0) = VOH – 0.5
4.1 x RE/(RE + Z0) = 3.6
RE = 7.33 Z0
As a general rule, we may then say that:
186 < RE < 7.33 Z0
Fig u re 5 -3
P u ll-Do w n a n d Te rm in a t io n fo r AC Co u p le d Lin k
TAXI TX
TAXI RX
VCC
R1
Pseudo-ECL
Driver
C
Copper Media
SEROUT
SERIN
R2
RE
12330E-10
Fig u re 5 -4
S e ria l Lin k w it h Ou t p u t Drive r Mo d e l
VCC
TAXI TX
TAXI RX
Pseudo-ECL
Driver (Model)
R1
R2
R0
C
Copper Media
B
SWI
VOH
SERIN
RE
12330E-11
5 .4 Ba s e lin e Wa n d e r a n d t h e AC Co u p lin g Ca p a c it o r
The 4B/5B and 5B/6B data encoding schemes which are used by the TAXIchip set are
run-length limited to a maximum of 3 consecutive LOW states (non-transitions in NRZI).
This type of encoding ensures that on average there will be less than ±10% variation in
the DC component of the encoded data.
When the encoded data is passed through an AC coupled link, the high-pass filtering of
the AC coupling will introduce jitter because of the fluctuating threshold caused by the
variation in DC component. This undesired side-effect of AC coupling is often described
64
TAXIchip Integrated Circuits Technical Manual
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