Pre-Emphasis Techniques - Xilinx RocketIO User Manual

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Table 3-1: Differential Transmitter Parameters

Pre-emphasis Techniques

In pre-emphasis, the initial differential voltage swing is boosted to create a stronger rising or falling
waveform. This method compensates for high frequency loss in the transmission media that would
otherwise limit the magnitude of this waveform. The effects of pre-emphasis are shown in four
scope screen captures,
The STRONG notation in
magnitude, at this point, than the LOGIC or normal level (i.e., no pre-emphasis).
A second characteristic of RocketIO transceiver pre-emphasis is that the STRONG level is reduced
after some time to the LOGIC level, thereby minimizing the voltage swing necessary to switch the
differential pair into the opposite state.
Lossy transmission lines cause the dissipation of electrical energy. This pre-emphasis technique
extends the distance that signals can be driven down lossy line media and increases the signal-to-
noise ratio at the receiver.
It should be noted that high pre-emphasis settings are not appropriate for short links (a fraction of the
maximum length of 40 inches of FR4). Excessive pre-emphasis can actually degrade the bit error
rate (BER) of a multi-gigabit link. Careful simulation and/or lab testing of the system should always
be used to verify that the optimal pre-emphasis setting is in use. Consult the Virtex-II Pro
RocketIO™ Multi-Gigabit Transceiver Characterization Summary for more detailed information
on the waveforms to be expected at the various pre-emphasis levels.
The four levels of pre-emphasis are shown in
Table 3-2: Pre-emphasis Values
102
Parameter
V Common mode output
TCM
voltage range (no
transmission line connected)
V Common mode output
TCM
voltage range (transmission
line connected)
V Differential output skew
ISKEW
Figure 3-2
Figure 3-3
Attribute Values Emphasis (%)
0
1
2
3
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Chapter 3: Analog Design Considerations
Min Typ Max
1.1 1.5
1.1 2.0
15
through Figure 3-5 on the pages following.
is used to show that the waveform is greater in voltage
Table 3-2.
10
20
25
33
Units Conditions
V
V
The common mode depends
on coupling (DC or AC),
VTTX, VTRX, and
differential swing. Spice
simulation gives the exact
common mode voltage for
any given system.
ps
RocketIO™ Transceiver User Guide
UG024 (v2.3.2) June 24, 2004