Virtex-5 RocketIO GTP Transceiver User Guide
227
UG196 (v1.3) May 25, 2007
Coupling
R
Several protocols including PCI Express and SATA specify ranges for blocking capacitors
in applications. This is done not only to simplify compliance but also to ensure that the link
presence detection features included in these specifications work correctly.
The blocking capacitor when combined with the termination resistance acts as a high-pass
filter.
shows a simplified circuit model for the link. The internal blocking
capacitors are not shown in this model because they do not play a significant role in
blocking DC currents from the external link as described in
.
Problems occur when the line is held in the on state for an extended period of time. When
this happens, charge accumulates on the blocking capacitors and a DC offset is added or
subtracted from V2. This offset results in what is known as baseline wander (see
).
The effect of baseline wander is to shift the signal with respect to the threshold points in
the receiver. This in turn skews the time at which transitions within the signal are
recognized. Pattern Dependant Jitter (PDJ) is the result of this skew.
shows an
overlay of V1 and V2 in the region of
where the jitter is greatest and shows
several key parameters.
Table 11-1:
PCI Express and SATA Blocking Capacitor Values
Specification Required
Range
PCI Express Base Specification Revision 1.1
75 to 200 nF
SATA Revision 2.5
0 to 12 nF
Figure 11-3:
Simplified Link Circuit Model
Figure 11-4:
Baseline Wander and PDJ
R
TERM
C
IN
C
IN
R
TERM
V
REF
RXP
RXN
+
–
+
V1
+
–
V2
+
–
TXP
Transmitter
Receiver
–
TXN
UG196_c11_03_091906
VTH
V1
V2
VTH
PDJ
UG196_c11_04_091906
Содержание Virtex-5 RocketIO GTP
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