PCK953_5 © NXP B.V. 2008. All rights reserved.
Product data sheet Rev. 05 — 9 October 2008 7 of 15
NXP Semiconductors
PCK953
20 MHz to 125 MHz PECL input, 9 CMOS output, 3.3 V PLL clock driver
11.2 Driving transmission lines
The PCK953 clock driver was designed to drive high speed signals in a terminated
transmission line environment. To provide the optimum flexibility to the user, the output
drivers were designed to exhibit the lowest impedance possible. With an output
impedance of less than 20 Ω, the drivers can drive either parallel or series terminated
transmission lines.
In most high performance clock networks, point-to-point distribution of signals is the
method of choice. In a point-to-point scheme either series terminated or parallel
terminated transmission lines can be used. The parallel technique terminates the signal at
the end of the line with a 50 Ω resistance to 0.5V
CC
. This technique draws a fairly high
level of DC current, and thus only a single terminated line can be driven by each output of
the PCK953 clock driver. For the series terminated case, however, there is no DC current
draw, thus the outputs can drive multiple series terminated lines. Figure 4 illustrates an
output driving a single series terminated line versus two series terminated lines in parallel.
When taken to its extreme, the fan-out of the PCK953 clock driver is effectively doubled
due to its capability to drive multiple lines.
The waveform plots of Figure 5 show the simulation results of an output driving a single
line versus two lines. In both cases, the drive capability of the PCK953 output buffers is
more than sufficient to drive 50 Ω transmission lines on the incident edge. Note from the
delay measurements in the simulations, a delta of only 43 ps exists between the two
differently loaded outputs. This suggests that the dual line driving need not be used
exclusively to maintain the tight output-to-output skew of the PCK953. The output
waveform in Figure 5 shows a step in the waveform; this step is caused by the impedance
mismatch seen looking into the driver. The parallel combination of the 43 Ω series resistor
plus the output impedance does not match the parallel combination of the line
impedances. The voltage wave launched down the two lines will equal:
(1)
Z
o
=50Ω||50 Ω
R
s
=36Ω||36 Ω
R
o
=14Ω
Fig 4. Single versus dual transmission lines
Z
o
= 50 Ω
002aae140
R
s
= 36 Ω
Z
o
= 50 Ω
R
s
= 36 Ω
PCK953
OUTPUT BUFFER
OutB1
OutB0
14 Ω
Z
o
= 50 Ω
R
s
= 36 Ω
PCK953
OUTPUT BUFFER
OutA
14 Ω
IN
IN
R
o
R
o
V
L
V
S
Z
o
R
s
R
o
Z
o
++
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