July 2005

ASM5I9658

rev 0.2

3.3V 1:10 LVCMOS PLL Clock Generator

9 of 14

Notice: The information in this document is subject to change without notice.

Driving Transmission Lines

The ASM5I9658 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 20Ω 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 50Ω

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 ASM59658 clock driver. For the series

terminated case however there is no DC current draw,

thus the outputs can drive multiple series terminated

lines. Figure 6. “Single versus Dual Transmission Lines”

illustrates an output driving a single series terminated line

versus two series terminated lines in parallel. When taken

to its extreme the fanout of the ASM5I9658 clock driver is

effectively doubled due to its capability to drive multiple

lines.

Figure 6. Single versus Dual Transmission Lines

The waveform plots in Figure 7. “Single versus Dual Line

Termination Waveforms” show the simulation results of

an output driving a single line versus two lines. In both

cases the drive capability of the ASM5I9658 output buffer

is more than sufficient to drive 50Ω transmission lines on

the incident edge. Note from the delay measurements in

the simulations a delta of only 43pS 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 ASM5I9658. The output

waveform in Figure 7. “Single versus Dual Line

Termination Waveforms” shows a step in the waveform,

this step is caused by the impedance mismatch seen

looking into the driver. The parallel combination of the

36Ω 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:

= 1.31V

At the load end the voltage will double, due to the near

unity reflection coefficient, to 2.6V. It will then increment

towards the quiescent 3.0V in steps separated by one

round trip delay (in this case 4.0nS).

Figure 7. Single versus Dual Waveforms

Since this step is well above the threshold region it will

not cause any false clock triggering, however designers

may be uncomfortable with unwanted reflections on the

line. To better match the impedances when driving

multiple lines the situation in Figure 8. “Optimized Dual

Line Termination” should be used. In this case the series

terminating resistors are reduced such that when the

parallel combination is added to the output buffer

impedance the line impedance is perfectly matched.

14

Ω + 22Ω || 22Ω = 50Ω || 50Ω

25

Ω = 25Ω

Figure 8. Optimized Dual Line Termination

ASM5I9658

OUTPUT BUFFER

14Ω

IN

OUTA

ASM5I9658

OUTPUT BUFFER

14Ω

IN

OUTB1

OUTB0

ASM5I9658

OUTPUT BUFFER

14Ω

IN