Agilent Technologies 93000 SOC Series Training Manual page 144

Defining the Levels
Why terminate ?
144
Unterminated Transmission Line
If the driver is switched off, you have an unterminated transmission
∞
line: R = , r
Term Comp
An unterminated transmission line distorts the pulse shape due to
reflections, which interferes with the signal at the device output. The
reflections
• broadens the pulse width,
• decreases the maximum amplitude of a pulse depending on the
pulse width.
That is, the comparator does not see a clear pulse. It is difficult to
predict what the driver actually sees and to adjust the compare levels
accordingly.
Terminated Transmission Line
Terminating the transmission line solves the above problems. With a
terminated transmission line, the comparator receives a well-defined
signal from the device output. The comparator sees the original pulse
shape with
• a lower amplitude due to voltage drops (scaling with a factor < 1),
• a voltage offset due to a termination voltage.
When you terminate the transmission line, you have to adjust your
compare level thresholds accordingly.
You can use the driver to terminate output and input/output pins:
Pin Type
output (o)
bidirectional (io)
bidirectional (io)
bidirectional (io)
In order to define a termination voltage you have to specify vt in a level
set:
vt = <expression>
You must not specify iol and ioh.
Agilent 93000 SOC Series User Training Part 1, October 2004
= 1.
Termination Voltage Voltage Source
arbitrary voltage level driver
drive level high driver
drive level low driver
arbitrary voltage level active load
8-1 AC Levels