Agilent Technologies J-BERT N4903B, User Manual, Page: 127 / 572

As shown in this figure, the signal output levels have the following components:
• V
hi
is the upper voltage level of the signal.
• V
lo
is the lower voltage level of the signal.
• V
of
is the offset of the average voltage level from 0 V.
• V
ampt
is the amplitude of the signal.
When adjusting the output levels, it is important to understand the concept of how
the Serial BERT handles voltages.
Typically, during tests, when you adjust the amplitude, you want to keep the offset
constant. This keeps the ideal sampling point within the eye. The Serial BERT
handles this by keeping V
of
constant when V
ampt
is changed.
On the other hand, you may want to adjust the output voltage level without
changing the amplitude. The Serial BERT handles this by keeping V
ampt
constant
when V
of
is changed and V
hi
/V
lo
are accordingly adjusted.
The Serial BERT cannot generate a signal that has voltage levels out of range. If
you try to enter a value for one parameter that would put another parameter out
of limits, the Serial BERT rejects the change. This could happen, for example, if
V
lo
is already at the minimum, and you try to lower either of V
hi
or V
of
, or increase
V
ampt
.
Understanding Delay and Crossover
The Serial BERT provides the possibility of modifying the output data signal by
varying the signal's delay to the clock signal, and the signal's crossover.
The exact time delay through a test setup can vary. The delay function allows you
to compensate for this by adjusting the frequency reference for the data outputs.
This varies the phase relationship of the data and clock outputs (causes the data
pulse to have a certain time delay after the clock pulse). The higher the delay, the
greater the time difference between the clock signal and the data signal.
Setting up the Pattern Generator
4
Agilent J-BERT N4903B High-Performance Serial BERT 127
Changing the Amplitude
Changing the Output Levels
Voltage Level Restrictions
Delay