Agilent Technologies J-BERT N4903, User Manual

Page: 78 / 434

De-emphasis is a method that reduces the voltage of a digital signal if
the generated level is high or low for more than one clock period. The
principle is illustrated in the following figure.
0 0 0 0 0 1 1 1 1 1 1
Output
Amplitude
(Vpp)
De-emphasis
Amplitude
Input
Signal
The de-emphasis amplitude is specified as a fraction of the output
amplitude (in percent or dB). Both can be set in the
PG Output Setup
window of the Serial BERT.
When you connect the N4916A in the PG Output Setup window, the N4916A
becomes completely transparent. Software adjusts the signal voltage
produced by the pattern generator, so that the N4916A provides the
indicated voltages to the DUT. This refers to V
low
, V
high
(or amplitude
and offset), and termination. When you disconnect the N4916A in the
same window, the data output voltages of the pattern generator are
reset and it generates the specified and indicated voltages as usual.
The bit rate setting of the Serial BERT is automatically transmitted to
the N4916A. Other functions of the pattern generator (for example,
adding jitter) are not affected.
The figure above refers to a so-called post-cursor de-emphasis. You may
wish to know how that is generated.
The N4916A De-Emphasis Signal Converter splits the incoming signal
into two branches:
•
One branch has a programmable amplifier to produce the desired
output voltage (V
pp
).
•
The other one has an adjustable delay (automatically set to one signal
clock period) and a programmable inverting attenuator/amplifier to
produce the delayed signal with a lower voltage swing.
Finally, the signals of both branches are added. This means, the delayed
signal voltage is subtracted from the specified peak-to-peak amplitude.
It is also possible to convert the input signal to a pre-cursor de-
emphasized signal.
3
Setting up the Pattern Generator
78 Agilent J-BERT N4903 High-Performance Serial BERT
Post-cursor de-emphasis
Pre-cursor de-emphasis