User guide

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 figure below.
0 0 0 0 01 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).
The figure above refers to a so-called post-cursor de-emphasis. You may wish to
know how that is generated.
The 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.
This can be done by setting the output voltage swing to the desired de-emphasis
amplitude and specifying a negative amplitude ratio (an amplification). This inverts
Setting up External Instrument(s) 2
Agilent J-BERT N4903B High-Performance Serial BERT 39
Post-cursor de-emphasis
Pre-cursor de-emphasis