Applying Digital Predistortion; Required Options; About Digital Predistortion - Emerson R&S SGT100A User Manual

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R&S SGT100A

6.6 Applying digital predistortion

Digital predistortion (DPD) is a method to improve the efficiency of RF power amplifi-
ers. In the R&S SGT, the generated digital signal can be deliberately AM/AM and
AM/PM predistorted.

6.6.1 Required options

The equipment layout for digital predistortion includes:
●
Option AM/AM AM/PM predistortion (R&S SGT-K541)
●
Optional option envelope tracking (R&S SGT-K540)

6.6.2 About digital predistortion

Power amplifiers are an essential part of any telecommunication systems. While ampli-
fying the transmitted signal, power amplifiers sometimes also distort this signal and
change its amplitude and/or phase characteristics. Such distortions result in undesired
effects like spectrum regrowth, harmonic generation, intermodulation (IM) products, or
increased bit error rate.
The principle of the digital predistortion
To compensate for the distortions caused by the transmission system, the signal is
deliberately digitally predistorted. Digital predistortion (DPD) is a method to apply wan-
ted and well-defined predistortion on the transmitted signal. When this signal is ampli-
fied, the resulting signal features the identical characteristics, as the initial signal before
the predistortion.
signal
Figure 6-23: Illustration of predistortion principle
DPD = Digital predistortion
PA
a
b
c
Digital predistortion models
When testing power amplifiers, it is important to measure and analyze signal distor-
tions.
User Manual 1176.8674.02 ─ 12
a
P
DPD
P
= Power amplifier
= Predistortion function
= Characteristic of the power amplifier, for example the non-linear input power vs. output power
(AM/AM) function
= Ideal linearized characteristic of the amplified signal
b c
P
out out
P
in
PA
P
out
in
Signal generator settings
Applying digital predistortion
P
in
198