Signal Generator Settings
R&S
®
SGT100A
179
User Manual 1176.8674.02 ─ 07
7.6.1 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)
7.6.2 About Digital Predistortion
Power amplifiers are an essential part of any telecommunication systems. While
amplify the transmitted signal, power amplifiers may 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 signal to be transmitted so that when this sig-
nal is amplified, the resulting signal features the identical characteristics, as the initial
signal before the predistortion.
P
in
P
out
P
in
P
out
PA
P
in
P
out
DPD
a
b
c
signal
Figure 7-21: Illustration of predistortion principle
DPD = digital predistortion
PA
= power amplifier
a
= predistortion function
b
= characteristic of the power amplifier, for example the non-linear input power vs. output power
(AM/AM) function
c
= ideal linearized characteristic of the amplified signal
Digital predistortion models
When testing power amplifiers, it is important to measure and analyze signal distor-
tions.
There are several known models used to describe distortions. This implementation
focuses on the following two types of distortion:
●
The AM/AM (amplitude-to-amplitude) distortion and
●
The AM/PM (amplitude-to-phase) distortion.
Applying Digital Predistortion
