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

®
R&S SGT100A

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.
signal
Figure 7-21: 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.
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.
User Manual 1176.8674.02 ─ 07
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
179