Signal Generator Settings
R&S
®
SGT100A
147
User Manual 1176.8674.02 ─ 07
The R&S
SGT generates the envelope signal directly from the baseband signal. The
envelope signal is a voltage signal, with voltage level V
out
proportional to the power of
the RF signal (
√
[I(t)
2
+Q(t)
2
]
) of the corresponding path. If you do not apply a shaping
function, the envelope signal follows the variation of the envelope of the RF signal line-
arly dependent.
The envelope signal is output at the I OUT and I BAR OUT rear panel connectors. This
envelope signal is then further fed to an external DC modulator.
The PA receives the RF input signal and the dynamically adapted supply voltage v
cc
.
Ideally, the PA gain should stay constant.
Suitable baseband signal to observe the effect of the envelope tracking settings
To simplify the explanation in the following sections, we use a simple ramp function as
a baseband signal modulated on the RF carrier.
Other suitable baseband signals are signals with relative constant envelope. You find a
choice of predefined signals in the "Baseband > Custom Dig Mod" dialog. For instance,
with the default settings in this dialog ("Custom Dig Mod > Set acc. to the standard >
GSM"), you can observe the generated envelope signal and the effects of enabled
shaping.
7.5.2.1
Envelope Voltage Adaptation Modes
In the R&S
SGT, you define the voltage of the generated envelope signal using one of
the following modes:
●
Auto Power/Normilized Envelope Voltage Adaptation:
In this mode, you define the desired input characteristics of the power amplifier.
Based on these values and depending on the crest factor of the currently gener-
ated signal, the R&S
SGT calculates:
–
The voltage on the I OUT/I BAR OUT connectors (
) and a bias
),
–
The RMS level of the RF signal
The auto voltage adaptation mode is a suitable choice, if you have knowledge on
the power amplifier components and characteristics, like for example the supply
voltage V
cc
, the input power PEP
in
required for working in the linear range, as well
as the gain characteristics of the external DC modulator.
You find the required values in the documentation of your power amplifier, for
example in its data sheet.
●
Manual Envelope Voltage Adaptation:
In this mode, you can additionally define the operating range of the power amplifier
based on a pre-gain and a post-gain range. Based on these values, the instrument
calculates the supply voltage V
cc
.
All modes support envelope shaping.
Generation of Envelope Tracking Signals
