R&S SGT100A, User Manual

Page: 212 / 593

Signal Generator Settings
R&S
®
SGT100A
181 User Manual 1176.8674.02 ─ 07
and
Δ
Phase) directly as it is in the look-up table, but you describe the predistortion
function and the R&S SGT derives the correction values out of it.
See Chapter 7.6.3.4, "Polynomial Coefficients Settings" , on page 193.
This implementation uses a polynomial with complex coefficients defined as follows:
P
DPD
(x) =
∑
[(a
n
+j*b
n
)*x n ],
where:
●
n = "Polynomial Order"
≤
10
●
x = P
in
/P
in
Max
●
a
n
and b
n
are user-defined coefficients, defined as Cartesian (polar) or Cylindrical
coordinates.
In Cartesian coordinates system, the coefficients b
n
are expressed in degrees.
The R&S SGT calculates the AM/AM and AM/PM predistortion functions as follows:
●
AM/AM(x) = abs[P
DPD
(x)]
●
AM/PM(x) = tan
-1
{Im[P
DPD
(x)]/Re[P
DPD
(x)]}
A dedicated graphical display visualizes the resulting functions, see Figure 7-23 .
The R&S SGT calculates the correction values (
Δ
AM/AM and
Δ
AM/PM functions) as
follows:
● Δ
AM/AM(x) = AM/AM(x) - x = abs[P
DPD
(x)] -x
● Δ
AM/PM(x) = AM/PM(x) = tan -1 {Im[P
DPD
(x)]/Re[P
DPD
(x)]}
A dedicated graphical display visualizes the calculated correction functions, see Fig-
ure 7-24 and compare with Figure 7-23 .
File format of the polynomial file
You can store a polynomial function in a file or even define the polynomial coefficients,
store them as a file and load this file into the instrument. The polynomial files are files
with the extension
*.dpd_poly
.
The file contains an optional header
# Rohde & Schwarz - Digital
Predistortion Polynomial Coefficients # a0,b0, a1,b1, a2,b2, ...
and a list of comma-separated coefficient value pairs, stored in Cartesian coordinates.
For values above the selected Input Range (PEP
in
) , the predistortion function assumes
a linear ratio of the input to output power.
Example: Polynomial function file content
# Rohde & Schwarz - Digital Predistortion Polynomial Coefficients
# a0,b0, a1,b1, a2,b2, ...
0,0,-0.25,0.2,0.6,-0.3,0.3,0.3,0.5,-0.4
Applying Digital Predistortion