Emerson R&S SGT100A User Manual page 200

®
R&S SGT100A
Polynomial function
The polynomial function is an analytical method to describe a predistortion function.
When using the polynomial function, you do not define the correction values (ΔPower
and ΔPhase) directly as it is in the look-up table. You describe the predistortion func-
tion and the R&S SGT derives the correction values out of it.
See
This implementation uses a polynomial with complex coefficients defined as follows:
P
DPD
Where:
● n = "Polynomial Order" ≤ 10
●
x = P
●
a
coordinates.
In Cartesian coordinates system, the coefficients b
The R&S SGT calculates the AM/AM and AM/PM predistortion functions as follows:
●
AM/AM(x) = abs[P
●
AM/PM(x) = tan
The R&S SGT calculates the correction values (ΔAM/AM and ΔAM/PM functions) as
follows:
● ΔAM/AM(x) = AM/AM(x) - x = abs[P
●
ΔAM/PM(x) = AM/PM(x) = tan
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
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
Shaping table
In the R&S SGT, there are two ways to define the predistortion function in form of a
shaping table:
●
Externally
User Manual 1176.8674.02 ─ 12
Chapter 6.6.3.4, "Polynomial coefficients
(x) = ∑[(a
+j*b )*x n ],
n n
/P Max
in in
and b are user-defined coefficients, defined as Cartesian (polar) or cylindrical
n n
(x)]
DPD
-1
{Im[P
DPD
settings",
(x)]/Re[P (x)]}
DPD
(x)] -x
DPD
-1
{Im[P (x)]/Re[P
DPD
Input Range (PEP ) From/To, the predistortion function
in
Signal generator settings
Applying digital predistortion
on page 211.
are expressed in degrees.
n
(x)]}
DPD
200