ISL5239
Pre-Distorter (PD)
HALFBAND FILTER 2 RESPONSE
0
-20
MAGNITUDE (dB)
-40
The function of the Pre-distorter is to compute the
magnitude of the input signal, look up a complex distortion
vector based on the magnitude, and apply that distortion to
the input signal.
The signal magnitude may be computed by any of three
different methods: log of power, linear magnitude or linear
power. The result is scaled and offset by programmable
amounts and becomes the address into a Look-up Table
(LUT).
Two LUTs are available, one of which is 鈥榣ive鈥?in the circuit
and the other is offline and can be loaded via the processor
interface. This configuration allows instantaneous switching
of pre-distortion characteristics without unpredictable
effects on the processed signal.
The LUTs contain a complex distortion vector, as well as
complex delta values which interact with an external
Thermal/Memory calculation circuit to predict the effects of
temperature changes on the RF amplifier鈥檚 behavior and
compensate. The average power into the amplifier is
computed and transmitted serially off chip. The external
circuits compute one or two memory effect coefficients
which are combined with the complex delta values in the
LUT to derive the final distortion vector. The distortion
vector is a rectangular complex value which is multiplied
with the input signal resulting in a magnitude based non-
linearity. Access to the LUT is optimized by the use of an
auto incrementing address register which allows the tables
to be updated with only one address register write
operation. Control words 0x10 through 0x1d apply to the
-60
-80
-100
-120
-140
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
NORMALIZED FREQUENCY (NYQUIST=1)
FIGURE 3A. X4, HB1 AND HB2 ENABLED FREQUENCY
RESPONSE
HALFBAND FILTER 3 RESPONSE
0
-20
MAGNITUDE (dB)
-40
-60
-80
-100
-120
-140
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
NORMALIZED FREQUENCY (NYQUIST=1)
FIGURE 3B. X8, HB1-HB3 ENABLED FREQUENCY RESPONSE
7