Detailed Description
Introduction to Predistortion Using the SC1894
Wideband signals in today’s telecommunications systems
have high peak-to-average ratios and stringent spectral
regrowth specifications. These specifications place high
linearity demands on power amplifiers. Linearity may
be achieved by backing off output power at the price of
reducing efficiency. However, this increases the compo-
nent and operating costs of the power amplifier. Better
linearity may be achieved through the use of digital pre-
distortion and other linearization techniques, but many of
these are time consuming and costly to implement.
Wireless service providers are deploying networks with
wider coverage, greater subscriber density, and higher
data rates. These networks require more efficient power
amplifiers. Additionally, the emergence of distributed
architectures and active antenna systems is driving the
need for smaller and more efficient power amplifier imple-
mentations. Further, there continues to be a strong push
toward reducing the total capital and operating costs of
base stations.
With the SC1894, the complex signal processing is done
in the RF domain. This results in a simple system-on-chip
that offers wide signal bandwidth, broad frequency of
operation, and very low power consumption. It is an ele-
gant solution that reduces development costs and speeds
time to market. Applicable across a broad range of signals
— including 2G, 3G, 4G wireless, and other modulation
types — the powerful analog signal-processing engine
is capable of linearizing the most efficient power ampli-
fier topologies. The SC1894 is a true RFin and RFout
solution, supporting modular power amplifier designs
that are independent of the baseband and transceiver
subsystems. The SC1894 delivers the required efficiency
and performance demanded by today’s wireless systems.
RF Power Management Unit (PMU)
Description
Analysis
The RFIN and RFFB log slope and intercept are derived
using a linear regression performed on data collected
under nominal operating conditions. The error from linear
response to the CW waveform is the dB difference in out-
put from the ideal output. This is a measure of the linearity
of the device response to both CW and modulated wave-
forms. Error from the linear response to the CW waveform
is a measure of relative accuracy because the system has
yet to be calibrated. However, it verifies the linearity and
the effect of modulation on the device response. Error
from the +25°C performance uses the performance of a
given device and waveform type as the reference. This
error is largely dominated by output variations associated
with temperature.
The PMU codes are represented as 16-bit signed integer
and are converted to dBm (referenced to the balun input)
using the following formula:
For RFIN:
P[Balun](dBm)
1024
OFFSET (dBm)
=
+
For RFFB:
RFFB PMU (CODE) 3.01
P[Balun](dBm)
1024
OFFSET (dBm)
×
=
+
The OFFSET
RFIN
and OFFSET
RFFB
are dependent on
end-system characteristics and also on the part-to-part
variation of the RFPAL. For absolute accuracy, the PMU
calibration procedure outlined in the release notes and
SPI programming guide must be followed.
Measurement Considerations
In order to provide sufficient integration samples to allow
precise measurements of signals, the default integration
time (measurement window) is fixed to 40ms. Note that if
the measurement window is not a multiple of the system
frame length, then the power-measurement window will
span an incomplete frame and cause a measurement
error. However; the synchronization of the frame and
measurement window is not required to achieve precise
measurements.
TDD Considerations—Operation with < 100%
PA Duty Cycle
The PMU fully supports accurate measurement of TDD
waveforms. The PMU does not differentiate between
samples taken when the PA is on versus when the PA is
off. Though easily compensated, this condition will affect
the reading for waveforms with less than 100% duty cycle
(e.g., TDD applications). For example, the PMU value
read for a 50% duty-cycle waveform will be 3dB lower
than the value for the same signal but with a 100% duty
cycle. Calculating the offset associated with TDD mea-
surements is straightforward and may be handled by the
PMU depending on the system requirements. Refer to the
Release Notes for additional details on different methods.
SC1894 225MHz to 3800MHz RF Power
Amplier Linearizer (RFPAL)
www.maximintegrated.com
Maxim Integrated
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