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:

RFIN

RFIN PMU (CODE) 3.01

P[Balun](dBm)

1024

OFFSET

(dBm)

×

=

+

For RFFB:

RFFB

RFFB PMU (CODE) 3.01

P[Balun](dBm)

1024

OFFSET

(dBm)

×

=

+

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

Amplifier Linearizer (RFPAL)

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