原创 超线性技术

MW4IC001 Pre Distortion System for
2.1GHz WCDMA SCPA application
MOTOROLA JAPAN
SPS NCSG RF/DSP
Hideaki Okubo
Naoya Saito
Bron Robinett
David Runton
INTRODUCTION
In recent years, low cost has become a high priority for development and deployment of
cellular base stations. To achieve this requirement, base station amplifier manufacturers use the
smallest RF power transistor possible to achieve the highest efficiency in the basic amplifier
design and then adopt other high efficiency and distortion canceling amplifier topologies.
The selection of the error correction methodology is the main factor in defining the overall
system efficiency. Today, most amplifier manufacturers are mainly using feed-forward error
correction systems. This type of system needs an error amplifier that has very low distortion
characteristics, which results in a very low efficiency amplifier. Due to the error amplifier
power consumption and parts cost, as well as the cost of other control functions, the total system
cost for feed forward can be very high. For this reason, other error correction methods are being
considered. In this article, another distortion cancellation technique called analog Pre Distortion
is discussed. The Pre Distortion system proposed uses a new device from Motorola, the
MW4IC001. In a lineup consisting of the MW4IC001, the MW4IC2230, and the MRF5P21180,
use of this system gives the user nearly 3dB more output power with an 11% points efficiency
improvement for the overall system lineup.
SYSTEM　INFORMATION
The prototype Pre distortion system described below is a basic Cuber [1] type system. The
system uses the main amplifier path to cancel distortion (ACP and IMD). This is done by
adding a 180 phase shifted distortion signal, which has been generated by another “distortion
generator device,” to the signal before feeding the main amplifier. The Pre Distortion system
has to have a variable phase shifter and attenuator to control phase angle and amplitude of
distortion.
Conventional Pre Distortion systems adopt a diode as the non-liner “distortion generator
device.” However, the distortion generated by a diode has different distortion characteristics
from the main amplifier path. This is due to the fact that a diode has mostly 3rd order distortion
characteristics. The result is that the diode type pre distortion system does not get good
cancellation performance under wider output power dynamic range. This means that it is very
difficult to meet SEM (Spectrum Emission Mask) specifications in a SCPA (Single Carrier
Power Amplifier) with this type of system.
The MW4IC001, which is a low power RF LDMOS device made by Motorola, has been
specifically designed for this type of pre distortion system. This system has advantages
compared to conventional diode type pre distortion systems as outlined below.
1. Proven to be effective for higher order distortion cancellation.
2. Very wide output power dynamic range of cancellation.
3. Better thermal and noise tracking of the main amplifier path

CIRCUIT CONFIGULATION
The basic block diagram of a pre distortion system using the MW4IC001 is shown in Figure 1
below.
Figure 1
The circuit is very basic [1]. It uses a two-loop configuration. A distortion signal is generated
and the fundamental frequency is cancelled in the 1st loop. The distortion signal is then added to
the fundamental signal with controlled phase angle and signal amplitude at the 2nd loop. The
fundamental cancellation is 35 to 40dB at the 1st loop. The MW4IC001 has to match the
operating power at the same output power back off point as the final stage of the main amplifier.
This can be difficult, so, an attenuator between the output of MW4IC001 and the input of the
90 coupler adjusts the back off point of the MW4IC001.
It is important to pay special attention to the distortion level occurring at both the variable
phase shifter and attenuators since the MW4IC001 output power can be much higher than other
Diode type distortion generators. In this system, we are using a GSC355 90 coupler made by
SOSHIN combined with two varactor diodes to create a variable phase shifter and two PIN
diodes for variable attenuators. Power splitting and combining is done using the GSC355.
The MW4IC001 is the latest device designed using Motorola LDMOS HV4 IC process. It has
P1dB=900mW, Gain="12dB" (typ) at Vdd="28V" and Idq="10mA" at 2.17GHz. The MW4IC001 is
designed to be usable over a very wide bandwidth through the use of an integrated internal input
matching circuit. The same part can be used from 800MHz to 2.2GHz.
The Output Power vs. Distortion characteristic for the MW4IC001 is shown in Figure 2.