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Keysight Technologies
Solutions and Measurement Tools for Use
in Average Power and Envelope Tracking Design
Application Note
Introduction
With the advent of LTE, the transmission system in a cellular smartphone must process
signals with both a very wide range of average power, to suit the user's location in the cell,
and a high (8 dB) short term peak to average power ratio due to the nature of the LTE signal.
Depending on the ampliier technology, the device may have to operate several dB below the
level it would be capable of providing with a W-CDMA waveform. This comes at a time when
battery capacity is already at a premium, heat dissipation within the handset is based on a
budget and frequency coverage of multiple spectrum bands is required.
The RF power ampliiers account for a signiicant portion of both the energy consumed and
heat generated by the RF front end. Individual ampliiers may be required for each band. For-
tunately, combining a multi-phase switched mode power supply with a high frequency linear
ampliier enables an "on demand" approach to the supply voltage that can be used to deliver
a useful reduction in both energy used and heat generated.
With ampliier technologies having a soft AM/AM compression characteristic, supply
modulation may also be used to signiicantly improve linearity. This reduces in-band and
out-of-band intermodulation signals at higher powers. The use of boost mode in the SMPS
to give transient increases in supply voltage enables further linearity improvements.
This document provides an outline of the techniques involved and the solutions Keysight
Technologies, Inc. provides for RF, baseband and system developers.
Envelope
Shaping
PA supply
PA input
Magnitude
ACP
dB
RF signal RF PA