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Keysight Technologies
Solutions for Implementing Envelope Tracking in
Power Amplifiers




                                      Application Note
02 | Keysight | Solutions for Implementing Envelope Tracking in Power Amplifiers - Application Note


Fast and Efficient Configuration and Test of Envelope Tracking Systems--From
R&D to Design Verification and Into Production--Using a Range of Flexible and
Accurate Hardware and Software Solutions

Overview
By their very nature, Power Amplifiers (PAs) are power hungry, nonlinear devices. They are also a
critical component in modern mobile communication devices like smartphones and tablets, which
hold increased battery capacity and power efficiency at a premium. PA techniques like Crest Factor
Reduction (CFR), Digital Pre-Distortion (DPD) and Envelope Tracking (ET) were introduced as a way
to overcome these issues, while at the same time reducing problems associated with distortion and
nonlinearity. ET offers significant advantages in terms of improved battery life and RF PA perfor-
mance, along with reduced heat dissipation. Also, it's an increasingly important capability for PAs
that support high peak/average modulation schemes.

With smartphones and tablets increasingly employing wider bandwidths, Multi-Input Multi-Output
(MIMO) technology, and higher order Orthogonal Frequency Division Multiplexing (OFDM) modulation
formats to achieve higher data rates, the use of ET is projected to become even more critical.


Problem
While a PA operating in ET mode does exhibit greater power efficiency and better performance, these
benefits come at the price of complexity. And it's this complexity that makes configuring and testing
an ET system so challenging.

Consider a basic ET system block diagram as shown in Figure 1. Here, the envelope detector gener-
ates the envelope by taking the magnitude of the IQ waveform and then applying a shaping table to
determine the actual voltage supplied to the PA. Next, a shaped envelope is supplied to the Envelope
Tracking Power Supply (ETPS), which modulates the Vcc so that the bias voltage tracks changes to
the RF input waveform. Finally, the PA outputs the amplified RF signal.

Unlike polar modulation, ET does not apply a level limiter or operate with a fixed amplitude PA input
signal. Instead, it uses a mixture of open and closed loop feedback, with delays introduced to the IQ
or RF path to match those in the supply modulation path. In an open loop system, the shaping curve
is applied to the envelope signal to match the supply voltage versus RF gain in the PA. Pre-distortion
may also be used.

                                                                                    DC Power




               Envelope              Shaping                             Vin    Envelope tracking
               detector               table                                       power supply
                  |X|                 (LUT)                                          (ETPS)

                                                       Time alignment
                                                          required


                                                                                         I, Vcc


                   IQ                                      Upconverter           ETPA
                                                                                                      RFout
                                                                         RFin



                   Baseband                                                         ETPA Front-end


Figure 1. Shown here is a basic ET system block diagram.
03 | Keysight | Solutions for Implementing Envelope Tracking in Power Amplifiers - Application Note


 What makes the use of ET mode so challenging is that the PA has to be treated as a 3-termi-
 nal active device, and that's just the beginning. A low noise, high bandwidth power supply is
 needed, which usually operates in a combination of switched and linear modes. Additionally, a
 shaping curve or table (which determines characteristics of the ET system) has to be designed
 and optimized--a process that requires multiple difficult steps and often takes a great deal of
 time and effort (Figure 2).


                        

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