Challenges for Power Amplifier Testing with Envelope Tracking

Challenges for Power Amplifier

Testing with Envelope Tracking -

concepts, pit falls, state of the

art and trends

Cambridge Wireless Workshop

Advance Techniques for Radio Transmitters

September 25th, 2014

Rainer Lenz

Product Manager Signal Generators

2Improving the Efficiency of Your Envelope Tracking Test Setup

Agenda

ı Envelope Tracking – Overview

ı Envelope Tracking - Test Challenges

ı Measurement Setup

ı Summary


Envelope Tracking – Overview

Low battery. Connect charger.

ı Beside display and processor, a main contributor to the power consumption of a

modern smartphone is the TX power amplifier (PA)

ı Envelope tracking is a technique to improve the efficiency of PAs, thus

increasing battery lifetime



ı Modern communication standards, such as LTE, show a high peak-to-average

ratio (PAR), i.e. up to 7 dB for LTE uplink signals

ı PAs are most efficient when operating near its maximum output power

ı For signals with high crest factor, PAs operate in low efficiency regions most of

the time

typical power added efficiency (PAE)

signal amplitude probability



DC supply

BasebandUp -

converterI/Q RF

Constant supply voltageEnergy

dissipated

as heat

RF signal

Conventional approach

PA

ı With a constant supply voltage, high amount of

energy is dissipated as heat

PINPOUT



DC supply

BasebandUp -

converterI/Q RF

RF signal

DC modulator

Envelope tracking

PA

Modulated supply voltage

Envelope

ı Envelope is calculated from the baseband

signal with (𝐼2 + 𝑄2)

ı DC supply voltage is modulated according to

the envelope of the RF signal

ı As a result the PA is operated in high efficient

regions most of the time

PIN

VCC

POUT

Envelope

detector


Envelope Tracking – Test Challenges

DC supply

BasebandUp -

converterI/Q RF

Envelope

detectorShaping

Delay

adjustmentDC modulator

PAPIN

VCC

POUT

Signal Generator

Envelope

RF

analysis

Signal Analyzer



DC supply

BasebandUp -

converterI/Q RF

Envelope

detectorShaping

Delay


PAPIN

VCC

POUT

Signal Generator

Synchronization

ı Synchronization between RF signal and modulated supply voltage VCC

RF

analysisRF

Envelope

Signal Analyzer



DC supply

BasebandUp -

converterI/Q RF

Envelope

detectorShaping

Delay


PAPIN

VCC

POUT

Signal Generator

RF

analysisRF

Envelope

Signal Analyzer


ı Apply shaping curve



DC supply

BasebandUp -

converterI/Q RF

Envelope

detectorShaping

Delay


PAPIN

Voltage

POUT

Signal Generator

𝐏𝐀𝐄 in percentage =RF Power out − RF Power in

DC Power%

RF

analysis

DC power

analysis

RF

Envelope

Signal Analyzer



ı Instantaneous power added efficiency (PAE) calculation

Current



DC supply

BasebandUp -

converterI/Q RF

Envelope

detectorShaping

Delay


PA

Signal Generator

PIN

VCC

POUT

RF and

distortion

analysis

DC power

analysis

RF

Envelope

Signal Analyzer




ı Optional: distortion analysis



DC supply

Baseband

and

pre-

distortion

Up -

converterI/Q RF

Envelope

detectorShaping

Delay


PA

Signal Generator




ı Optional: distortion analysis

ı Optional: predistortion

PIN

VCC

POUT

RF and

distortion

analysisRF

Envelope

Signal Analyzer

DC power

analysis


Envelope Tracking – Measurement Setup

DC modulator

PIN

VCC

POUT

Signal Generator Signal Analyzer

R&S®SMW200AR&S®FSW

LAN

Generate RF and

envelope signalRF and

Voltage/current

analysis

PA

DUT

RF

Envelope

14

Signal GenerationAvailable Options

R&S SMW200A

Vector Signal Generator

R&S SMW-K540

R&S SMW-K541

Improving the Efficiency of Your Envelope Tracking Test Setup

Envelope Tracking - User Interface

15

eTrak

ı Preset configuration for envelope signal

ı Specific DC modulator parameters

like Gain, V CC Offset or Impedance are taken into account


Envelope Settings - Synchronization

ı Envelope to RF synchronization below 1 ns is required

ı Can be adjusted in SMW with ps resolution in real time

RF Envelope


Different shaping modes

ı Table (Look-Up Table with up to 4000 comma separated value pairs)

ı Polynomial (up to 10 polynomial coefficients)

ı Detroughing (3 different functions)

Real time Shaping capabilities

Envelope

RF

Inv. Env.

Shaping setting Resulting Envelope


18

Signal Analysis

R&S FSW

Signal- and Spectrum-Analyzer FSW-K18 (Amplifier Measurements)


FSW-K18 PAE measurement (1)

PAE calculation:

DC modulator

PIN

VCC

POUT

Signal Generator Signal Analyzer

R&S®SMW200AR&S®FSW

LAN

Generates RF and

envelope signalRF and

Voltage/current

analysis

PA

DUT

RF

Envelope

ı RF power in -> from SMW

ı RF power out -> measured at RF port from FSW

ı DC power in -> measured at Baseband ports

(2 ports for 2 independent voltages)

DC Power

PAE calculation requires synchronous information of:


FSW-K18 PAE measurement (2)

ı Oscilloscope probes for easy connection

ı Usage of shunt resistor for supply current

measurement

RF Output

(to FSW)

RF Input

(from SMW)

ı PAE graphical display


FSW-K18 Digital Predistortion

21

Measure

AM/AM AM/PM

with FSW-K18

Generate

DPD Tables

Transfer

Tables to

SMW-K541

Activate DPD

on

SMW-K541

Activate DPD

in

SMW-K541

Generate DPD

Tables

(automatically)

Transfer Tables

from FSW-K18

to SMW-K541

Maximum Output

Power of the PA


22

FSW-K18

SMW-K541

Automatic Transfer

and configuration via

LAN

FSW-K18 Digital Predistortion

Interface to SMW-K541



SMW Digital Predistortion implementation

ı DPD applies amplitude and phase offset on every IQ sample in real time

ı DPD alters the signal statistics and the signal level

ı PEP, Level and Crest Factor before and after DPD are shown

ı Adjustable mean Level Reference Mode (Before or After DPD)

24

Envelope Tracking – Summary and Status

R&S®SMW200A

SMW-K540 Envelope tracking Available

SMW-K541 Digital

Predistortion

Available

R&S®FSW

FSW-B71 Synchronous analog

baseband inputs

Available

i.e. RT-

ZP10

Oscilloscope probe

connection to FSW

Available

FSW-K18 Amplifier Measurements Available

ı RF and envelope signals out of

one box

ı Realtime shaping and delay

adjustment

ı Power sweeps with automatic

recalculation of the envelope signal

ı Realtime Digital predistortion

ı RF and envelope signal analysis

in one box

ı Fastest measurement speed for

EVM/ACLR, Distortion Analysis

and PAE calculation

ı Highest dynamic range,

simplifying the test setup


Thank you

for your attention!


Technology

Challenges for Power Amplifier Testing with Envelope Tracking