Introduction to Envelope Tracking

G J Wimpenny

Snr Director Technology, Qualcomm UK Ltd

• EER first proposed by Leonard Kahn in 1952 to

improve efficiency of SSB transmitters

• ET offers similar efficiency enhancement to EER

but has fewer drawbacks

• Technique not widely adopted for many years due

to difficulty of implementation, particularly for wide

bandwidth signals

• In the last few years the implementational issues

have been overcome and ET is now widely used to

improve PA efficiency in Cellular Handsets

Envelope Tracking Historical Context

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• Modern high spectral efficiency wireless communications standards have high Peak to Average Power

Ratio (PAPR) e.g. 4G/LTE, 802.11ac WiFi

• Conventional fixed supply Power Amplifier has to use supply voltage high enough to support Peak Power,

but is only energy efficient at the peaks. Most of the time the voltage is much higher than needed, resulting

in high PA heat dissipation

• In ET, the PA supply voltage is dynamically adjusted to the instantaneous amplitude of the signal, resulting

in high PA efficiency at all times

Envelope Tracking Overview

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• The efficiency of a Fixed Supply PA is much lower at average Pout than at peak Pout.

• ET PA efficiency is only slightly lower at average Pout

• The difference in average PAPR between fixed supply and ET PAs increases with signal

PAPR

PA Efficiency Curves: ET vs Fixed supply

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• Need for PA efficiency enhancement techniques such as ET continues to grow

• Envelope Tracking Implementation difficulty increases with carrier bandwidth

Wireless standard trends

Increasing

Spectral

Efficiency

Increasing

PAPR

Standard Launched

Typical

Carrier BW

(MHz)

Typical Spectral

Efficiency (bps/Hz)

Approx

PAPR(dB)

1G cellular AMPS 1983 0.03 0.046 0

2G cellular GSM 1991 0.2 0.17 0

Digital TV DVB-T 1997 8 0.55 8

3G cellular WCDMA FDD 2001 5 0.51 7

2.75G cellular GSM + EDGE 2003 0.2 0.33 3.5

Wi-Fi IEEE 802.11a/g 2003 20 0.9 9

WiMAX IEEE 802.16d 2004 20 1.2 8.5

Wi-Fi IEEE 802.11n 2007 20 2.4 9

3.5G cellular HSDPA 2007 5 2.88 8

Digital TV DVB-H 2007 8 0.28 8

4G cellular LTE 2009 20 16 5-10

WiFi IEEE 802.11ac 2012 80 14 10

4G cellular LTE-A 2013 20 30 5-10

5G cellular 5G-NR 2018 100 - 5-10

Decreasing

Conventional

PA efficiency

5

ET System Elements

Envelope detection: most

accurate if performed in digital

domain

Envelope shaping: Determines

relationship between RF power

and PA supply voltage

Envelope Amplifier: High BW,

Low Noise, High efficiency

Amplifier used to generate PA

supply voltage

ET PA: ET can be applied to

standard fixed supply PA.

Improved performance possible by

optimising PA for ET operation

PA operates in ‘polar’ mode at high

instantaneous power and ‘linear’

mode at low instantaneous power

Delay Alignment: ET requires accurate (~ns) timing

alignment between envelope and RF paths. Most accurate /

repeatable if performed in digital domain

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• BW of Envelope path needs to be 2-3x RF bandwidth

• Delay alignment between RF and Envelope paths at PA (interfaces B and C) is essential to achieve

good RF linearity (ACLR / EVM). Sub-sample timing alignment ( <ns) required for wideband signals

• Analogue imperfections in both Envelope and RF paths must be corrected

• Gain / DC Offset in Envelope path

• Gain vs frequency in RF path

Envelope Tracking Signal Processing

Differential Analogue

Envelope Interface

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Envelope Processing Basics

• Swing Range

• Optimise efficiency of combined modulator /PA

• Prevent gross PA nonlinearity due IV curve ‘knee’

• Envelope ‘Shaping’

• Control envelope bandwidth

• Optimise efficiency

• Can be used to linearise PA

• Timing Alignment

• Timing error leads to ‘memory effect’ (AM-PM)

• Fine adjustment necessary (~1ns)

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• Minimum PA supply voltage (Vmin) set by Envelope shaping table

• Determined by PA technology e.g GaAs HBT

• Vmin chosen to ensure the PA never operates in its highly non-linear region

• At high power, PA supply voltage swings between Min voltage defined in shaping table up to

Max voltage supported by PA

• As power is backed off, Vmax falls but Vmin remains unaltered – i.e ET swing range reduces

ET High Power operation

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• ET generally used for highest 10dB operating power range

• At lower power there is insufficient swing range to significantly improve PA efficiency

• Static power dissipation of ET modulator starts to outweigh PA efficiency benefit

• Average Power Tracking (APT) typically used at lower powers

• PA DC supply voltage varied with average slot power

ET Lower Power operation

10

• Envelope Tracking PA can be considered to be a 3 port network

• RF Input, Supply Input, RF output

• PA Gain, Phase, Efficiency influenced by

• PA Device technology

• PA Circuit design (matching, biasing)

• Instantaneous Supply Voltage

• 3D Characterisation of PA ‘surfaces’ allows ET system performance to be predicted

• Gain, Phase, Efficiency vs Instantaneous (Pin,Vsupply)

ET PA = 3 Port Device

ET PARFin RFout

Vsupply

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• Envelope Shaping table defines Rfin to Vsupply mapping

• Once shaping is defined the ET subsystem is reduced to a 2 port network

• Envelope Shaping determines key ET PA ‘black box’ metrics

• Linearity (AM/AM and AM/PM)

• Efficiency

• Gain

• System Linearity metrics for a wide range of waveforms can then be derived

• Allows EVM / ACLR / Efficiency trade-offs to be explored

• PA / system memory effects are not captured – but good predicted vs measured results can still be

obtained for low-med BW waveforms

ET PA + Shaping Table = 2 Port Device

ET PA

RFin RFout

Vsupply

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Characterising the ET PA

Test methodology

PA current measurement

Supply impedance

Supply bandwidth

requirements

ET Efficiency prediction

ET Linearity

prediction

Parameters measured

Swept CW testing

Bench PSU Low

(decoupling Capacitor)

Low (Bench PSU)

Poor, due to PA die heating

Poor, due to PA die heating

Gain (AM:AM), Efficiency

Pulsed RF /DC testing

Instrumentation grade current

probe, ~5 us

resolution

Low (decoupling Capacitor)

Low (Bench PSU)

Good, if short

pulses (~10 us, 10% duty

cycle).

Fair Gain

(AM:AM), Efficiency

Dynamic supply

modulation

Challenging – high BW with high common mode voltage current sense

Requires low impedance

dynamic supply (no decoupling)

High (~60 MHz

BW) V. Good V. Good

Gain (AM:AM),

Phase (AM:PM), Efficiency

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PA Surface Characterisation setup

Supply Modulator: High BW /

Low output impedance

PA Supply Current: High BW /

High CM rejection

Dynamic measurement of:

Gain, Phase, Efficiency

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Example 3D surface model of ET PA

Trajectory across surfaces set by choice of shaping table – shown by black lines

Gain Surface Phase Surface Efficiency Surface

Ga

in (

dB

)

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Alternative 2D views of 3D ET Gain Surface

2D Shaping table view

(Colour = Gain)3D view

2D ‘Waterfall’ view

Gain vs Power

parameterised by Vsupply

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• ET PA shows gain expansion

• ‘Kink’ in AM/AM characteristic is

very difficult to linearise – high

BW required

ET Shaping Table - Optimum Efficiency vs Isogain

Optimum Efficiency Shaping Isogain shaping

Effic

iency

Gain

• Flat gain characteristic

• No DPD required if ET PA has

low AM/PM

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Isogain Shaping Families

Higher Gain

Low swing range

Low Efficiency

Lower Gain

High swing range

High Efficiency

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• Unlike traditional PAs, low voltage gain reduction is desirable as it allows isogain operation

with high gain compression at high voltages (= high efficiency)

• ‘Gain peaking’ with fixed voltage supply is unimportant – these points are not ‘visited’ when in

ET mode

• Flat gain at lowest voltage is required (PA operates in linear mode at low voltages)

• Best ET efficiency achieved if high peak efficiency can be maintained at low supply voltages

Ideal ET PA Gain characteristic

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• Different Phase optimisation required for ET PA

• Best ET PA performance achieved by designing PA for ET to meet ET performance objectives

from the outset, rather than by ‘adapting’ an APT PA

ET PA AM/PM optimisation

APT optimised PA

(Flat AM/PM with fixed supply)

ET optimised Phase Characteristic

(Flat AM/PM with isogain shaping)

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• When operating in ET mode a PA is in

compression and acts as a mixer

• Noise and distortion on ET supply mixes with

RF generating unwanted sidebands

• Supply noise transfer is significantly higher than

for fixed supply or APT PA

• Supply noise transfer is controlled by PA

compression level set by ET shaping table

• To maintain full ET efficiency benefit, supply noise

needs to sufficiently low to avoid need to ‘back

off’ compression level

• Some distortion mechanisms can be

corrected at system level using DPD, others

cannot

• Correctable distortion: Frequency

Response, ET modulator Output

Impedance

• Non-correctable noise/distortion: Switcher

noise, Slew rate limiting, Thermal noise

ET Noise and Distortion mechanisms in ET

90% Supply Sensitivity

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Supply Sensitivity Determines Supply ‘Noise’ – RF conversion

Partial derivative of PA RF output voltage wrt PA supply voltage 𝑆 =𝜕𝑉𝑟𝑓𝑜𝑢𝑡

𝜕𝑉𝑠𝑢𝑝𝑝𝑙𝑦

Ideal AM mixer has S=1 (100% supply sensitivity)

ET PA in hard compression S ~ 1, Fixed Supply Linear PA S ~ 0.2-0.3

Average DC drain voltage 2.62V

Measured 40MHz injected tone level 17.3mV rms

Calculated RF sideband level for ideal AM modulator -49.6dBC

Measured RF sideband level -51dBC

PA Supply Sensitivity (dB) -1.4dB

PA Supply Sensitivity (%) 85%

env

env

rf

rf

V

V

V

V

env

env

V

V

rf

rf

V

V

dBm

0

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SoftPlot Measurement Presentation

Trace A

Start: 0 Hz Stop: 200.0000 MHz

Res BW: 100 kHz Vid BW: 300 kHz Sweep: 200.00 ms

17/11/2010 18:44:43 FSEB 30

Skyworks 174 isogain 25dB 28.8dBm Avg DC drain

volts = 2.62V peak = 3.9V

Trace A 40MHz 0dBm injected tone

Measurement Parameter Value

Channel bandwidth 3.84 MHz

Channel spacing 40.00 MHz

On- channel power -65.53 dBm

Adjacent channel power (channel -1) 4.45 dB

Adjacent channel power (channel +1) 0.09 dB

Mkr Trace X-Axis Value Notes

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1 Trace A 39.2786 MHz -64.56 dBm

2-1

2-1 Trace A 801.6032 kHz -22.25 dBm{sum}

dBm

0

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SoftPlot Measurement Presentation

Trace A

Start: 1.8500 GHz Stop: 2.0500 GHz

Res BW: 100 kHz Vid BW: 300 kHz Sweep: 200.00 ms

17/11/2010 18:36:58 FSEB 30

Skyworks 174 ET Iso gain 25dB 5MHz WCDMA

28.8dBm

Trace A Injected CW tone (0dBm @ 40MHz)

Mkr Trace X-Axis Value Notes

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1 Trace A 1.9474 GHz -37.61 dBm

2-1

2-1 Trace A 1.9522 GHz 8.54 dBm{sum}

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3 Trace A 1.9875 GHz -72.81 dBm

4-3

4-3 Trace A 4.8096 MHz -41.89 dBm{sum}

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5 Trace A 1.9073 GHz -74.51 dBm

6-5

6-5 Trace A 4.8096 MHz -42.89 dBm{sum}

40MHz ‘test tone’ added to Envelope Amplifier O/P

(whilst amplifying 5MHz WCDMA signal)Corresponding RF sidebands

40MHz

40MHz

Supply Spectrum RF Spectrum

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Envelope Tracking Modulator Requirements

‘Hybrid’ Switch-mode / Linear-mode

ET Modulator

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✓Very simple hardware

✓ET much more amenable to open loop

correction than fixed supply PA – operates in

compression over most of envelope cycle

✓Moderate linearity performance

✓Moderate bandwidth capability

× No direct phase correction– relies on PA

designed for low ET AM/PM

× No memory correction – high ‘intrinsic’ ET

modulator performance needed

ET CFR Via Shaping Table

Configurable soft

clipping provides

simple CFR ‘for free’

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IQ CFR

|r|

PA

Envelope

Detection

Envelope

Shaping

Envelope

Amplifier

Memoryless

DPD

DPD

ET PA Subsystem

ET and Digital PreDistortion

CFR applied to source waveform

(both Envelope and RF are clipped)

DPD only needed to clean up residual

AM/AM errors and correct ET PA’s AM/PM

(if required)

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Thank You

Questions