Envelope Amplifier from digits to PA
pushing the envelope of PA efficiency
ET Envelope Path
from digits to PA
Gerard Wimpenny
Nujira Ltd
ARMMS Conference
19th/20th November 2012
Agenda
• Envelope Processing
• ET PA Characterisation
• Isogain shaping
• CFR shaping
• Envelope Amplifier Design Requirements
• Sources of Impairment
• Integrated Modulator
• Distributed Modulator
2
ET System Anatomy
Envelope detection: most
accurate if performed in digital
domain
Envelope shaping: Determines
relationship between RF power
and PA supply voltage
Delay Alignment: ET requires accurate (~ns) timing
alignment between envelope and RF paths. Most accurate /
repeatable if performed in digital domain
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
3
ET PA System Principles
In compressed region O/P
power is determined by
supply voltage – RF input
power has little influence
70
60
50
Efficiency (%)
4.5V
4.0V
40
3.5V
3.0V
2.5V
30
2.0V
Fixed Supply
20
1.5V
ISO26dB
In transition region O/P
power is determined by
both supply voltage and RF
input power
Fixed
10
0
15
20 Pout
35
(dBm) 25
30
In linear region O/P power
is determined by RF input
power – supply voltage has
little influence
4
Envelope Processing Basics
1.5
•
•
•
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)
1
0.5
0
-0.5
-1
1.5
-1.5
1
0.5
0
-0.5
-1
-1.5
5
PA Characterisation Methods
PA Characteristics must be known to determine Shaping table
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
(if device has
low AM/PM)
Gain (AM:AM),
Efficiency
Dynamic
supply
modulation
Challenging –
high BW with
high common
mode voltage
current sense
Requires low
impedance
dynamic supply
(no decoupling)
V. Good
V. Good
(if device has
low memory
effects)
Gain (AM:AM),
Phase (AM:PM),
Efficiency
High
(~60 MHz BW)
Phase measurement possible in principle – but accuracy poor due
to heating effects and phase reference ‘wander’
No phase
measurement
6
AM/PM Input Surfaces
Fixed Supply Voltage
Gain contours
Input Gain Surface
Fixed Supply Voltage
Phase contours
Input Phase Surface
7
Isogain Contours
27dB IsoGain
shaping contour
Phase peak flattening
c.f. fixed supply
25dB IsoGain
shaping contour
Low voltage phase
collapse
Input Gain Surface
Input Phase Surface
8
Isogain Shaping Functions
27dB gain
Input Gain Surface
25dB gain
Isogain Shaping Functions
9
Useful 2D Slices - Efficiency
25/27dB IsoGain
contours
Output Efficiency Surface
Fixed Supply
contours
Output Efficiency locus
10
2D Slices AM/AM, AM/PM
AM/AM distortion removed
Output Gain
Residual PM distortion
Output Phase
11
Predicted Performance
AM/AM
ACPR
AM/PM
Predicted Efficiency = 67.7%
Waveform = HSUPA / 5.4dB PAPR
Shaping = Isogain 24dB
12
Measured Performance
AM/AM
ACPR
AM/PM
Predicted Efficiency = 67.7%
Measured Efficiency = 67.6%
Waveform = HSUPA / 5.4dB PAPR
Shaping = Isogain 24dB
13
Shaping Table based CFR
Envelope Amplifier
Max Vout
Isogain shaping modified to introduce soft
clipping e.g using ‘Rapp’ function (VCFR)
Unmodified Isogain
shaping (Viso)
Desired PA gain profile
V
CFR
=
V
iso
æ æV ö
ç1 + ç iso ÷
ç çV ÷
è è pk ø
p
ö
÷
÷
ø
1
p
p = Smoothness factor
Vpk = Limiting voltage
14
Increased Pout using CFR
Increased Mean Power
Isogain shaping CCDF
10
80
0
Increased Efficiency
70
10
10
10
60
-1
Efficiency (%)
Probability
10
-2
-3
6.4dB
50
40
4.9dB
30
20
Modified Isogain
shaping CCDF
LTE Power pdf
10
-4
0
1
2
3
4
dB above mean
5
6
7
0
5
10
15
20
RF Output power (dBm)
25
30
Output Signal Statistics
Controlled use of CFR allows Increased mean power and
efficiency for given PA device periphery
15
‘Software Defined PA’
RF Spectrum
Soft clipping
80
80
80
80
80
60
70
60
60
60
40
60
40
40
40
20
50
20
20
20
0
40
0
0
0
-20
0
80
30 2
4
x 10
20
7
Reference
Spectrum
-20
-20
0
2
4
0 2 4 x 10
(No
clipping)
Offset from Carrier x(Hz)
10
0
40
-10
20
-20
0
0 1 2 3 4 x 10
7
-20
0
2
7
10
60
85% Clipping level
(1.4dB CFR)
PSD (dB)
PSD (dB)
75% Clipping level
(2.5dB CFR)
Hard clipping
4
x 10
80
80
80
60
60
60
40
40
40
20
20
20
0
0
0
7
7
Offset from Carrier (Hz)
-20
0
2
x 10
p=4
-20
4
0
2
7
x 10
p=6
-20
4
0 2 4 x 10
7
-20
0
2
7
4
x 10
p=10
7
p=100
Shaping Table based CFR allows dynamic configuration of PA’s
Power / ACPR / Efficiency characteristics
16
Agenda
• Envelope Processing
• ET PA Characterisation
• Isogain shaping
• CFR shaping
• Envelope Amplifier Design Requirements
• Sources of Impairment
• Integrated Modulator
• Distributed Modulator
17
Envelope Amplifier
Requirements
High Bandwidth
(e.g 4ch WCDMA, 20MHz LTE, 2x 10MHz WiMAX)
• Envelope Bandwidth ~3x RF Bandwidth
• Cannot be achieved with ‘switcher only’
architecture
Low Noise
Low Noise / Distortion
•
Required to meet ACPR specifications
•
•
Power
Many factors to consider
Requires high Tracking Accuracy
High Efficiency
High
Bandwidth
•
•
High
Efficiency
Must consider combined PA / modulator
efficiency
Linear supply would be pointless
Power
•
Must maintain BW and Noise at
increased power levels
18
ET Impairment Categories
•
System (Env & RF paths)
•
•
•
•
RF/Env Delay match
RF/Env Gain match
PA AM/AM and AM/PM
RF Path
•
Noise
•
•
•
•
Linearity
PA Memory effects
•
•
•
Thermal
Quantisation
Bias
Thermal
Envelope Path
19
Envelope Path Impairments
•
•
Shaping Accuracy
Tracking Accuracy
•
Noise
•
•
•
•
•
Frequency Response
•
•
•
Amplitude
Group Delay flatness
Env Amp Distortion
•
•
•
DAC Quantisation
Env Amp Thermal
Switcher breakthrough
Linear Amp PSRR
Harmonic
Crossover
Env Amp to PA Interaction
•
•
•
Env Amp Output Impedance
PA Interconnect Impedance
PA Non Linear Load Impedance
20
Tracking Accuracy Explained
Residual
Gross
Gross
Small
modulator
gain timing
and timing
tracking
errorerror
error
60
10
8
50
6
4
40
2
30
V
•
The difference between ideal and measured supply waveform after removal
of DC offset, gain and timing errors
Analogous to EVM for modulated signals
• Tracking error analysis is useful diagnostic tool: RMS, Peak, Spectrum
V
•
0
-2
20
10
-4
-6
-8
0
Ideal and measured waveforms
-10
Tracking Error
21
Supply ‘Noise’ – RF Conversion
• PA in compression – Supply Noise & Distortion modulates RF carrier
• PA can be considered as mixer
•
O/P spectrum is convolution of Supply and PA input Spectra
• Conversion factor (Supply Sensitivity) for noise on supply to RF
sidebands is similar to ideal AM modulator (mixer)
0
0
-10
-10
-20
40MHz
Supply Spectrum
-20
RF Spectrum
2-1
-30
-30
-40
-40
-50
1
40MHz
-50
1
2-1
-60
-60
-70
-70
-80
-80
-90
-90
-100
5 6-5
3 4-3
-100
Start: 0 Hz
Stop: 200.0000 MHz
40MHz ‘test tone’ added to Envelope Amplifier O/P
(whilst amplifying 5MHz WCDMA signal)
Start: 1.8500 GHz
Stop: 2.0500 GHz
Corresponding RF sidebands
22
Measured Supply Sensitivity
y(t ) = [ A + M cos(wmt )]sin(wct )
An ideal AM modulator is described by:
where modulation index
h=
M
A
This can be re-expressed in terms of carrier and LSB and USB components
y(t ) = A sin(wct ) + R[sin((wc + wm )t )+ sin((wc - wm )t )]
where for an ideal AM modulator
Average DC drain voltage
Measured 40MHz injected tone level
Calculated RF sideband level for ideal AM modulator
Measured RF sideband level
PA Supply Sensitivity (dB)
PA Supply Sensitivity (%)
R=
M
2
2.62V
17.3mV rms
-49.6dBC
-51dBC
-1.4dB
85%
DVenv
Venv
DVrf
Vrf
DVrf
Vrf
DVenv
Venv
23
Integrated Modulator Example
–Coolteq.L
•
Boost and Buck capable
• Battery depletion resilience
• Increased PA peak Power
•
•
•
Slow switching Buck converter
provides LF power
Fast switching multilevel converter
provides HF power
Error Amplifier ‘cleans up’ output
24
Distributed Modulator Example
- Coolteq.u
4 x Coolteq.u High Accuracy Tracking module (HAT®)
Exciter
HAT
DC
input
•
•
PA
•
Modulation
Digital
Pre
Distortion
HAT
•
PA
HAT
PA
RF
Upconvert
RF COMBINER
Coolteq.u
Power
Supply
Module
(PSM)
RF SPLITTER
Envelope
signal
generation
Envelope
input
Scalable O/P Power
Allows multiple PAs per
Power Supply Module
(PSM)
Allows Envelope path
Linear Amplifier to be
placed close to PA
PA supply impedance
minimised
RF out
400-475W
@7.5 dB
PAPR
HAT
RF in
RF
Driver
PA
25
Conclusions
• Understanding of PA characteristics key to
achieving good ET performance.
• Careful selection of shaping table contents
allows optimisation key ET system performance
metrics
• ET is a simple concept, but attention must be
paid to multiple potential sources of impairment
to realise full potential
26
pushing the envelope of PA efficiency
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