high speed devices group rf pulse modulation and the digitally driven class c power amplifier r....
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High Speed Devices Group
RF Pulse Modulation and the Digitally Driven Class C Power Amplifier
R. Uang, J. Keyzer, A. Dalvi, Y. Sugiyama,
M. Iwamoto, I. Galton and P. Asbeck, UCSD
High Speed Devices Group
Introduction
• Digital RF generation – DSP’s are approaching the microwave
regime:• Intel Pentium 4 ALU operates at 4 GHz.
– CMOS processes continue to be scaled in size, reducing power and increasing speed.
• We can use digital RF to improve modern wireless communication systems by:– Reducing power dissipation using high efficiency
power amplifiers.– Using DSP to generate signals such that system can
overcome limitations of PA alone.– Developing versatile “mostly-CMOS” transmitters with
programmable functionality.
High Speed Devices Group
Overview
• System Overview – RF Digital Pulsewidth Modulation
• The Digitally Driven Class C Power Amplifier
• Simulations and Experimental Results
• Conclusions
High Speed Devices Group
Future RF Transmitters
PresentMicrowaveTransmitter
DAC
DAC
xI
Q x+ Filter x Filter VGA PA
fofIF(sin)
DSP
fIF(cos)
FutureMicrowaveTransmitter
I
Q
fbb=fs/N
Analogfilter
Pulse
Modulator PA
fclk
DSPDigital
Upconverter
DSPx 8
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Desired RF signal with AM and PM
RF pulse modulated waveform
RF pulse modulated waveform
Concept: Digital RF Pulse Modulation
t j t j
e e t A t RF
Re
High Speed Devices Group
Concept: Digital RF Pulse Modulation
t j t j
e e t A t RF
Ret
RF(t)
t
tA 1 13
3 4 6
(Pulse Position)
(Pulse Wdith)
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Pulse Modulator Block Diagram
Modulator
in(t)
fclk
Digital pulse Position
Modulator
3 Bit Word
PulseGenerator
8 fclk
Ain(t)
Digital Pulse Width
Modulator
1.5 Bit Word
PhaseModulation
EnvelopeModulation
Modulator
fclk
8 fclk
RF Output
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Simulations
• Single tone: spur level typically below - 55 dBc in desired band.
• Delta-Sigma modulator shapes quantization noise away from signal of interest.
CDMA: Linearity Test
Meets IS-95 ACPR Specs
CDMA (broad band view)
Single Tone
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The Digitally Driven Class C Power Amplifier
t
Vin
RF Pulse Digital Input
• Replace modulated sinusoidal input with RF digital pulse modulated signal• Input is digital pulse train with varying
pulse width and position• ID assumes pulse shape
• Desired output is modulated sinusoid
Vout
t
Vdd
LoadGaAsFET
RFC
CL
Shunt LCResonator
ID
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Digitally Driven Class C PA - Analysis
)V
V(
T
Wη
dd
min1)sinc( 1
min
I
VVR ddLopt
@
T
WIVP dddc max )sin(
)( minmax1 T
WVVIP dd
VD
ωt2π0 π
Vdd
Vmin
t
ID
W < T/2
T/2 T0
Imax
T/4
Po
wer
at
fun
da
men
tal
(W)
Efficiency and Output Power vs. Conduction Angle
0
10
20
30
40
50
60
70
80
90
100
04590135180225270315360Conduction Angle (Degrees)
Dra
in E
ffic
ien
cy (
%)
0
0.1
0.2
0.3
0.4
0.5
Class C
Traditional PA Pout Drain Efficiency
Drain Efficiency Pout Digitally Driven PA
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Simulated PA Performance:
RL,opt
Rl,opt coincides with max voltage swing at output, max power at fundamental.
Pattern Rlopt ADS Rlopt Analysis
1’s 49Ω 41Ω
3’s 16Ω 13Ω
10 20 30 40 50 60 70 80 900 100
0.05
0.10
0.15
0.20
0.25
0.30
0.00
0.35
Rl
Pow
er
of
fun
dam
enta
l (W
)Power of fundamental vs. Load Resistance
Rlopt for 3’s
Rlopt for 1’s
3’s
1’s
• Peak efficiency does not occur at fixed Rl for varying pulse widths.
• Optimum load occurs before maximum drain efficiency is reached.
10 20 30 40 50 60 70 80 900 100
10
20
30
40
50
60
0
70
Rl
Dra
in E
ffici
ency
Drain Efficiency vs. Load Resistance
1’s
3’s
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Experimental Test Setup
•Pattern generator outputs looping bit sequence•Serializer multiplexes to high data rate (3.2 Gbps), 900mV signal swing•Laser driver amplifies signal 2x to 2.5V max swing (adjustable)
MatlabGeneratedSequences
1 bit
3.2 Gbps
Conexant CX60061OC-192 Serializer
16 bits
200 Mbps
HP16522APattern
Generator
Synchronized200 MHz /3.2GHz Clocks
16:1
Conexant CX60077Laser Driver
Class C PAunder testing
AmplifiedOutput
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Experimental Results: Pulse Train
• Measured efficiency for 3’s exceeds calculated value.
• Measured efficiency for 1’s is much less than expected.
0
20
40
60
80
100
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Pout (Normalized)
Dra
in E
ffici
ency
Drain Efficiency vs. Output Powerfor ideal amplifierswith modulatedsignals
Switching Mode
Digitally Driven Class C
Class B
Class A
Pattern
Drain Efficiency
Calculated
Simulated
Measured
1’s 73% 55% 39%
3’s 59% 63% 74.5%
Prototype Class C PA
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Experimental Results: CDMA
Linearity Results to Date
“CDMA” signal - OQPSK at 400MHz
Po = +17.1 dBm, 22% Drain Efficiency
IS-95 ACPR specificationsoverlaid in red
Input to PA Output
5dB/div30 kHz BW
5dB/div30 kHz BW
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Conclusions
• A novel Digitally Driven Class C Power Amplifier has been demonstrated using a commercial power GaAsFET.
• This new variant of the Class C PA presents many unique challenges in its design and implementation.
• RF Digital Pulsewidth Modulation combined with the Digitally Driven Class C Power Amplifier may potentially provide a means for high efficiency, high linearity, easily reconfigured RF and microwave transmitters.
High Speed Devices Group
Simulated PA Performance:
RL,opt
Rl,opt coincides with max voltage swing at output, max power at fundamental.
Pattern Rlopt ADS Rlopt Analysis
1’s 49Ω 41Ω
3’s 16Ω 13Ω
10 20 30 40 50 60 70 80 900 100
0.05
0.10
0.15
0.20
0.25
0.30
0.00
0.35
Rl
Pow
er
of
fun
dam
enta
l (W
)Power of fundamental vs. Load Resistance
Rlopt for 3’s
Rlopt for 1’s
including package model3’s
1’s
10 20 30 40 50 60 70 80 900 100
10
20
30
40
50
60
0
70
Rl
Dra
in E
ffici
ency
Drain Efficiency vs. Load Resistance
1’s
3’sincluding package model
• Peak efficiency does not occur at fixed Rl for varying pulse widths.
• Optimum load occurs before maximum drain efficiency is reached.