Polar Loop Transmitter
T. Sowlati, D. Rozenblit, R. Pullela, M. Damgaard, E.
McCarthy, D. Koh, D. Ripley, F. Balteanu, I. Gheorghe
Content
• Introduction
• Architectures for EDGE Transmitter
• Polar Loop Transmitter
• Measurement results
• Conclusion
2 3/14/2016
Introduction
GMSK
EDGE modulation
• 8-PSK with Gaussian filtering
• Data rate 3X compared to GMSK
• Spectrum very similar to GMSK
• 17 dB amplitude variation
Q
EDGE
Q
I
I
0
2
0
-20
-54dBc
-30
-60dBc
-40
Power [dB]
Power [dB]
-10
-2
-4
-6
-50
-8
-60
-10
-70
-12
-80
-600
-14
-400
-200
0
200
Frequency [kHz]
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400
600
1000
1050
1100
1150
1200
Time [us]
1250
1300
Introduction (ii)
• Low Efficiency PA!
-10
-20
Power [dB]
• Unlike GMSK, EDGE cannot use
nonlinear PA
• For EDGE, PA operating point
should be ~ 6dB bellow 1dB
compression point.
• This operating point provides only
3dB margin on Spectral Mask
0
Failure
-30
-40
-50
Spectral failure due to
Compression
& AM-to-PM of 0.8°/dB
-60
-70
-80
-600
-400
-200
0
Motivation for our GSM / GPRS / EDGE Transmitter
+ Preserve high efficiency of the TX chain in GMSK mode
+ No SAW Filter in TX chain
+ No mode change in PA between GMSK & EDGE
+ Improve system efficiency in EDGE
4 3/14/2016
200
Frequency [kHz]
400
600
TX Architectures with Linear PA
1) Superhet up conversion with IQ vector modulator
2) Direct up conversion with IQ vector modulator
I
I
90°
+
Q
IF VGA
PLL
SAW
Filter
PA
+
90°
Q
Isolator
Driver
PA
SAW
Filter
PLL
PLL
IF VCO
UHF VCO
VCO
+ Well known techniques with simple interface
between Transceiver and PA
3) Polar modulator prior to the PA
Isolator
– Low Efficiency Linear PA for EDGE mode
– External Filter to meet noise in RX band
– Isolator to maintain EVM under VSWR
– Tend to be expensive and bulky solutions
5 3/14/2016
Driver
PLL
VCO
SD
Modulator
Digital
Phase
D/A
Digital
Amplitude
Modulator
PA
Polar with open loop PA amplitude modulation
+ Very high PA efficiency is possible
PA
Isolator
– Requires linear amplitude control
PLL
– Sensitive to AM-to-PM in PA
– Sensitive to load variations, temperature,
supply
– Requires isolator after PA to maintain EVM
under VSWR
– Accurate alignment of AM and PM
components is very critical
– Dynamic range for power control and power
ramping is an issue
6 3/14/2016
VCO
SD
Modulator
Digital
Phase
D/A
Amplitude
Modulation
Modulator
Required amplitude control range:
• DCS / PCS
> 50 dB
• GSM 850/900
>48 dB
Polar Loop Transmitter
PA
+Very high PA efficiency is possible
+Phase and amplitude feedback from
PA output
VCO
gain control
+Insensitive to AM-to-PM in PA
PLL
+Insensitive to load variations,
temperature, supply, etc.
IF
+No isolator required to maintain
good EVM under VSWR
+No pre- or post-PA filtering required
to meet TX Noise in RX band
7 3/14/2016
Down-conversion
I
Q
IF
Amplitude
control
loop
Modulator
Main Challenges:
– Stability
– Noise
Power
control
Polar Loop Transmitter Block Diagram
UHF VCO
/M
PA
RF VCO
/N
PFD
CP
T/R
Switch
LPF1
Limiter2
Error
BPF2
LO2
IQ
Modulator
BPF1
Baseband I & Q input
BB
VGA
D1
Vcntrl
LPF2
D2
IF
VGA
Limiter1
Vapc
+ Closed Loop Power Control
+ High Linearity: IM3 < -40dBc
+ Wide Control Range > 65dB
8 3/14/2016
LO1
PM
AM
AM & PM
+ Constant Gain over Control Range
+ Low AM-PM
+ Low Noise
Characteristics of AM / PM loops
Loop Gain
Loop BW
Gain Margin
Phase Margin
Attenuation at 20MHz
AM Loop
40 dB
1.8MHz
> 14dB
> 65°
> 38dB
PM Loop
1.8MHz
> 19dB
> 65°
> 40dB
• Dominant poles / zeros are set by external components
• Variations in loop bandwidth are mostly due to gain variations
• Loop BW ↑ : Design Tolerance↑
• Loop BW ↓ : Noise ↓
9 3/14/2016
System Simulations
AM Loop gain (dB)
• Polar Loop Transmitter modeled
and simulated using MATLAB,
MATHCAD and ADS
• EVM is not sensitive to mismatch
between AM / PM Loops
• Spectral mask degrades due to
mismatch between loops
Loop Gain
Nominal
Nom. – 7dB
Nom. + 10dB
10 3/14/2016
Spectrum at 400KHz
EVM (RMS)
( Icp , Gain , vz)
offset, Spec: -54dBc
Spec: 9%
-63dBc
1.4%
-55dBc
2.1%
-55dBc
0.8%
Spectral Mask
at 400kHz
Offset (-dBc)
EVM (Peak)
Spec: 30%
2.9%
4.3%
1.9%
Power Amplifier - EDGE Specific requirements
• Same GaAs die as GSM PA.
Control Gain
• No Mode Select between GMSK / EDGE.
Control gain variation
at max power
• Modified Local Power Controller:
– Linear Gain Control
– Wide Bandwidth Gain Control
– Low Noise
– No Efficiency Degradation
11 3/14/2016
AM to PM
20 dB (10V/ V)
+/- 3 dB
2.5 ° /dB max
Process Info & Die Photos
•
BiCMOS Process: Transceiver / PA Controller
30GHz Ft NPN, 0.35 mm CMOS, 3-layer metal
• GaAs HBT Process: PA
25GHz Ft, 2 mm Feature size, 2-layer gold metal
Transceiver: 5 mm x 4 mm
PA Controller
2 mm x 2 mm
12 3/14/2016
PA Module
8 mm x 10 mm
Quad Band GSM / GPRS / EDGE RF Subsystem
• Transceiver:
– RX = 40mA GSM; 50mA DCS
– TX = 85mA
– SYNTH = 35mA
PA Module
Transceiver
• PA Controller
– TX = 50mA
• PA Module – Typical performance
Low Band
– GSM @ 34.5 dBm = 54%
– EDGE @ 28.5 dBm = 35%
High Band
– GSM @ 31.5 dBm = 45%
– EDGE @ 27.5 dBm = 35%
13 3/14/2016
PA Controller
Quad Band Evaluation Board
GSM Band – EDGE: Max. Required Power at
Antenna (27 dBm)
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GSM Band – EDGE: Max. Required Power at
Antenna (27 dBm)
15 3/14/2016
GSM Band – EDGE: Max. Required Power at
Antenna (27 dBm)
16 3/14/2016
GSM Band – EDGE: Max. Required Power at
Antenna (27 dBm)
17 3/14/2016
GSM Band – EDGE: 2dB above Max Required Power
at Antenna!
18 3/14/2016
19 3/14/2016
960 MHz
935 MHz
914.8MHz
TX Noise in RX Band: Highest Channel & Highest
Power Level
-79dBm
Performance under VSWR
Requirements:
• 6:1 - Maintain the link, EVM
and RF spectrum allowed to
be corrupted
• 10:1 - No device damage
28
2:1
Pout Delivered (dBm)
• 3:1 - EVM meet spec, RF
spectrum allowed to fail spec
Measured power at Antenna under VSWR
3:1
4:1
24
5:1
6:1
8:1
20
10:1
16
0
90
180
VSWR Angle (degrees)
20 3/14/2016
270
VSWR Measurement EDGE Mode
EVM under VSWR Variation
2:1
Spec
8
3:1
4:1
6
5:1
4
6:1
2
8:1
10:1
0
0
90
180
270
Max
Spec
VSWR Angle (degrees)
600KHz Offset (dBc)
-45
2:1
-50
3:1
4:1
Spec
5:1
-60
6:1
-65
8:1
10:1
-70
0
21 3/14/2016
90
180
270
VSWR Angle (degrees)
2:1
3:1
-50
Spec
-55
4:1
5:1
6:1
-60
8:1
10:1
-65
0
90
180
270
Max
Spec
VSWR Angle (degrees)
Spectral Mask at 600KHz Offset
-55
-45
400KHz Offset (dBc)
EVM (% rms)
10
Spectral Mask at 400KHz Offset
Max
Spec
Without Isolator:
+ No degradation of EVM !
+ Modulation spectrum within
spec up to 4:1 VSWR
System Performance Summary – Low Band
Modulation /
Band
Parameter
Maximum Output Power
GMSK Mode
900 MHz
Band
EDGE Mode
900 MHz
Band
22 3/14/2016
Modulation Spectrum
400 kHz offset, 30 kHz BW
600 kHz offset, 30 kHz BW
RMS Phase Error
Peak Phase Error
Noise 10MHz offset, 100KHz BW
Noise 20MHz offset, 100KHz BW
Maximum Output Power
Modulation Spectrum
400 kHz offset, 30 kHz BW
600 kHz offset, 30 kHz BW
RMS EVM
Peak EVM
95th percentile EVM
Noise 10MHz offset, 100KHz BW
Noise 20MHz offset, 100KHz BW
Measured
Performance
33.5 dBm
GSM
Requirement
31 dBm min /33 dBm nom
-65 dBc
-68 dBc
1.4
4.8
-75 dBm
-80.5 dBm
29 dBm
-60 dBc max
-60 dBc max
5 max
20 max
-71dBm
-79 dBm
24 dBm min / 27 dBm nom
-60 dBc
-65 dBc
2.9%
11%
5.5%
-78 dBm
-83.5 dBm
-54 dBc max
-60 dBc max
9% max
30% max
15% max
-71 dBm
-79 dBm
System Performance Summary – High Band
Modulation /
Band
Measured
Performance
30.5 dBm
GSM
Requirement
28 dBm min / 30 dBm nom
Modulation Spectrum
400 kHz offset, 30 kHz BW
600 kHz offset, 30 kHz BW
RMS Phase Error
Peak Phase Error
Noise 20MHz offset, 100KHz BW
Maximum Output Power
-64 dBc
-67 dBc
1.6
6.0
-75 dBm
26.5 dBm
-60 dBc max
-60 dBc max
5 max
20 max
-71 dBm
22 dBm min / 26 dBm nom
Modulation Spectrum
400 kHz offset, 30 kHz BW
600 kHz offset, 30 kHz BW
RMS EVM
Peak EVM
95th percentile EVM
Noise 20MHz offset, 100KHz BW
-60 dBc
-64 dBc
3.8%
13%
7.3%
-78 dBm
-54 dBc max
-60 dBc max
9% max
30% max
15% max
-71 dBm
Parameter
Maximum Output Power
GMSK Mode
1800 MHz
Band
EDGE Mode
1800 MHz
Band
23 3/14/2016
Conclusions
• A new transmitter architecture has been presented.
• Closed AM and PM feedback loops ensure very robust
performance.
• High PA efficiency by use of saturated operation.
• No SAW filter needed to meet TX noise in RX band.
• No need for Isolator to maintain good EVM under VSWR.
• Meet or exceed all GSM requirements in Quad Band with both
GMSK and EDGE modulated signals.
24 3/14/2016