polar loop transmitter t. sowlati, d. rozenblit, r. pullela, m. damgaard, e. mccarthy, d. koh, d....
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Polar Loop Transmitter
T. Sowlati, D. Rozenblit, R. Pullela, M. Damgaard, E. McCarthy, D. Koh, D. Ripley, F. Balteanu, I. Gheorghe
2 04/18/23
Content
• Introduction• Architectures for EDGE Transmitter• Polar Loop Transmitter • Measurement results• Conclusion
3 04/18/23
Introduction
1000 1050 1100 1150 1200 1250 1300
-14
-12
-10
-8
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-4
-2
0
2
Time [us]
Po
wer
[d
B]
• 8-PSK with Gaussian filtering• Data rate 3X compared to GMSK• Spectrum very similar to GMSK• 17 dB amplitude variation
EDGE modulation
-600 -400 -200 0 200 400 600-80
-70
-60
-50
-40
-30
-20
-10
0
Frequency [kHz]
Po
wer
[d
B]
-54dBc
-60dBc
GMSK EDGE
Q
I
Q
I
4 04/18/23
-600 -400 -200 0 200 400 600-80
-70
-60
-50
-40
-30
-20
-10
0
Frequency [kHz]
Po
wer
[d
B]
• 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
• Low Efficiency PA!
Introduction (ii)
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
Spectral failure due to Compression
& AM-to-PM of 0.8°/dB
Failure
5 04/18/23
TX Architectures with Linear PA
2) Direct up conversion with IQ vector modulator
3) Polar modulator prior to the PA
1) Superhet up conversion with IQ vector modulator
+ Well known techniques with simple interface between Transceiver and PA
– 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
+90°
VCO
PLL
I
Q PADriver SAWFilter
DigitalPhase
VCO
PLL
Modulator D/A
DigitalAmplitude
PA
Isolator
Modulator
Driver
SAW Filter
6 04/18/23
Polar with open loop PA amplitude modulation
+Very high PA efficiency is possible
– Requires linear amplitude control
– 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
AmplitudeModulation
D/A
PA
DigitalPhase
VCO
PLL
Modulator
Modulator
Isolator
Required amplitude control range:• DCS / PCS > 50 dB• GSM 850/900 >48 dB
7 04/18/23
Polar Loop Transmitter
+Very high PA efficiency is possible+Phase and amplitude feedback from
PA output +Insensitive to AM-to-PM in PA+Insensitive to load variations,
temperature, supply, etc.+No isolator required to maintain
good EVM under VSWR+No pre- or post-PA filtering required
to meet TX Noise in RX band
Main Challenges:– Stability– Noise
PA
VCO
PLL
Modulator
IFAmplitude
controlloop
Down-conversion
gain control
IF
I
Q
Powercontrol
8 04/18/23
Polar Loop Transmitter Block Diagram
+ Closed Loop Power Control + High Linearity: IM3 < -40dBc+ Wide Control Range > 65dB
+ Constant Gain over Control Range+ Low AM-PM + Low Noise
/M /N PFD CP
IQModulator
Baseband I & Q input
Vapc
IFVGA
BBVGA
PA
LO1
UHF VCO
Limiter1
Limiter2
D2
D1
Error Vcntrl
RF VCO
BPF1
BPF2
LPF2
LPF1
LO2
T/RSwitch
PM AM AM & PM
/M /N PFD CP
IQModulator
Baseband I & Q input
Vapc
IFVGA
BBVGA
PA
LO1
UHF VCO
Limiter1
Limiter2
D2
D1
Error Vcntrl
RF VCO
BPF1
BPF2
LPF2
LPF1
LO2
T/RSwitch
PM AM AM & PM
9 04/18/23
Characteristics of AM / PM loops
• 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 ↓
AM Loop PM LoopLoop Gain 40 dBLoop BW 1.8MHz 1.8MHzGain Margin > 14dB > 19dBPhase Margin > 65° > 65°Attenuation at 20MHz > 38dB > 40dB
10 04/18/23
Icp Gain vz( )
Loop Gain
Spectrum at 400KHz offset, Spec: -54dBc
EVM (RMS)
Spec: 9%
EVM (Peak)
Spec: 30%
Nominal -63dBc 1.4% 2.9%Nom. – 7dB -55dBc 2.1% 4.3%Nom. + 10dB -55dBc 0.8% 1.9%
System SimulationsAM Loop gain (dB)
PM Loop BW
(Normalized)
Spectral Mask at 400kHz Offset (-dBc)
• 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
11 04/18/23
Power Amplifier - EDGE Specific requirements
• Same GaAs die as GSM PA.
• No Mode Select between GMSK / EDGE.
• Modified Local Power Controller: – Linear Gain Control – Wide Bandwidth Gain Control– Low Noise – No Efficiency Degradation
Control Gain 20 dB (10V/ V)
Control gain variation at max power
+/- 3 dB
AM to PM 2.5 ° /dB max
12 04/18/23
Process Info & Die Photos
• BiCMOS Process: Transceiver / PA Controller 30GHz Ft NPN, 0.35 m CMOS, 3-layer metal• GaAs HBT Process: PA 25GHz Ft, 2 m Feature size, 2-layer gold metal
Transceiver: 5 mm x 4 mm
PA Controller2 mm x 2 mm
PA Module8 mm x 10 mm
13 04/18/23
Quad Band GSM / GPRS / EDGE RF Subsystem • Transceiver:
– RX = 40mA GSM; 50mA DCS– TX = 85mA– SYNTH = 35mA
• PA Controller– TX = 50mA
• PA Module – Typical performanceLow Band– GSM @ 34.5 dBm = 54%– EDGE @ 28.5 dBm = 35%High Band– GSM @ 31.5 dBm = 45%– EDGE @ 27.5 dBm = 35%
TransceiverPA Module
PA Controller
Quad Band Evaluation Board
14 04/18/23
GSM Band – EDGE: Max. Required Power at Antenna (27 dBm)
15 04/18/23
GSM Band – EDGE: Max. Required Power at Antenna (27 dBm)
16 04/18/23
GSM Band – EDGE: Max. Required Power at Antenna (27 dBm)
17 04/18/23
GSM Band – EDGE: Max. Required Power at Antenna (27 dBm)
18 04/18/23
GSM Band – EDGE: 2dB above Max Required Power at Antenna!
19 04/18/23
TX Noise in RX Band: Highest Channel & Highest Power Level
-79dBm
935
MHz
960
MHz
914.
8MHz
5dB/div
20 04/18/23
Performance under VSWR
Requirements:
• 3:1 - EVM meet spec, RF spectrum allowed to fail spec
• 6:1 - Maintain the link, EVM and RF spectrum allowed to be corrupted
• 10:1 - No device damage
Measured power at Antenna under VSWR
21 04/18/23
Without Isolator:
+ No degradation of EVM !
+ Modulation spectrum within spec up to 4:1 VSWR
VSWR Measurement EDGE Mode
0
2
4
6
8
10
0 90 180 270
VSWR Angle (degrees)
EV
M (
% r
ms
)
2:1
3:1
4:1
5:1
6:1
8:1
10:1
MaxSpec
EVM under VSWR Variation
Spec
Spectral Mask at 400KHz Offset
Spec
Spectral Mask at 600KHz Offset
Spec
22 04/18/23
System Performance Summary – Low Band
23 04/18/23
System Performance Summary – High Band
24 04/18/23
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.