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

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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

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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

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Frequency [kHz]

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[d

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• 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

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GSM Band – EDGE: Max. Required Power at Antenna (27 dBm)

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GSM Band – EDGE: Max. Required Power at Antenna (27 dBm)

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GSM Band – EDGE: Max. Required Power at Antenna (27 dBm)

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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

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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

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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.