lo harmonic content dependency on mod/demod performance

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LO Harmonic Content Dependency on Mod/Demod Performance

RFG

August 14th, 2009

2

How does LO harmonics affect direct conversion solutions?

We usually assume a synthesizer presents a sinusoidal waveform which we use to apply to a mixer/mod/demod. More often the synth presents a square-wave with potentially non-50% duty cycle which is rich in harmonic content. How do these LO harmonics influence sideband-suppression/image-rejection?

3

Consider Square-wave spectral characteristics

A perfect 50% duty cycle square wave will have an infinite amount of odd order harmonic content with no even order terms.

FFT Result

Note odd harmonics only, and the harmonics are -20Log(n) below the fundamental

4

Results for non 50% duty cycle and softened rising and falling edges

Vload VARVAR1

dutycorrection=(risefall/period)-dutyerrorrisefall=4 {t}dutyerror=0.0319 {t}period=100

EqnVar

VtPulseSRC1

Period=period nsecWidth=(0.5-dutycorrection)*period nsecFall=risefall nsecRise=risefall nsecEdge=linearDelay=0 nsecVhigh=1 VVlow=-1 V

t

RR1R=50 Ohm Tran

Tran1

MaxTimeStep=1 nsecStopTime=1000 nsec

TRANSIENT

0.2 0.4 0.6 0.80.0 1.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

-2.0

2.0

time, usec

Vlo

ad,

V

Eqn VOUT=spectrum_analyzer(Vload)

m1freq=dBm(VOUT)=12.032

10.00MHz

m2freq=dBm(VOUT)=-8.033

20.00MHz


30.00MHz

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90.0 1.0

-90

-80-70

-60-50

-40

-30-20

-100

10

-100

20

freq, GHz

dBm

(VO

UT

)

Readout

m1

Readout

m2

Readout

m3 m1freq=dBm(VOUT)=12.032

10.00MHz

m2freq=dBm(VOUT)=-8.033

20.00MHz


30.00MHz

m4freq=phase(VOUT)=141.260

11.00MHz

m5freq=phase(VOUT)=-25.884

20.00MHz


30.00MHz

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90.0 1.0

-100

0

100

-200

200

freq, GHz

phas

e(V

OU

T)

Readout

m4

Readout

m5

Readout

m6

m4freq=phase(VOUT)=141.260

11.00MHz


20.00MHz


30.00MHz

Eqn VOUT1=spectrum_analyzer(Vload)

With ~4nsec rise fall time and ~3% duty cycle error the HD2 is -20dBc and the HD3 is -10dBc. This is close to the harmonic distortion levels present at the VCO divider outputs of the ADF4350.

5

Replacing Square-wave generator with multi-tone source

10 20 30 40 50 60 70 80 90 1000 110

-400

-300

-200

-100

-500

0

freq, MHz

dBm

(VLO

sour

ce)

10.00M-4.776

m4

20.00M-20.00

m5

30.00M-10.00

m6m4freq=dBm(VLOsource)=-0.006

10.00MHz

m5freq=dBm(VLOsource)=-20.006

20.00MHz

m6freq=dBm(VLOsource)=-10.006

30.00MHz

m7freq=phase(VLOsource)=141.260

10.00MHz

m8freq=phase(VLOsource)=-25.884

20.00MHz


30.00MHz

20 40 60 80 1000 120

-100

0

100

-200

200

freq, MHz

phas

e(V

LOso

urce

)

10.00M141.3

m7

20.00M178.5

m8

30.00M47.60

m9

m7freq=phase(VLOsource)=141.260

10.00MHz


20.00MHz


30.00MHz

VLOsource

Amplifier2AMP1

S12=0S22=polar(0,180)S11=polar(0,0)S21=dbpolar(4.77,0)

P_1TonePORT1

Freq=LO_FreqP=polar(dbmtow(0),141.26)Z=50 OhmNum=1

RR4R=50 Ohm

P_1TonePORT5

Freq=LO_Freq3P=polar(dbmtow(-10),231.174)Z=50 OhmNum=5

P_1TonePORT4


PwrSplit3PWR2

S41=0.577S31=0.577S21=0.577

Approach provides an accurate harmonically defined waveform which is similar to the output of an integrated PLL/VCO.

6

Simulation Setup for SSB Modulator with Multi-tone LO source and Perfect Quadrature

Note zero phase error in quadrature splitter

LO Leakage set for -40dBm

VloadVIF

Amplifier2AMP2


RR3R=50 Ohm

P_1TonePORT5


P_1TonePORT1


P_1TonePORT4


P_1TonePORT2

Freq=IF_FreqP=polar(dbmtow(0),0)Z=50 OhmNum=2

HarmonicBalanceHB1

Order[6]=3Order[5]=3Order[4]=3Order[3]=3Order[2]=3Order[1]=3Freq[6]=LSB_FreqFreq[5]=USB_FreqFreq[4]=LO_Freq3Freq[3]=LO_Freq2Freq[2]=LO_FreqFreq[1]=IF_Freq

HARMONIC BALANCEVARVAR1

LSB_Freq=LO_Freq-IF_FreqUSB_Freq=LO_Freq+IF_FreqLO_Freq3=3*LO_FreqLO_Freq2=2*LO_FreqLO_Freq=10E6IF_Freq=1E5

EqnVar

PwrSplit3PWR2

S41=0.577S31=0.577S21=0.577

Amplifier2AMP1


Mixer2MIX2

SP23=dbpolar(-40,0)ConvGain=dbpolar(0,0)SideBand=BOTH

Mixer2MIX3


Hybrid90HYB2

PhaseBal=0GainBal=0 dBLoss=0 dB

-90

0IN

ISO

RR2R=50 Ohm

PwrSplit2PWR1

S31=0.707S21=0.707R

R1R=50 Ohm

Hybrid90HYB1


-90

0IN

ISO

Input IF set for 100kHz with LO of 10MHz. Using ideal quadrature hybrids to generate IQ baseband input and LO quadrature

7

Testing Quadrature Modulator Sideband Suppression with Perfect LO Source

10 20 30 40 50 60 70 80 90 1000 110

-400

-350

-300

-250

-200

-150

-100

-50

0

-450

50

freq, MHz

dBm

(Vlo

ad)

9.900M-250.1m

m1

Readout

m10

Readout

m11

m1freq=dBm(Vload)=2.760

9.900MHzm10freq=dBm(Vload)=-22.771


29.90MHz

9.6 9.7 9.8 9.9 10.0 10.1 10.2 10.3 10.49.5 10.5

-400

-350

-300

-250

-200

-150

-100

-50

0

-450

50

freq, MHz

dBm

(Vlo

ad)

Readout

m2

10.00M-43.09

m3

Readout

m12




10.10MHz

LO harmonics with linear modulator and perfect quadrature results in no sideband. LO harmonics do result in nxLO-IF mixing products as expected.

8

Simulation Setup for SSB Modulator with Multi-tone LO source and Imperfect Phase Quadrature on LO harmonics

Note: Quadrature phase error for 2nd and 3rd LO harmonics deliberately set for horrible quadrature (10degrees error applied for 2nd, and 30degrees applied to 3rd)

VloadVIF

Hybrid90HYB4


-90

0IN

ISO

Hybrid90HYB3


-90

0IN

ISO

P_1TonePORT5


RR5R=50 Ohm

P_1TonePORT4


RR4R=50 Ohm

PwrSplit3PWR3

S41=0.577S31=0.577S21=0.577

Amplifier2AMP3


PwrSplit3PWR2

S41=0.577S31=0.577S21=0.577

Amplifier2AMP1


RR2R=50 Ohm

Hybrid90HYB2


-90

0IN

ISO

Amplifier2AMP2


RR3R=50 Ohm

P_1TonePORT1


P_1TonePORT2


HarmonicBalanceHB1




EqnVar

Mixer2MIX2


Mixer2MIX3


PwrSplit2PWR1

S31=0.707S21=0.707R

R1R=50 Ohm

Hybrid90HYB1


-90

0IN

ISO

9

Testing Quadrature Modulator Sideband Suppression with Imperfect LO Source (phase error only)

10 20 30 40 50 60 70 80 90 1000 110

-400

-350

-300

-250

-200

-150

-100

-50

0

-450

50

freq, MHz

dBm

(Vlo

ad)

9.900M-250.1m

m1

Readout

m10

Readout

m11




29.90MHz

9.6 9.7 9.8 9.9 10.0 10.1 10.2 10.3 10.49.5 10.5

-400

-350

-300

-250

-200

-150

-100

-50

0

-450

50

freq, MHz

dBm

(Vlo

ad)

Readout

m2

10.00M-43.09

m3

Readout

m12




10.10MHz

Similar result as before except sidebands show up around 2xLO and 3xLO. Lesson learned: Phase impairments on the LO path is NOT causing poor sideband suppression at fundamental LO output.

10

Simulation Setup for SSB Modulator with Multi-tone LO source and Imperfect Magnitude Quadrature on LO harmonics

VloadVIF

Hybrid90HYB4


-90

0IN

ISO

Hybrid90HYB3


-90

0IN

ISO

P_1TonePORT5


RR5R=50 Ohm

P_1TonePORT4


RR4R=50 Ohm

PwrSplit3PWR3

S41=0.577S31=0.577S21=0.577

Amplifier2AMP3


PwrSplit3PWR2

S41=0.577S31=0.577S21=0.577

Amplifier2AMP1


RR2R=50 Ohm

Hybrid90HYB2


-90

0IN

ISO

Amplifier2AMP2


RR3R=50 Ohm

P_1TonePORT1


P_1TonePORT2


HarmonicBalanceHB1




EqnVar

Mixer2MIX2


Mixer2MIX3


PwrSplit2PWR1

S31=0.707S21=0.707R

R1R=50 Ohm

Hybrid90HYB1


-90

0IN

ISO

Note: Quadrature mag error for 3rd LO harmonic deliberately set for 1dB of error

11

Testing Quadrature Modulator Sideband Suppression with Imperfect LO Source (1dB error on 3rd LO harmonic only)

10 20 30 40 50 60 70 80 90 1000 110

-400

-350

-300

-250

-200

-150

-100

-50

0

-450

50

freq, MHz

dBm

(Vlo

ad)

9.900M-250.1m

m1

Readout

m10

Readout

m11




29.90MHz

9.6 9.7 9.8 9.9 10.0 10.1 10.2 10.3 10.49.5 10.5

-400

-350

-300

-250

-200

-150

-100

-50

0

-450

50

freq, MHz

dBm

(Vlo

ad)

Readout

m2

10.00M-43.09

m3

Readout

m12




10.10MHz

Somewhat unexpected result. HD3 magnitude quadrature is important for good sideband suppression.

12

Simulation Setup for SSB Modulator with Multi-tone LO source and Imperfect Magnitude Quadrature on LO harmonics

Note: Quadrature mag error for 2nd LO harmonic deliberately set for 1dB of error

VloadVIF

Hybrid90HYB4


-90

0IN

ISO

Hybrid90HYB3


-90

0IN

ISO

P_1TonePORT5


RR5R=50 Ohm

P_1TonePORT4


RR4R=50 Ohm

PwrSplit3PWR3

S41=0.577S31=0.577S21=0.577

Amplifier2AMP3


PwrSplit3PWR2

S41=0.577S31=0.577S21=0.577

Amplifier2AMP1


RR2R=50 Ohm

Hybrid90HYB2


-90

0IN

ISO

Amplifier2AMP2


RR3R=50 Ohm

P_1TonePORT1


P_1TonePORT2


HarmonicBalanceHB1




EqnVar

Mixer2MIX2


Mixer2MIX3


PwrSplit2PWR1

S31=0.707S21=0.707R

R1R=50 Ohm

Hybrid90HYB1


-90

0IN

ISO

13

Testing Quadrature Modulator Sideband Suppression with Imperfect LO Source (1dB error on 2nd LO harmonic only)

10 20 30 40 50 60 70 80 90 1000 110

-400

-350

-300

-250

-200

-150

-100

-50

0

-450

50

freq, MHz

dBm

(Vlo

ad)

9.900M-250.1m

m1

Readout

m10

Readout

m11




29.90MHz

9.6 9.7 9.8 9.9 10.0 10.1 10.2 10.3 10.49.5 10.5

-400

-350

-300

-250

-200

-150

-100

-50

0

-450

50

freq, MHz

dBm

(Vlo

ad)

Readout

m2

10.00M-43.09

m3

Readout

m12




10.10MHz

Surprisingly 2nd Harmonic has less impact than 3rd harmonic

14

Testing Quadrature Modulator Sideband Suppression with Imperfect LO Source (1dB error on 2nd and 3rd LO harmonics)

10 20 30 40 50 60 70 80 90 1000 110

-400

-350

-300

-250

-200

-150

-100

-50

0

-450

50

freq, MHz

dBm

(Vlo

ad)

9.900M-250.1m

m1

Readout

m10

Readout

m11




29.90MHz

9.6 9.7 9.8 9.9 10.0 10.1 10.2 10.3 10.49.5 10.5

-400

-350

-300

-250

-200

-150

-100

-50

0

-450

50

freq, MHz

dBm

(Vlo

ad)

Readout

m2

10.00M-43.09

m3

Readout

m12




10.10MHz

With 1dB of quadrature error at the 2nd and 3rd harmonics in the LO path the sideband suppression is reduced to -49dB. Increasing the error to 3dB results in -40dB of sideband suppression.

15

Sideband Suppression versus LO Harmonic Quadrature Magnitude Error

Mag Error on 2nd LO Harmonic - dB

Mag Error on 3rd LO Harmonic -

dB

Sideband Suppression -

dBc

0 0 -295.11 0 -69.93 0 -60.65 0 -56.7

10 0 -52.80 0 -295.10 1 -50.20 3 -40.90 5 -37.10 10 -33.10 0 -295.11 1 -49.33 3 -40.15 5 -36.2

10 10 -32.3-350.0

-300.0

-250.0

-200.0

-150.0

-100.0

-50.0

0.0

0 2 4 6 8 10 12

Quadrature Impairment - dB

Sid

eban

d S

up

pre

ssio

n -

dB

c

Magnitude Impairment on 2nd Harmonic Magnitude Impairment on 3rd LO Harmonic

Magnitude Impairment on 2nd and 3rd LO Harmonics

Note that poor magnitude imbalance on the 3rd LO harmonic is the dominate contributor to poor sideband suppression.

16

Conclusions

LO Harmonics can degrade Sideband-Suppression/Image-Rejection Performance in Direct Conversion Systems. The degradation is mainly due to quadrature amplitude mismatch through the typical polyphase structures employed in 1xLO designs (not really due to the phase mismatch as we may have thought). Using a simple 3rd Order lowpass LC filter with a cut-off of 1.5xfLO improves sideband suppression to ~72dBc even with 10dB of quadrature amplitude mismatch at 2nd and 3rd harmonics. Simple shunt-C series-L shunt-C filters should be enough to suppress LO harmonics for 1xLO mod/demods. When using 1xLO IQ Mod/Demod Components it is important to filter the LO harmonics for good sideband-suppression/image-rejection. When using 2xLO digital quadrature designs the magnitude and phase can be better matched over a broad range of frequencies and LO harmonics prove to be less critical.

lo harmonic content dependency on mod/demod performance

Documents

db of quadrature error

quadrature phase error

quadrature mag error

db error

db of sideband suppression

imperfect phase quadrature

imperfect magnitude

horrible quadrature