LO Harmonic Content Dependency on Mod/Demod Performance
description
Transcript of 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
m3freq=dBm(VOUT)=1.950
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
m3freq=dBm(VOUT)=1.950
30.00MHz
m4freq=phase(VOUT)=141.260
11.00MHz
m5freq=phase(VOUT)=-25.884
20.00MHz
m6freq=phase(VOUT)=-128.826
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
m5freq=phase(VOUT)=-25.884
20.00MHz
m6freq=phase(VOUT)=-128.826
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
m9freq=phase(VLOsource)=-128.826
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
m8freq=phase(VLOsource)=-25.884
20.00MHz
m9freq=phase(VLOsource)=-128.826
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
Freq=LO_Freq2P=polar(dbmtow(-20),334.116)Z=50 OhmNum=4
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
S12=0S22=polar(0,180)S11=polar(0,0)S21=dbpolar(3.01,0)
RR3R=50 Ohm
P_1TonePORT5
Freq=LO_Freq3P=polar(dbmtow(-10),231.174)Z=50 OhmNum=5
P_1TonePORT1
Freq=LO_FreqP=polar(dbmtow(0),141.26)Z=50 OhmNum=1
P_1TonePORT4
Freq=LO_Freq2P=polar(dbmtow(-20),334.116)Z=50 OhmNum=4
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
S12=0S22=polar(0,180)S11=polar(0,0)S21=dbpolar(4.77,0)
Mixer2MIX2
SP23=dbpolar(-40,0)ConvGain=dbpolar(0,0)SideBand=BOTH
Mixer2MIX3
SP23=dbpolar(-40,0)ConvGain=dbpolar(0,0)SideBand=BOTH
Hybrid90HYB2
PhaseBal=0GainBal=0 dBLoss=0 dB
-90
0IN
ISO
RR2R=50 Ohm
PwrSplit2PWR1
S31=0.707S21=0.707R
R1R=50 Ohm
Hybrid90HYB1
PhaseBal=0GainBal=0 dBLoss=0 dB
-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
19.90MHzm11freq=dBm(Vload)=-12.876
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
m2freq=dBm(Vload)=2.760
9.900MHzm3freq=dBm(Vload)=-40.008
10.00MHzm12freq=dBm(Vload)=-290.026
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
PhaseBal=30GainBal=0 dBLoss=0 dB
-90
0IN
ISO
Hybrid90HYB3
PhaseBal=10GainBal=0 dBLoss=0 dB
-90
0IN
ISO
P_1TonePORT5
Freq=LO_Freq3P=polar(dbmtow(-10),231.174)Z=50 OhmNum=5
RR5R=50 Ohm
P_1TonePORT4
Freq=LO_Freq2P=polar(dbmtow(-20),334.116)Z=50 OhmNum=4
RR4R=50 Ohm
PwrSplit3PWR3
S41=0.577S31=0.577S21=0.577
Amplifier2AMP3
S12=0S22=polar(0,180)S11=polar(0,0)S21=dbpolar(4.77,0)
PwrSplit3PWR2
S41=0.577S31=0.577S21=0.577
Amplifier2AMP1
S12=0S22=polar(0,180)S11=polar(0,0)S21=dbpolar(4.77,0)
RR2R=50 Ohm
Hybrid90HYB2
PhaseBal=0GainBal=0 dBLoss=0 dB
-90
0IN
ISO
Amplifier2AMP2
S12=0S22=polar(0,180)S11=polar(0,0)S21=dbpolar(3.01,0)
RR3R=50 Ohm
P_1TonePORT1
Freq=LO_FreqP=polar(dbmtow(0),141.26)Z=50 OhmNum=1
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
Mixer2MIX2
SP23=dbpolar(-40,0)ConvGain=dbpolar(0,0)SideBand=BOTH
Mixer2MIX3
SP23=dbpolar(-40,0)ConvGain=dbpolar(0,0)SideBand=BOTH
PwrSplit2PWR1
S31=0.707S21=0.707R
R1R=50 Ohm
Hybrid90HYB1
PhaseBal=0GainBal=0 dBLoss=0 dB
-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
m1freq=dBm(Vload)=2.760
9.900MHzm10freq=dBm(Vload)=-22.804
19.90MHzm11freq=dBm(Vload)=-13.177
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
m2freq=dBm(Vload)=2.760
9.900MHzm3freq=dBm(Vload)=-40.008
10.00MHzm12freq=dBm(Vload)=-292.340
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
PhaseBal=0GainBal=1 dBLoss=0 dB
-90
0IN
ISO
Hybrid90HYB3
PhaseBal=0GainBal=0 dBLoss=0 dB
-90
0IN
ISO
P_1TonePORT5
Freq=LO_Freq3P=polar(dbmtow(-10),231.174)Z=50 OhmNum=5
RR5R=50 Ohm
P_1TonePORT4
Freq=LO_Freq2P=polar(dbmtow(-20),334.116)Z=50 OhmNum=4
RR4R=50 Ohm
PwrSplit3PWR3
S41=0.577S31=0.577S21=0.577
Amplifier2AMP3
S12=0S22=polar(0,180)S11=polar(0,0)S21=dbpolar(4.77,0)
PwrSplit3PWR2
S41=0.577S31=0.577S21=0.577
Amplifier2AMP1
S12=0S22=polar(0,180)S11=polar(0,0)S21=dbpolar(4.77,0)
RR2R=50 Ohm
Hybrid90HYB2
PhaseBal=0GainBal=0 dBLoss=0 dB
-90
0IN
ISO
Amplifier2AMP2
S12=0S22=polar(0,180)S11=polar(0,0)S21=dbpolar(3.01,0)
RR3R=50 Ohm
P_1TonePORT1
Freq=LO_FreqP=polar(dbmtow(0),141.26)Z=50 OhmNum=1
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
Mixer2MIX2
SP23=dbpolar(-40,0)ConvGain=dbpolar(0,0)SideBand=BOTH
Mixer2MIX3
SP23=dbpolar(-40,0)ConvGain=dbpolar(0,0)SideBand=BOTH
PwrSplit2PWR1
S31=0.707S21=0.707R
R1R=50 Ohm
Hybrid90HYB1
PhaseBal=0GainBal=0 dBLoss=0 dB
-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
m1freq=dBm(Vload)=2.760
9.900MHzm10freq=dBm(Vload)=-22.771
19.90MHzm11freq=dBm(Vload)=-12.889
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
m2freq=dBm(Vload)=2.760
9.900MHzm3freq=dBm(Vload)=-40.008
10.00MHzm12freq=dBm(Vload)=-47.429
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
PhaseBal=0GainBal=0 dBLoss=0 dB
-90
0IN
ISO
Hybrid90HYB3
PhaseBal=0GainBal=1 dBLoss=0 dB
-90
0IN
ISO
P_1TonePORT5
Freq=LO_Freq3P=polar(dbmtow(-10),231.174)Z=50 OhmNum=5
RR5R=50 Ohm
P_1TonePORT4
Freq=LO_Freq2P=polar(dbmtow(-20),334.116)Z=50 OhmNum=4
RR4R=50 Ohm
PwrSplit3PWR3
S41=0.577S31=0.577S21=0.577
Amplifier2AMP3
S12=0S22=polar(0,180)S11=polar(0,0)S21=dbpolar(4.77,0)
PwrSplit3PWR2
S41=0.577S31=0.577S21=0.577
Amplifier2AMP1
S12=0S22=polar(0,180)S11=polar(0,0)S21=dbpolar(4.77,0)
RR2R=50 Ohm
Hybrid90HYB2
PhaseBal=0GainBal=0 dBLoss=0 dB
-90
0IN
ISO
Amplifier2AMP2
S12=0S22=polar(0,180)S11=polar(0,0)S21=dbpolar(3.01,0)
RR3R=50 Ohm
P_1TonePORT1
Freq=LO_FreqP=polar(dbmtow(0),141.26)Z=50 OhmNum=1
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
Mixer2MIX2
SP23=dbpolar(-40,0)ConvGain=dbpolar(0,0)SideBand=BOTH
Mixer2MIX3
SP23=dbpolar(-40,0)ConvGain=dbpolar(0,0)SideBand=BOTH
PwrSplit2PWR1
S31=0.707S21=0.707R
R1R=50 Ohm
Hybrid90HYB1
PhaseBal=0GainBal=0 dBLoss=0 dB
-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
m1freq=dBm(Vload)=2.760
9.900MHzm10freq=dBm(Vload)=-22.785
19.90MHzm11freq=dBm(Vload)=-12.876
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
m2freq=dBm(Vload)=2.760
9.900MHzm3freq=dBm(Vload)=-40.008
10.00MHzm12freq=dBm(Vload)=-67.112
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
m1freq=dBm(Vload)=2.760
9.900MHzm10freq=dBm(Vload)=-22.784
19.90MHzm11freq=dBm(Vload)=-12.889
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
m2freq=dBm(Vload)=2.760
9.900MHzm3freq=dBm(Vload)=-40.008
10.00MHzm12freq=dBm(Vload)=-46.572
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.