Introduction to CMOS RF Integrated Circuits...
Transcript of Introduction to CMOS RF Integrated Circuits...
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-1
Introduction to CMOS RF Integrated Circuits Design
IV. Mixers
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-2
•Functionality•Figures of Merit•Passive Mixer Design•Active Mixer Design
•Single-Balanced Mixer•Double-Balanced Mixer (Gilbert Cell)
•Design Example
OutlineOutline
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-3
)()()( tvtKvtv LORFIF
fRF
fLO
fIF
•To perform frequency translation (Up-conversion and Down-Conversion)•Linearity must be good to avoid SNR degradation due to interference•Positive gain is preferred
fRF
fLO
fIF
Mixer FunctionalityMixer Functionality
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-4
Frequency TranslationFrequency Translation
)()()(OUT tvtKvtv LORF)0cos()()( tttAtvRF
)cos()( tLOLOAtvLO
OUT
( )( ) {cos(( ) ( )) cos(( ) ( ))}0 02
KA t ALOv t t t t tLO LO
•Low-pass filter for down-conversion•High-pass filter for up-conversion
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-5
Image RejectionImage Rejection
LO
IF
ageLOLO Im0
We need Image Rejection Filter!
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-6
•Frequency and Bandwidth•Linearity (P-1dB, IIP3)•Noise Figure •Conversion Gain•Power Consumption•Supply Voltage•Isolation (RF-LO, LO-RF, LO-IF)
Mixer Figures of MeritMixer Figures of Merit
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-7
IIP3 ~ 10 dBm P-1dB ~ 0 dBm Gain 10 dB NF ~ 12 dB Port Isolation < -20 dB Current ~ 3 mA
Typical ValuesTypical Values
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-8
LO
IF
LO
IF
Single-Sideband (SSB) NF Double-Sideband (DSB) NF
Mixer Noise FigureMixer Noise Figure
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-9
•SSB NF Applied if Desired Signal Exists at Only One Frequency•DSB NF Applied if Desired Signals Exist at Both Image and Desired Frequencies•SSB NF = DSB NF + 3 dB•Typically, Desired Signal Only Exists at One Frequency => SSB NF•For Direct Conversion, No Image Noise => DSB NF
SSB SSB vsvs DSB NFDSB NF
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-10
•Input matching is needed if an off-chip image-rejection filter is used in front to:
•Maximizes Power Transferred•Preserves Characteristics of the Filter
•Output matching is needed if an off-chip channel-selection filter is used
Input/Output MatchingInput/Output Matching
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-11
The conversion gain may be measured as voltage or power gain. Voltage Conversion Gain=rms voltage of the IF signal/rms voltage of the RF signalPower Conversion Gain=IF Power delivered to the load/rms available RF power from the source
Conversion GainConversion Gain
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-12
Signals may leak through different paths from one port to the other.•LO-to-RF leakagecauses self-mixing (problem for zero-IF)•RF-to-LO feedthrough allows interferers and spurs present in the RF signal to interact with the LO.• LO-to-IF feedthroughmay cause desensitization of consequent blocks • RF-to-IF feedthrough causes problems in some architectures such as zero-IF because of the leakage of low-frequency even-order intermod. products (even-order distortion).
PortPort--toto--Port IsolationPort Isolation
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-13
Multiplication: NonMultiplication: Non--LinearityLinearity
The multiplication can be implemented with a multiplier or nonlinear circuit
....44
33
2210 inVainVainVainVaaoutV
)cos()cos()( tLOLOVtRFRFVtvin
)]cos()cos([1)( tLOLOVtRFRFVatvout
)](2cos2)(2cos2[2 tLOLOVtRFRFVa
2* 2[ cos( ) cos( )] ...a V t V tRF RF LO LO
2 [cos( ) cos( ) ]a V V t tRF LO RF LO RF LO
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-14
Implementation of a Simple MixerImplementation of a Simple Mixer
IFV
Biasv
LOv
RFvn OX LO
WG C VL
For VLO >>Vdsat,
*No RF-LO port isolation
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-15
Implementation of DualImplementation of Dual--Gate MixerGate Mixer
IFV
Biasv
RFv
LOv
2D
1D
1S
2S2G
1G
2M
1M
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-16
DualDual--Gate MixerGate Mixer
The LO signal is large enough to push M1 into triode during part of the operating cycle.The gm of M1 is therefore modulated periodically
( )n OXm GS Tsat
C Wg V VL
DS
OXnTriodem V
LWCg
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-17
Implementation of DualImplementation of Dual--Gate MixerGate Mixer
)()( 11
TGSOXn
VVVVV
LWCtg
TGSD
VGS2 is roughly constant since M1 acts like a current source.
1 2 2 0 0 2cosD LO GS B GSV v V V V t V
12 2 0( ) ( 0cos )
D GS T
n OXB GSV V V
C Wg t V V V tL
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-18
RF
LO
RL
IF
LO
RL
IFRFV
Passive Passive vsvs Active MixersActive Mixers
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-19
•MOS Devices Operate in Triode Region•High Linearity•High Frequency•No Gain or even Loss•In Practice, Buffer Needed to Isolate or to Convert Output Current to Voltage
Passive MixersPassive Mixers
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-20
Voltage Switching MixersVoltage Switching Mixers
•Switching of voltage•Requires good switches that turn on hard (low resistance) and turn off well (good insolation)
During the +LO cycle, feeds the RF to the output directly. In the -LO cycle, feeds an inverted RF signal to the output. We see that the RF signal is effectively multiplied by ±1 with a rate determined by the LO signal. A differential RF signal is created using a balun or fed directly from a balanced LNA.
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-21
Voltage Switching MixersVoltage Switching Mixers
When the device is “on”, it’s in the triode region. Due to the low on-resistance, the coupling through the substrate and LO path is minimal.When the device is “off”, the RF and LO leak into the IF through the overlap and substrate capacitances.
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-22
Voltage Switching MixersVoltage Switching Mixers
•MOS passive mixer is very linear. The device is either “on” or “off” and does not impact the linearity too much. Since there is no transconductance stage, the linearity is very good.•The downside is that the MOS mixer is passive, or lossy. •There is no power gain in the device.•Need large LO drive to turn devices on/off Need to create a differential RF and LO signal. This can be done using baluns or by using a differential LNA and LO buffer.
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-23
Passive Mixer LO PowerPassive Mixer LO Power
RF
LO
IF
•Large capacitive load P=CV2LOfLO large power
•Large inverters or tuned buffer
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-24
Passive Current MixerPassive Current Mixer
•The input stage is a Gm stage similar to a Gilbert cell mixer. The Gilbert Quad, though, has no DC current and switches on/off similar to a passive mixer.•The output signal drives the virtual ground of a differential op-amp. The current signal is converted into a voltage output by the op-amp.
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-25
Passive Current MixerPassive Current Mixer
•No DC current in quad implies that there is no flicker noise generated by the switching quad. This is the key advantage.•The linearity is very good since the output signal is a current. The voltage swing does not limit the linearity of the mixer. This is to be contrasted to a Gilbert cell mixer where the voltage swing is limited due to the headroom of the switching mixer and the transconductance stage.•The op-amp output stage can be converted into an IF filter (discussed later)
•Need large LO drive compared to the active Gilbert cell mixer.•Need an op-amp. This requires extra power consumption and introduces additional noise.•Need a common mode feedback circuit at the input of the op-amp.
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-26
Passive Current MixerPassive Current Mixer
Note that the Gilbert quad is really a folded ring. Thus the passive and active mixers are very similar. The main difference is how the quad devices are biased. In the Gilbert cell they are biased nominally in saturation and have DC current. In the passive mixers, they are biased near the threshold.
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-27
Passive Current MixerPassive Current Mixer
The op-amp input referred noise is amplified to IF. The resistance seenat the op-amp input terminals is actually a switched capacitor resistor!The parasitic capacitance at the output of the transconductance stage is charged and discharged at the rate of the LO.
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-28
Passive Current MixerPassive Current Mixer
Note that the parasitic capacitances are charged at the rate of the LO to the input voltage Vx, and then to the -Vx, every cycle.The total charge transferred during a period is given byQtot = CpVx-(-CpVx)= 2CpVxThe net current is given byIx = Qtot/TLO= 2CpVxfLO
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-29
Passive Current MixerPassive Current Mixer
Since there are two differential pairs connected to the op-amp terminals in parallel, the total charge is twice. So the effective resistance seen at this node is given by:Rp =Vx/2Ix
The effective resistance is therefore given by:Rp =1/4fLOCpThis is a switched capacitor “resistors”.
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-30
Passive Current MixerPassive Current Mixer
The noise is thus transferred to the output with transfer function given by:
To minimize this noise, we have to minimize the parasitic capacitance Cp and the op-amp noise.
2 2 2(1 )fo amp
p
Rv v
R
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-31
Passive Current MixerPassive Current Mixer
Since a down-conversion mixer will naturally drive a filter, we see that the output current can be used directly to drive a current mode filter.For instance, the op-amp can be absorbed into the first stage of a multi-stage op-amp RC IF filter. The feedback resistor Rf is shunted with a capacitor Cf to produce a pole.
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-32
Implementation of Active MixerImplementation of Active Mixer
)(tVRF
)(tVLO)(tVLO
)(tVIF
1,3,5...
( ) ( )cos ω ω
1 2 1cos ω [ cos( ω )]2
1 1 1cos ω cos(ω ω ) cos ω 3ω )2 π 3
RF LO
RF RF LO
RF RF LOn
RF RF RF RF LO RF LO
V t V tA t sq t
A t n tn
A t A t t
+1
0
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-33
IFV
RFV
LO
IRF=IBias+gmVrf
IRF
IFR
LORFIF VIV
IRF IFV IFV
Phase 1 Phase 2
MultiplierMultiplier--Based MixersBased Mixers
+1
0
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-34
MultiplierMultiplier--Based MixersBased Mixers
4 4 4[1 cos cos3 cos5 ....] cos2 3 5 2bias m
IF LO LO LO RF RFI gI t t t A t
tAV RFRFRF ωcos
])cos(1)cos(1[ ttAg RFLORFLORFm
])3cos(31)3cos(
31[ ttAg RFLORFLORFm
1 1[ cos(5 ) cos(5 ) ]5 5m RF LO RF LO RFg A t t
DC nωLO n=1,3,5
nωLO+- ωRF n=1,3,5ωRF
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-35
MultiplierMultiplier--Based MixersBased Mixers
•LO-IF feedthroughA spectra component at wLO appears at the IF-port. If the LO frequency is not far enough from the desired RF, it may be difficult to attenuate the LO component enough via filtering.• RF-IF feedthrough (or direct feedthrough).An RF spectra components shows at the IF-port. Directfeedthrough worsens the NF of the mixer because it allows the noise at the RF-port at the desired IF frequency to leak to the IF-port.
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-36
SingleSingle--Balanced MixersBalanced Mixers
IFV
RFV
LOIRF=IBias+gmVrf
IRF
IFRIFR
Switching between IRF and -IRF
IRF
IFV
IRF+1
-1
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-37
SingleSingle--Balanced MixersBalanced Mixers
4 4 4[ cos cos3 cos5 ....]3 5IF bias LO LO LOI I t t t
tAV RFRFRF ωcos
])cos(2)cos(2[ ttAg RFLORFLORFm
])3cos(32)3cos(
32[ ttAg RFLORFLORFm
2 2[ cos(5 ) cos(5 ) ]5 5m RF LO RF LO RFg A t t
nωLO n=1,3,5
nωLO+- ωRF n=1,3,5
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-38
DoubleDouble--Balanced MixersBalanced Mixers
RF
LO+
IF-
RF
LO+
IF+
LO-
VB
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-39
DoubleDouble--Balanced MixersBalanced Mixers
tAV RFRFRF ωcos
2 2[ cos( ) cos( ) ]IF m RF LO RF LO RFI g A t t
])3cos(32)3cos(
32[ ttAg RFLORFLORFm
2 2[ cos(5 ) cos(5 ) ]5 5m RF LO RF LO RFg A t t
nωLO+- ωRF n=1,3,5
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-40
LinearizationLinearization
Vbias
VRF
VRF
R
R
Common-Gate Input Common-Source Input
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-41
LinearizationLinearization
Common-Gate Linearization:•The transconductance of the common-gate at the RF port is:Gm=IM RF/VRF=gm/(1+gmR)1/Rcan be made is R is large enoughThe disadvantage is the additional noise due to R.•The conversion transconductance is Gc=2Gm/π
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-42
Noise in MixersNoise in Mixers
RF
LO+
IF-
RF
LO+
IF+
LO-
VB
Noise
Vni
•Noise Contributors:•Loads•Transconductance FETs•Switches
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-43
Noise in MixersNoise in Mixers
•Loads1/f noise•Transconductance FETs1/f noise and white noise
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-44
Noise in MixersNoise in Mixers
fc fLO-fLO -fcf
1/f
3fLO
fc fLO-fLO -fcf
1/f
3fLO
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-45
Noise in MixersNoise in Mixers
•Switch Noise
Assume that all input phase noise is reflected to one side of the differential pair and is given by:vid = vn(t)+2AsinωLOtEach time the Vid crosses zero the iod switches from –I to I or vice-versa.
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-46
Noise in MixersNoise in Mixers
Random current pulses due to vn(t). The width of each pulse is:Δt= vn(t)/s
S is the slope of LOAnd its height=2 IFrequency=2fLO
The average current noise pulses is:
AtIV
sTtIV
TtIi nn
nav )(
2/)(2
2/2 0
,0 22 fAs
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-47
Noise in MixersNoise in Mixers
SNR due to the switch noise:SNR=i0/ino
The SNRs can be improved by:•In crease LO swing•Trade off with frequency •(reduce over-ride voltage)•(Reduce Nn by increase WL of the switch)
inm
inmVgVGsignal 2
n
in
TGSn
inm
VV
VVA
AIV
VgSNR
2
112
TGSm VVg
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-48
Simulation of Mixers in Simulation of Mixers in SpectreSpectre--RFRF
•Periodic Transfer Function (PXF)conversion gain•Periodic Steady State (PSS)1dB compression point and conversion gain•Periodic Distortion (Pdisto) analysisIIP3 •Periodic Noise (Pnoise)noise figure.
Introduction to CMOS RF Integrated Circuits DesignFall 2012, Prof. JianJun Zhou IV-49