PCIRF_5_MIXER_6

7/29/2019 PCIRF_5_MIXER_6

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Mixers



2

Outline

• Functionality• Figures of Merit

• Passive Mixer Design

• Active Mixer Design

– Single-Balanced Mixer

– Double-Balanced Mixer (Gilbert Cell)

– Low-Voltage Mixer Design

• Up-Conversion Mixer

• Image-Rejection Mixer



3

Mixer Functionality

• To perform frequency translation, i.e. movedesired signals from RF to IF

• Linearity must be good to avoid SNR

degradation due to interference• Positive gain is preferred not to degrade NF

)()()( t vt Kvt v LO RF IF f RF

f LO

f IF



4

Mixer Figures of Merit

• Frequency and Bandwidth

• Linearity (P-1dB, IIP3)

• Noise Figure

• Conversion Gain

• Power Consumption

• Supply Voltage

• Isolation (RF-LO, LO-RF, LO-IF)



5

Mixer Noise Figure

wLO

wIF

wLO

wIF

Single-Sideband SSB Double-Sideband SSB



6

Mixer Noise Figure

• SSB NF Applied if Desired Signal Exists atOnly 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



7

Typical Figures of Merit

IIP3 ~ 10 dBm

P-1dB ~ 0 dBm

Gain > 0 dB

NF ~ 15 dB

Port Isolation < -20 dB

Supply Voltage ~ 1.5 V

Current ~ 3 mA



8

Input/Output Matching

• Input matching is needed if an off-chip

image-rejection filter is used in front:

– Maximizes Power Transferred

– Preserves Characteristics of the Filter

• Output matching is needed if an off-chip

channel-selection filter is used



9

Passive Mixer Design

RF

LO

R L

IFRF+

RF-

RF-

RF+

LO-LO+

IF+

IF-



10


• 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



11


RF+

RF-

RF-

RF+

LO-LO+

IF+

IF-

R L

R L



12

Active Mixer - Single-Balanced

RF

LO+

R L

IF+

LO-

IF-



13

Active Mixer - Single-Balanced

• Simple

• Few Devices

• Low Noise for Same Power • No Rejection of Even-Order Distortion



14

Active Mixer - Double-Balanced

RF

LO+

IF-

RF

LO+

IF+

LO-

VB



15

Active Mixer - Double-Balanced

• Most Popular and Known as Gilbert Cell• Higher Linearity

• High Rejection of Even-Order Distortion

• High LO-IF Isolation

• Minimize Substrate Noise Coupling

• Double Power for Same NF

• More Stacked Devices => High Minimum

Supply Voltage



16

Low-Voltage Mixer Techniques

• Operate RF Devices as Source Follower

• Current Steering from Loading

• Use PMOS in Triode

• Use PMOS Differential Loading w/ CMFB

• Use Inductors as Bias Devices



17

Low-Voltage Mixer - Source-

Follower RF [Kan]

RF+LO+

IF-

RF-LO+

IF+

LO-

VB VB



18

Low-Voltage Mixer - Source-

Follower RF• Lower Supply Voltage

• High Linearity due to Source Follower

• Lower Gain

• Higher NF

• Higher Power Consumption



19

RF

LO+

R L

IF+

LO-

IF-

Low-Voltage Mixer – Current

Steering



20

Low-Voltage Mixer – Current

Steering

• Facilitate Bias

• Minimize Current to Load to Maximize R Lfor High Gain and Low Noise Contribution

• Maximize Bias Current for RF Devices to

Minimize their Noise Contribution



21

Low-Voltage Mixer – PMOS

Loading

IF+

IF

-

• High R L

• High Gain

• Low Supply Voltage

• CMFB

• More Noise

• Smaller BW

R L



22

Low-Voltage Mixer – Inductor

Bias

RF

LO+

IF-

RF

IF+

LO-

LB



23


Bias• Allow More Voltage Headroom

• Maintain Good Common-Mode Rejection

• Bias-Voltage-Controlled Operation and

Performance

• Inductance Can Be Minimized if ACapacitor is Connected in Parallel for

Resonance at Interested Frequency



24


Loading

• Not Suitable for Downconversion Mixers

with Low IF• Ideal and Necessary for Upconversion

Mixers to Resonate Output Capacitance for

High Output Frequency and High Gain



25

Upconversion Mixers

• Low Input Frequency => Large Input

Devices for Small NF

• High Output Frequency => Inductor

Loading to Resonate Output Capacitance

for High Gain and High Bandwidth

• Linearity is NOT a Concern



26

Image-Rejection (IR) Mixers

• Can Be Used to Help Reject Image inHeterodyne Receivers

• Can also Be Used to Separate In-Phase and

Quadrature-Phase Signals in Zero-IFReceivers

• Can also Be Used to Select Upper or Lower

Band for Dual-Band Operation• Limited Image Rejection due to Mismatches



27

IR Mixer - Hilbert Filter

90ovin(t) vo(t)

t AT

t At v

t At v

o

in

w w

w

sin)4

(cos)(

cos)(

0 wowo 0

wo

wo ww

1/2 1/2

0 w

j

-j -j/2

j/2

0 w

j

-j

h(t) H(w)=-j*sgn(w)



28

IR Mixer - Hartley Architecture

vIF

sinwLOt

90o

0o

coswLOt

vQ

vI vI90

vRF



29


t At v

t

A

t

A

t v

t A

t A

t v

t A

t A

t v

t At At v

RF RF IF

IF

IM

IF

RF

Q

IF IM

IF RF

I

IF IM

IF RF

I

IM IM RF RF RF

w

w w

w w

w w

w w

cos)(

cos2cos2)(

cos2

cos2

)(

sin

2

sin

2

)(

coscos)(

90



30


vI-

vRF

0 wLOwLO

0 0

j/2 j/2

-j/2-j/2

vI+



31


vI90-

0 0

vI90+

vQ-

0 0

1/21/2 vQ+

vIF-

0 0

1vIF

+

1/21/2

-1/2

1/2 1/2

-1/2

1



32

Image-Rejection Mixer

• Ideally, Image Rejection is Infinitely Large

• In Practice, Limited Rejection due to Gainand Phase Mismatches (LO, Mixers, Filters,

Phase Shifters)

• Cascaded Polyphase Filters can be Used for Better Matching and Image Rejection

[Behbahani]



33

Image-Rejection Mixer

• Gain Mismatch DA/A ~ 0.1 dB, PhaseMismatch ~ 1o => IRR ~ 30 dB

4

)/(

)(cos)(2)(cos)(2

/

/

22

2

2

2

2

22

22

D

A A

A A A A A A A A

A A

A A IRR

LO LO LO LO

LO LO LO LO

input RF IM

output RF IM



34


sinwLOt

45o

-45o

coswLOt

vQ

vI

vRF



35

IR Mixer - Weaver Architecture

sinwLO1t

coswLO1t

vQ

vI

vRFsinwLO2t

coswLO2t



36

I-Q Generation

• Synthesizers with Quadrature Outputs

• Use Divider-By-2 Before Mixers

• 90-Degree RC-CR Phase Shifter • Polyphase Filter



37

90-Degree Phase Shifter

• RC-CR Network

• Polyphase Filter

in

outI

outQ



38

90-Degree Phase Shifter

• Sensitive to Mismatches of Components

• Good Matching only at a Single Frequency

• Introduces Loss and Degrades NF• Small Resistor for Low Noise, but Large

Resistor for Low Loss

• Large Components and Chip Area for GoodMatching



39

Polyphase Filter

in+

in-

out(-90)

out(180)

out(0)

out(90)



40

Polyphase Filter

• Accommodate and Provide Differential

Input/Output Signals

• Same Challenges as RC-CR Network

• Can Be Cascaded to Allow Good Matching

and Good Image Rejection over Wide

Bandwidth

• May Include Gain Stages Between to

Compensate for Loss and Improve NF



41

References

• B. Razavi, RF Microelectronics, Prentice Hall,1998

• T. Lee, The Design of CMOS Radio Frequency Integrated Circuits, Cambridge, 1998

• J. Crols and M. Steyaert, CMOS WirelessTransceiver Design, Kluwer 1997

• T. Kan, et al, "Design Theory and Performance of

1-GHz CMOS Downconversion & UpconversionMixers," Analog Integrated Circuits and Signal Processing , pp. 101-111, August 2000



References

• C. Hull and R. Meyer, “ A Systematic Approach tothe Analysis of Noise in Mixers,” TCAS-I, pp.909-919, December 19993.

• J. Crols and M. Steyaert, “A 1.5-GHz HighlyLinear CMOS Downconversion Mixer,” JSSC, pp.736-742, July 1995

• Behbahani, et al., “CMOS Mixers and Polyphase

Filters for Large Image Rejection,” JSSC, pp.879-887, June 2001

PCIRF_5_MIXER_6

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