Chapter5 Mixer and VCO

Dr. Cuong HuynhTelecommunications DepartmentHCMUT 1

Huynh Phu Minh Cuong [email protected]

Department of Telecommunications

Faculty of Electrical and Electronics Engineering

Ho Chi Minh city University of Technology

Chapter 5

Mixer and Oscillator

MICROWAVE INTERGRATED CIRCUITS

mailto:[email protected]


Mixer

• Frequency translation device

– Convert RF frequency to a lower IF or base band for easy signal processing in receivers

– Convert base band signal or IF frequency to a higher IF or RF frequency for efficient transmission in transmitters

• Creative use of nonlinearity or time-variance

– These are usually harmful and unwanted

– They generates frequencies not present at input

• Used together with appropriate filtering

– Remove unwanted frequencies

What is a mixer

Dr. Cuong HuynhTelecommunications DepartmentHCMUT

An ideal nonlinearity mixer x(t)

y(t)

x(t)y(t) If

tBty

tAtx

2

1

cos)(

cos)(

Then the output is

tAB

tAB

tBtA )cos(2

)cos(2

coscos 212121

down convert up convert


Mixer


Mixer


Mixer

Mixer Fundamentals

IF

LO

Mixer

IF IF IFV = A cos(ω t)

LO LO LOV = A cos(ω t)

RFVRF

IF LORF IF LO IF LO

A AV = cos(ω +ω ) + cos(ω -ω )

2

High sideband Low sideband


Mixer

Mixers can be implemented using: Nonlinear circuits

Commutating (switching) circuits.

+-

+-VIF

VLO

V I VRF

RL

Non-linear

component

VRF

RL+-

VIF

VLO

Switch

2 3

0 1 2 3I = a + a V + a V + a V + ·· · LO LO LO

4 4 4S t Cos( t) Cos(3 t) Cos(5 t) . . .

3 5

switching circuit


Mixer


• Conversion gain – lowers noise impact of following stages

• Noise Figure – impacts receiver sensitivity • Port isolation – want to minimize interaction between

the RF, IF, and LO ports • Linearity (IIP3) – impacts receiver blocking

performance • Spurious response • Power match – want max voltage gain rather than

power match for integrated designs • Power – want low power dissipation • Sensitivity to process/temp variations – need to make

it manufacturable in high volume

Mixer Metrics

Mixer


Conversion Gain • Conversion gain or loss is the ratio of the desired IF

output (voltage or power) to the RF input signal value ( voltage or power).

signal RF theof voltager.m.s.

signal IF theof voltager.m.s.Gain Conversion Voltage

source thefrompower Available

load the todeliveredpower IF Gain ConversionPower

If the input impedance and the load impedance of the mixer are

both equal to the source impedance, then the voltage conversion

gain and the power conversion gain of the mixer will be the

same in dB’s.

Mixer


Port-to-Port Isolations

RF IF

LO

• Isolation

– Isolation between RF, LO and IF ports

– LO/RF and LO/IF isolations are the most important features.

– Reducing LO leakage to other ports can be solved by filtering.

Mixer


Mixer


LO Feed through

• Feed through from the LO port to IF output port due to parasitic capacitance, power supply coupling, etc.

• Often significant due to strong LO output signal – If large, can potentially desensitize the receiver due to the extra

dynamic range consumed at the IF output – If small, can generally be removed by filter at IF output

Mixer


Reverse LO Feed through

• Reverse feed through from the LO port to RF input port due to parasitic capacitance, etc. – If large, and LNA doesn’t provide adequate isolation, then

LO energy can leak out of antenna and violate emission standards for radio

– Must insure that isolation to antenna is adequate

Mixer


Self-Mixing of Reverse LO Feedthrough

• LO component in the RF input can pass back through the mixer and be modulated by the LO signal – DC and 2fo component created at IF output

– Of no consequence for a heterodyne system, but can cause problems for homodyne systems (i.e., zero IF)


Noise Figures: SSB vs DSB

Image band

Signal band

Thermal noise

LO

IF

Signal band

Thermal noise

LO

0

Single side band Double side band

Mixer

RF

IF

SNRNF =

SNR


SSB Noise Figure

• Broadband noise from mixer or front end filter will be located in both image and desired bands

• Noise from both image and desired bands will combine in desired channel at IF output – Channel filter cannot remove this

Mixer


• For zero IF, there is no image band – Noise from positive and negative frequencies combine, but the signals

combine as well

• DSB noise figure is 3 dB lower than SSB noise figure – DSB noise figure often quoted since it sounds better

DSB Noise Figure

Mixer


Mixer Classification • Diode – MOSFET • Active - Passive • Single-balanced – Double-Balanced • Discrete implementations:

– Single-diode and diode-ring mixers • IC implementations:

– MOSFET passive mixer – Active mixers – Gilbert-cell based mixer – Square law mixer – Sub-sampling mixer – Harmonic mixer

Mixer


Mixer

Single-Ended Diode Mixer


Mixer

Single-Ended FET Mixer

2

.ds GS TI Iss V V


Mixer

Single-Ended FET Mixer

VRF

Rb

VBB1

Cl earg

2

0.ds SQ GSQ TI K V V

VLO

Cl earg

Le

LgRS

RLO

VBB2

VDD

Cmatch

RL

IF Filter

Matching Network

MOSFET Mixer (with impedance matching)


Mixer


Mixer


Mixer


Mixer

Adding these two currents at the input to the low-pass filter gives

Note that the DC components of the diode currents cancel upon combining. After low-pass filtering, the IF output is


Mixer

Single-Balanced Active Mixer

VLO

RL RL

VLO

VRF

Vout

I IDC RF

M1

M2 M3

• The transistor M1 converts the RF voltage signal to the current signal.

• Transistors M2 and M3 commute the current between the two branches.

DC1 DC 1,RF m RFI = I + I = I +g .V

The mixing operation of the mixer is performed by turning on and off two transistors Q2 and Q3 in the switching stage using the large LO signal, VLO. The LO signal can be a square wave or sinusoidal signal.


Mixer


VLO

RL RL

VLO

VRF

Vout

I IDC RF

M1

M2 M3

VIF = -RL(IDC + gmVRF) , VLO > 0

+ - VIF = RL(IDC + gmVRF) , VLO < 0

It is observed that the IF signal is obtained

from toggling the signal term RL(IDC+gmVRF)

between positive and negative values. This is

equivalent to multiplying the term

RL(IDC+gmVRFF) with a bipolar square wave

S(t) having the amplitude of 1 and frequency

of fLO.

S(t)

t

+1

-1

1/FLO The IF output signal is then

expressed as

VIF (t) = RL(IDC + gmVRF)S(t)


Mixer


LO LO LO

4 4 4S t Cos( t) Cos(3 t) Cos(5 t) . . .

3 5

LO LO LO

4 4 4Cos( t) Cos(3 t) Cos(5 t) . . .

3 5

In form of Fourier series, S(t) is expressed as

Then, VIF(t) is expressed as

VRF(t) = RL(I0 + GmVIF)

IF L DC m RF RF LO

4V (t) R I g A Cos( t) Cos( t)

m L RF m L RFIF DC L LO LO RF LO RF

4 2g R A 2G R AV (t) I R Cos( t) Cos( t) Cos( t)

With the first harmonic LO mixing, the fundamental component of S(t) is considered.

m Lv

2g RA =

Gain: Disadvantage:

LO feedthrough

S(t)

t

+1

-1

1/FLO


Double Balanced Mixer – Gilbert Mixer

• Strong LO-IF feed suppressed by double balanced mixer.

• All the even harmonics cancelled.

• All the odd harmonics doubled (including the signal).

VLO

RL RL

VLOM2 M3

VRF

VLOM2 M3

VRF

VOUT

I IDC RF I IDC RF

Mixer


Mixer

Ze Ze

LO+

LO-

LO+

RL

Q4Q3

Q1Q2

Q5 Q6

VIF+VIF-

VRF+ VRF-

RL

VDD

2I0

IF

1 0 m

V I = I + G

2

IF

2 0 m

V I = I -G

2

Double Balanced Mixer – Gilbert Mixer

IF1 0 m

V I =I +G

2

IF2 0 m

V I =I -G

2

IF L 2 1 m L RFV R (I -I ) g R V , VLO > 0

IF L 1 2 m L RFV R (I -I ) g R V , VLO < 0

RF m L IFV (t) G R V (t)S(t)

RF m L IF IF LO

4V (t) G R A Cos( t)Cos( t)

m L IF m L IFRF LO IF LO IF

2G R A 2G R AV (t) Cos( t) Cos( t)


Oscillator

Introduction RF and microwave oscillators provide signal sources for frequency

conversion and carrier generation.

A solid-state oscillator uses an active nonlinear device, such as a diode

or transistor, in conjunction with a passive circuit to convert DC to a

sinusoidal steady-state RF signal.

Diodes or transistors are biased to provide a negative resistance then are

used with resonators using cavity, transmission line, or dielectric to

produce oscillations.

Frequency multipliers, in conjunction with a lower frequency source,

can be used to produce power at millimeter wave frequencies.

Because of the requirement of a nonlinear active device, the rigorous

analysis and design of oscillator circuits can be difficult, and is usually

carried out today with sophisticated CAD tools.

At startup, oscillation is triggered by transient or noise and then it

reaches a stable oscillation state.


Oscillator

Introduction Important considerations for oscillators used in RF and

microwave systems :

Tuning range (specified in MHz/V for voltage-tuned

oscillators)

Frequency stability (specified in PPM/◦C) ppm: parts-

per million, typical 2 ~ 0.5 PPM/oC

Phase noise (specified in dBc/Hz below carrier, offset

from carrier), typical –80 ~ -110 dBc/Hz @ 10 KHz offset Harmonics (specified in dBc below carrier)

http://www.rapidtables.com/math/number/PPM.htm







Oscillator

Various types of RF oscillators: Hartley, Colpitts, Clapp, and

Pierce, using transistor and a feed back network.

Focus on MICROWAVE OSCILLATORS


Oscillator


Oscillator


Oscillator The process of oscillation is critically dependent on the nonlinear behavior of Zin, as follows.


Oscillator Oscillation stability requires that any variation in current or frequency will be

damped out, allowing the oscillator to return to its original state.


Oscillator


Oscillator


Oscillator


Oscillator


Oscillator


Oscillator


Oscillator


Oscillator


Oscillator


Oscillator


Oscillator


Oscillator


Oscillator


Oscillator


Oscillator

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Chapter5 Mixer and VCO