Microwave Integrated Circuits (MIC)

Microwave circuits exist in three different forms:

Discrete circuitPackaged diodes/transistors mounted in coaxial and waveguide assemblies. Devices can usually be removed from the assembly and replaced

Hybrid MICDiodes/transistors, resonators, capacitors, circulators, … are fabricated separately on most appropriate material and then mounted into the microstrip circuit and connected with bond wires

MMIC Diodes/transistors, resistors, capacitors, microstrip,…all fabricated simultaneously, including their interconnections, in semiconductor chip

Advantages and Disadvantages of HMIC

Advantages:

1- Each component can be designed for optimal performance:

Each transistor can be made of the best material.

Other devices can be made of the most appropriate material.

The lowest loss microwave components can be made by choosing the optimal microstrip substrate.

2- It has high power capability since the high power generating elements can be optimally heat-sinked

3- Standard diodes and transistors can be used and made to perform different functions by using different circuit design.

4- Special-purpose devices for each function are not required.

5- Trimming adjustments are possible

6- The most economical approach when small quantities, up to several hundred, of the circuits are required.

Disadvantages:

1- Wire bonds cause reliability problems. Each circuit element that is not part of the microstrip assembly must be attached to the microstrip by a wire bond.

2- The number of devices that can be included is limited by the economics of mounting the devices onto the circuit and attaching them by a wire bonds. The circuit is usually limited to a few dozen compartments.

Advantages and Disadvantages of MMICs

Advantages:1- Minimal mismatches and minimal signal delay

2- There are no wire bond reliability problems

3- Up to thousands of devices can be fabricated at one time into a single MMIC.

4- It is the least expensive approach when large quantities are to be fabricated.

Disadvantages:

1- Performance compromised, since the optimal materials cannot be used for each circuit element.

2- Power capability is lower because good heat transfer materials cannot be used

3- Trimming adjustments are difficult or impossible.

4- Unfavorable device-to-chip area ratio in the semiconductor material.

5- Tooling is prohibitively expensive for small quantities of MMIC.

Materials used for MIC

The basic materials for fabricating MICs, in general are divided into four categories:

1- Substrate materials sapphire, alumina, ferrite/garnet, silicon, RT/duroid, quartz, GaAs, Inp, etc.,

2- Conductor materials-copper, gold, silver, aluminum, etc.

3- Dielectric films SiO, SiO2,…etc

4- Resistive films- Nichrome (cNiCr), tantalum (Ta)

Substrate Materials:

1- The cost of the substrate must be justifiable for the application2. Is the technology to be thin- or thick film?3- The choice of thickness and permittivity determines the achievable

impedance range and the usable frequency range.4- There should be low loss tangent for negligible dielectric loss

5- The substrate surface finish should be good (~ 0.1 m), with relative freedom from voids, to keep conductor loss low and yet maintain good metal-film adhesion

6- There should be good mechanical strength and thermal conductivity.7- No deformation should be occur during processing of circuit8- A substrates with sufficient size are for the particular application

and complexity should be available

Conductor Materials:

High conductivity, low temperature coefficient of resistance, low RF resistance, good adhesion, good etch- ability and solder-ability, and be easy to deposit.

Dielectric Material:

Used as insulators for capacitors, protective layer for active devices, and insulating layer for passive circuits.

The desirable properties:

Reproducibility, high breakdown voltage, low loss tangent, and the ability to under go processing without developing pin holes

Resistive Films:Required for fabricating resistors for terminations, attenuators, and for bias networks.

The properties required for resistive material are: Good stability, low temperature coefficient of resistivelySheet sensitivities in the range of 10 to 2000 /square1% accuracy is achievable

The creation of these resistive films demands additional processes of deposition and etching beyond those of the thin-film metallization. This complexity may be obviated by bonding directly chip resistors onto the conducting pattern (ex. using surface mount).

Planar and Uniplanar Transmission lines

Microstrip TL Coplanar Wave-Guide (CPW)

Slot line

 

Material r Tan Ther. Cond. Tmax during Fab. W/inoC (Co)

Teflon- 2.5 10×10-4 0.007 200

fiberglass

Epsilam 10 10 15×10-4 0.01 150

Alumina 10 1×10-4 0.1 500

Beryllia 6 2×10-4 1 500

Ferrite 15 2×10-4 0.1 500

Silicon 12 30×10-4 0.4 400

GaAs 12 16×10-4 0.1 400

Microstrip Circuit elements commonly used in HMIC

The components that can be fabricated as part of the microstrip transmission line are:

Matching stubs and transformers

Directional couplers

Combiners and dividers

Resonators

Filters

Inductors and capacitors

Thin film resistors

Microstrip coupler

Coupled line filter

Hybrid coupler Branch line coupler

Typical spiral inductor and interdigitated capacitor

Loop inductor High impedance transmission line inductor

Figure: Microstrip elements used in HMIC

Components Added After Microstrip Fabrication

The MIC Components that are fabricated separately and added to the microstrip circuits are:

Bond wire

Chip resistor

Chip capacitors

Dielectric resonators

Circulators

Diodes and transistors

Bond wires

Chip capacitor and resistorDielectric resonator

Passive Microwave Components (PMC)

(The circuits that does not contain any active device such as diode or transistor)

PMC are used extensively in any microwave communication system

Passive microwave components include:

• Terminations & attenuators

• Switches

• Couplers

• Isolators & Circulators

• Combiners & Dividers

• Phase shifters

• Filters

Terminations

Absorb all the power at the end of transmission line in order to terminate a microwave equipment without allowing the power to escape into surroundings or to be reflected back into the equipment.

Termination can be found in the form of:

Waveguide,

Coaxial line

Microstrip

Some Types of Terminations

Important specifications:

SWR (or S11) Power-handling capability

In waveguide form it contains a tapered absorber, usually consisting of a carbon-impregnated dielectric material that absorbs the microwave power

GHz7 - 10

watt300

8.2 – 12.4 GHz handles 75 watts

Coaxial terminations

50 SMA

75 BNC

50 N-type

High power 50 W) GHzDC- 3 Type C Cwwat600

100

Strip Line Load

Attenuators

Used to adjust the power level of microwave signals.

Attenuators Types:

Fixed (Pads)

Mechanically adjustable

Electronically Controlled

Coaxial attenuators cover the frequency range from dc to 18 GHz, and they can have any value of attenuation. Typical values are 3, 6 10, and 20 dB.

Fixed coaxial attenuator

The lossy material extending from the center to the outer conductor and along the center conductor. This lossy material forms a resistive T, which absorbs some of the microwave power without reflecting any type

Coaxial Attenuators

3 dB 1 W DC- 2 GHz

N-Type

30 dB 100 W DC- 21GHz

N-Type QC

Mechanical variable attenuator

A van of absorbing material inserted into the waveguide through a slot on the broad wall. The greater the penetration of the vane the greater the attenuation. The dial can be calibrated in dB

8.2 – 12.4 GHz 0 - 20 dB

12.4 – 18 GHz 0 - 50 dB

Electronically variable attenuator

Achieved with PIN diodes

Will be covered in active circuits

SwitchesDirect s microwave power from one transmission line to another or turns microwave power on and off. Switches can be mechanically or electronically. Here we discuss some types of mechanical switchs. Electronically switches will be introduced in active devices section.

Directional Couplers

Important specifications: Coupling Factor (dB) C = 10 log Pi/Pc

How much of the input power is sampled Insertion Loss IL = 10 log Pi/Po

Specify the output power relative to the input power Directivity D = 10 log Pc/Pwrong

No coupler is perfect i.e Pwrong 0 Isolation I = 10 log Pi/Pwrong

= D + C dB The amount of power sampled in the wrong direction

Pi Po

PcPwrong

Directional couplers sample the power traveling in only one direction down a transmission line.

Typical values are 3, 6, 10, 20, 30, 40, and 50 dBDirectional coupler can also be used as an attenuator and to measure the reflected power from a mismatch

g/4

Input power Output

power

Coupled power

Coupling Loss vs Coupling Factor

Directional Coupler Signal Paths

D is not critical for sampling microwave power

D is extremely important for a return loss measurement, to measure the small power reflected from the mismatch. Microstrip coupler

Waveguid coupler

High power

High directivity

limited in BW

Wide band

Poor directivity

Limited power

Wave guide coupler Coaxial and microstrip coupler

Coaxial coupler

3-dB Quadrature Hybrid (Hybrid Coupler)

Power combiners and dividers De(modulators)

Balanced Mixers Image rejection mixers

Balanced amplifiers Feed network in antenna arrays

/4

2(Input)

(output)

(output)

(Isolated)

/4

3

1

4

The 3-dB quadrature hybrids are used as components, in almost every RF

System, such as:

With all ports matched, power entering port 1 is divided between ports 2 and 3, with 90o phase shift between these outputs. No power is coupled to port 4. Ports 1

and 4 as well as ports 2 and 3 are isolated .

The [S] matrix is 0 j 1 0j 0 0 11 0 0 j 0 1 j 0

-1

[S] =

2

Small size coupler

The most important parameters of the hybrid are

Isolation between isolated ports

SWR at the input ports

Phase difference between the two coupled ports

Insertion loss between the input and the coupled ports

(2) 180o Hybrid Ring:The 0o/180o hybrid coupler is preferred in some applications, namely,

Mixers Modulators Isolated power splitters since the isolation between

its input ports may be independent of the value of the two balanced impedance loads.

1

2

3

4g/4

g/4

The [S] matrix is 0 1 1 0

1 0 0 - 1

1 0 0 1

0 -1 1 0

-j

[S] =

2

Some Small size couplers configurations

Combiners and Dividers

Lossless junctions

Can not be matched simultaneously at all ports

No isolation between the two input lines

The T- Junction Power Dividers (simplest configuration)

H-plane waveguide T

E-plane waveguide T Microstrip T-junction

Combiners are used to combine two or more transmission lines into one transmission line. They can also be used to divide the microwave signal from one transmission line into two transmission lines

Matching T-junctions is possible if a lossy components is inserted in series to all branches at the junction

Dissipate half of the supplied power and the two output ports may not be isolated

Resistive Divider

Wlikinson Power divider

In-phase Wilkinson divider

Isolation is achieved between ports by terminating resistors. Any unequal mismatch or out-of-phase condition that would couple power from one line to the other is attenuated by the resistor.

Disadvantages:

The termination must be mounted inside the coupler, which limits its power handling capability

Wilkinson power divider, is a wide band circuit (2:1 or more), can be matched at all ports and lossless when the output ports are matched. It can also be designed to give arbitrary power division. This divider is often made in microstrip or stripline form .

Multi-channel Combiner

LossyVery little selectivitySmall sizeWide band

The 180o hybrid can also be implemented in waveguide form as shown in the Figure. The waveguide magic-T hybrid junction has terminal properties similar to those of the ring hybrid. In practice, tuning posts irises are often used for matching.

Magic-T

1

23

4

The Lange coupler

/4

1

4 2

3

Tight coupling 3 or 6 dB

Wide band (as high as 4:1)

It is a type of quadrature coupler (90o phase shift between 2 and 3)

Lines are very narrowBonding wires are needed

Phase Shifters

Microwave signals are characterized by amplitude and phase. The amplitude is controlled with amplifiers and attenuators. The phase can be controlled by phase shifters. Phase shifters like attenuators, can be mechanically or

electronically adjustable

Mechanically adjustable phase shifters

It is a line stretchers. The phase shift can be adjusted by changing the signal path .

Isolators and Circulators

An isolator allows microwaves to pass in one direction but not the other, so it has unidirectional transmission characteristics. This isolating effect is achieved with ferrites

Isolators are usually used to protect high power microwave sources from possible reflection that may cause source damage. It can also be used in place of matching networks.

[S = ] 0 0

1 0

The most important specifications for isolators are the isolation which is the insertion loss in the reverse direction and the forward insertion loss. The isolation should be high and the insertion loss should be low. Typical values are 20 dB for isolation and 0.5 dB for insertion loss.

•Purpose of Isolator

Low insertion loss in the normal or forward path

High isolation in the reverse path

•Uses

Circulators providing input and output isolation for a one port amplifier

Isolator minimizing the pulling effect of an oscillator

Isolator reducing the power reflected back to a mixer

Reduce VSWR interactions between RF components

Circulator Circulator route microwave signals from one port of the device to another. For

example, a microwave signal entering port 1 is directed out of the circulator at port 2. A signal entering port 2 is routed to leave the circulator at port 3 and does not get back into port 1. A signal entering port 3 does not get into port 2, but goes out through port 1. The S matrix of an ideal circulator is

[S] =

0 0 1

1 0 0

0 1 0 1

2

3

Isolators are not a broadband as attenuators

The important specifications of a circulator are the insertion loss, which is the loss of signal as it travels in the direction that it is supposed to go, and the directivity, which is the loss in the signal as it travel in the wrong direction. Insertion loss is typically 0.5 dB and the directivity is 20 dB. Circulator enable the use of one antenna for both transmitter and receiver of communication system.