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.
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