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Microwave Integrated Circuits (MIC) Microwave circuits exist in three different forms: Discrete circuit Packaged diodes/transistors mounted in coaxial and waveguide assemblies. Devices can usually be removed from the assembly and replaced Hybrid MIC Diodes/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 application 2. 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 mm), 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 circuit 8- 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 etchability 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 resistively Sheet sensitivities in the range of 10 to 2000 W/square 1% accuracy is achievable The creation of these resistive films demands additional processes of deposition and etching beyond those of the thinfilm 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 Slot line Coplanar Wave-Guide (CPW) Material er Tan d Ther. Cond. Tmax during Fab. W/inoC (Co) Teflon- 2.5 10×10-4 0.007 200 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 fiberglass 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 Coupled line filter Hybrid coupler Branch line coupler Microstrip 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 Dielectric resonator Chip capacitor and resistor 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 In waveguide form it contains a tapered absorber, usually consisting of a carbon-impregnated dielectric material that absorbs the microwave power Some Types of Terminations 8.2 – 12.4 GHz handles 75 watts GHz7 - 10 watt300 Important specifications: SWR (or S11) Power-handling capability Coaxial terminations 50 W N-type 50 W SMA 75 W BNC Strip Line Load 100 W High power 50 W) GHzDC- 3 Cwwat600 Type C 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. Coaxial Attenuators 3 dB 1 W DC- 2 GHz N-Type 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 30 dB 100 W DC- 21GHz N-Type QC Mechanical variable attenuator 8.2 – 12.4 GHz 0 - 20 dB 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 12.4 – 18 GHz 0 - 50 dB Electronically variable attenuator Achieved with PIN diodes Will be covered in active circuits Switches Direct 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 Directional couplers sample the power traveling in only one direction down a transmission line. Pc Pwrong Pi Po 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 Typical values are 3, 6, 10, 20, 30, 40, and 50 dB Directional coupler can also be used as an attenuator and to measure the reflected power from a mismatch lg/4 Coupled power Input power Output power Coupling Loss vs Coupling Factor Directional Coupler Signal Paths Wave guide coupler Coaxial and microstrip coupler High power Wide band High directivity Poor directivity limited in BW Limited power Waveguid coupler 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. Coaxial coupler Microstrip coupler 3-dB Quadrature Hybrid (Hybrid Coupler) (Input) 1 l/4 2 (output) l/4 (Isolated) 4 3 (output) The 3-dB quadrature hybrids are used as components, in almost every RF System, such as: Power combiners and dividers Balanced Mixers Balanced amplifiers De(modulators) Image rejection mixers Feed network in antenna arrays 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 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 The [S] matrix is -1 [S] = 2 0 j 1 0 j 0 0 1 1 0 0 j 0 1 j 0 Small size coupler (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. 3 1 lg/4 4 2 3lg/4 The [S] matrix is -j [S] = 2 0 1 1 0 1 0 0 -1 1 0 0 1 0 -1 1 0 Some Small size couplers configurations Combiners and Dividers 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 The T- Junction Power Dividers (simplest configuration) E-plane waveguide T Microstrip T-junction H-plane waveguide T Lossless junctions Can not be matched simultaneously at all ports No isolation between the two input lines Resistive Divider 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 Wlikinson Power divider 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. 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 Multi-channel Combiner Lossy Very little selectivity Small size Wide band Magic-T 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 4 used for matching. 3 2 1 The Lange coupler 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 narrow Bonding wires are needed 4 2 l/4 1 3 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 [S] = 0 0 1 0 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. 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 Isolators are not a broadband as attenuators 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 3 0 0 1 [S] = 1 0 0 0 1 0 2 1 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.