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Chapter 3

Chapter 3. Transmission Lines. Contents. Features of Transmission Lines Low Frequency Characters of Microstrip Line High Frequency Characters of Microstrip Line Discontinuities of Microstrip Line. Features of Transmission Lines. Microwave Integrated Circuit (MIC).

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Chapter 3

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  1. Chapter 3 Transmission Lines

  2. Contents Features of Transmission Lines Low Frequency Characters of Microstrip Line High Frequency Characters of Microstrip Line Discontinuities of Microstrip Line

  3. Features of Transmission Lines

  4. Microwave Integrated Circuit (MIC) • The current trend of circuit design is toward miniaturization and integration. • An MIC consists of an assembly that combines different circuit functions that are connected by transmission lines. • The advantages of MIC compare to traditional circuit using printed circuit • Higher reliability • Reproducibility • Better performance • Higher Integrated • Smaller size • Two classes of MIC • HMIC • MMIC • Planar configuration • Easy fabrication • Lower cost • Lighter weight

  5. Hybrid Microwave Integrated Circuit (HMIC)

  6. Photograph of one of the 25,344 hybrid integrated T/R modules used in Raytheon’s Ground Based Radar system. This X-band module contains phase shifters, amplifiers, switches, couplers, a ferrite circulator, and associated control and bias circuitry.

  7. Monolithic Microwave Integrated Circuit (MMIC)

  8. Photograph of a monolithic integrated X-band power amplifier. This circuit uses eight heterojunction bipolar transistors with power dividers/combiners at the input and output to produce 5 watts.

  9. Material selection is an important consideration for any type of MIC; characteristics such as electrical conductivity, dielectric constant, loss tangent, thermal transfer, mechanical strength, and manufacturing compatability must be evaluated. • Features of HMICs: • Alumina, quartz, and Teflon fiber are commonly used for substrates. • During HMICs testing, tuning or trimming for each circuit is allowed to adjust components values. • Features of MMICs: • The substrate of an MMIC must be a semiconductor material to accommodate the fabrication of active devices. Hence GaAs is the most common substrate. Besides, Si, sapphire, and InP are also used. • All passive and active components are grown or implanted in the substrate. A single wafer can contain a large number of circuits. • Circuit trimming after fabrication will be difficult, even impossible.

  10. Conventional coaxial lines and waveguides are remain useful in : • High power transmission (e.g. KW~MW transmitters) • High Q component needed (e.g. low loss filter) • Some millimetric–wavelength systems (e.g. MW automotive radar) • Very low loss transmission systems • Precision instrumentation equipment • Planar technology are already tending to overcome problems in areas (2) and (3), but not (1) or (4).

  11. Transmission Line and Waveguide Structures

  12. Transmission Line and Waveguide Comparisons

  13. Planar Transmission Line Structures

  14. Modifications of Planar Transmission Line Structures

  15. Image Line • Behavior likes a dielectric slab waveguide (thick strip) for use at operation frequency into hundreds GHz. • Several thousand unloaded Q-factor. But fop  Q. • Poor compatibility with active devices, mutual coupling, and radiation from discontinuities and bends. • Microstrip • The most popular MIC TL with a very simple geometric planar structure. • Advantage: Zero cutoff frequency , light weight, small size, low cost, easy fabrication and integration, low dispersion , and broadband operation (frequency range from a few GHZ, or even lower, up to at least many tens of GHz). • At millimetre-wave range, problems such as loss, higher-order modes, and fabrication tolerances become exceedingly difficult to meet using HMICs.

  16. Finline (E-plane circuit) • Advantage: 1) Low loss (typically a factor about three better than microstrip. 2) Simpler fabrication in comparison with inverted and trapped-inverted microstrip. 3) Operation frequency up to 100GHz. • Disadvantage in biasing problem. • Application in compatibility with solid-state device is fairly good, especially in the case of beam-lead devices, 10% bandwidth of band pass filters, quadrature hybrids, waveguide transitions, and balanced mixer circuits. • Inverted Microstrip (IM) • Advantages in comparison with microstrip : 1) Wider line width for the same Z0, and this both reduces conductor dissipation and relaxes fabrication tolerances.

  17. 2) Structure utilizing air between the strip and ground plane gives higher Q, wavelength, operation frequency, and avoids interference. • Slotline • Guide mode of architecture makes it particularly suitable for applications where substrate is ferrite (components such as circulators and isolators). • Disadvantages : 1) Z0 below 60 are difficult to realize. 2) Q factor is significantly lower than other structures considered here. 3) Circuit structures often involve difficult registration problems ( especially with metallization on the opposite side to the slot). • Trapped Inverted Microstrip (TIM) • Advantages is similar to that of IM; moreover, a ‘slot’ or ‘channel’-shaped ground plane provides inherent suppression of some higher-order modes • Manufacturing difficulties are particularly significant with HMICs.

  18. Coplanar Waveguide (CPW) • Advantages in comparison with microstrip : 1) Easier grounding of surface-mounted ( or BGA mounted) component. • 2) Lower fabrication costs. • 3) Reduced dispersion and radiation losses. • 4) Photolithographically defined structures with relatively low • dependence on substrate thickness. • The major problem is non-unique Z0 because infinite range of ratio between centre strip width and gap width (In micrpstrip, Z0 is unique decided by strip width, substrate height, and substrate permittivity). • Coplanar Strip (CPS) and Differential Line • CPS : one of the conductors is ground; Differential line : neither of the conductors is grounded. • Advantage of differential line: 1) It is suitable for RFICs and high-speed digital ICs (but not for HMIC due to radiation losses and most passive components are single-ended). 2) This line is popular for use in long bus lines and clock distribution nets on chip as the signal return path.

  19. The differential line has a virtual ground itself, which means that a real metallic ground is not necessary. • Stripline • Completely filled microstrip, i.e. a symmetrical structure results in TEM transmission • Advantages : 1) lower loss. 2) Fairly high Q-factor. 3) Waveguide modes can easily to exited at higher frequencies. • Disadvantages: 1) Insufficient space for the incorporation of semiconductor devices. 2) Mode suppression gives rise to design problem. 3) Not compatible with shunt-mounted devices.

  20. Summary of TL Properties • Z0 and Q-factor are criterion for circuit applications.

  21. Substrate Choice for HMIC • Many factors, mechanical, thermal, electronics, and economic, leading to the correct choice of substrate deeply influence MIC design. • The kinds of questions include: 1) Cost 2) Thin-film or thick-film technology 3) Frequency range 4) Surface roughness (this will influence conductor losses and metal-film adhesion) 5) Mechanical strength, flexibility, and thermal conductivity 6) Sufficient surface area

  22. Commonly used substrate materials • Organic PCBs (Printed Circuit Boards) • FR4 1) Low cost, rigid structure, and multi-layer capability. 2) Applications for operation frequency below a few GHz. fop  Loss  • RT/Duroid 1) Low loss and good for RF applications. 2) Board has a wide selected range for permittivity. e.g. RT/Duroid 5870 with r =2.33, RT/Duroid 5880 with r=2.2, and RT/Duroid 6010 with r=10.2. 3) Board is soft leading to less precise dimensional control. • Softboard 1) Plastic substrate with good flexibility. 2) This board is suitable for experimental circuits operating below a few GHz and array antennas operating up to and beyond 20 GHz.

  23. Ceramic Substrate (Alumina) 1) Good for operation frequency up to 40 GHz. 2) Metallic patterns can be implemented on ceramic substrate using thin-film or thick-film technology. 3) Passive components of extremely small volume can be implemented because the ceramic substrate can be stacked in many tens of layers or more, e.g. low temperature co-fired ceramic (LTCC). 4) Good thermal conductivity. 5) Alumina purity below 85% should result in high conductor and dielectric losses and poor reproducibility. • Quartz 1) Production circuits for millimetric wave applications from tens of GHz up to perhaps 300 GHz, and suitable for use in finline and image line MIC structures. 2) Lower permittivity of property allows larger distributed circuit elements to be incorporated.

  24. Sapphire • The most expensive substrate with following advantages: 1) Transparent feature is useful for accurately registering chip devices. 2) Fairly high permittivity (r=10.1~10.3), reproducible ( all pieces are essentially identical in dielectric properties), and thermal conductivity (about 30% higher than the best alumina). 3) Low power loss. • Disadvantages: 1) Relatively high cost. 2) Substrate area is limited (usually little more than 25 mm square). 3) Dielectric anisotropy poses some additional circuit design problems.

  25. Properties of Some Typical Substrate Materials

  26. MIC Manufacturing Technology • Thin-Film Module • Circuit is accomplished by a plate-through technique or an etch-back technique. • Thick-Film Module 1) Thick-film patterns are printed and fired on the ceramic substrate. 2) Printed circuit technique is used to etch the desired pattern in a plastic substrate. • Medium-Film Module • Above technologies are suitable for HMIC productions. • Monolithic Technology • This technology is suitable for MMIC productions.

  27. Properties of Various Manufacturing Technology

  28. Multi-Chip Modules (MCM) • MCM provides small, high precision interconnects among multiple ICs to form a cost-effectively single module or package. • Four dominant types of MCM technologies: 1) MCM-L having a laminated PCB-like structure. 2) MCM-C based on co-fired ceramic structures similar to thick-film modules. 3) MCM-D using deposited metals and dielectrics in a process very similar to that used in semiconductor processing. 4) MCM-C/D having deposited layers on the MCM-C base • Advantages of an MCM over a PCB are : 1) Higher interconnect density. 2) Finer geometries enables direct chip connect. 3) Finer interconnect geometries enables chips placed closer together and it results in shorter interconnect lengths.

  29. Comparison of MCM Technologies

  30. Low Frequency Characters of Microstrip Line

  31. Microstrip Line • Microstrip line is the most popular type of planar transmission lines, primarily because it can be fabricated by photolithographic processes and is easily integrated with other passive and active RF devices. • When line length is an appreciable fraction of a wavelength (say 1/20th or more), the electric requirements is often to realize a structure that provides maximum signal, or power, transfer. • Example of a transistor amplifier input network • Microstrip components • Transmission line • Discontinuities • Step • Mitered bend • Bondwire • Via ground

  32. The most important dimensional parameters are the microstrip width w, height h (equal to the thickness of substrate), and the relative permittivity of substrate r. • Useful feature of microstrip : • DC as well as AC signals may be transmitted. • Active devices and diodes may readily be incorporated. • In-circuit characterization of devices is straightforward to implement. • Line wavelength is reduced considerably (typically 1/3) from its free space value, because of the substrate fields. Hence, distributed component dimensions are relatively small. • The structure is quite rugged and can withstand moderately high voltages and power levels. • Although microstrip has not a uniform dielectric filling, energe transmission is quite closely resembles TEM; it’s usually referred to as ‘quasi-TEM’.

  33. Electromagnetic Analysis Using Quasi-Static Approach (Quasi-TEM Mode) • The statically derived results are quite accurate where frequency is below a few GHz. • The static results can still be used in conjunction with frequency-dependent functions in closed formula when frequency at higher frequency. • Characteristic Impedance Z0 For air-filled microstrip lines, For low-loss microstrip lines, We can derive

  34. w er h w er h Procedure for calculating the distributed capacitance: • Effective Dielectric Constant e For very wide lines, w / h >> 1 For very narrow lines, w / h << 1

  35. We can express eeff as where filling factor qrepresents the ratio of the EM fields inside the substrate region, and its value is between ½ and 1. Another approximate formula for q is (provided by K.C. Gupta, et. al.) • Planar Waveguide Model (Parallel-Plate Model)

  36. Conductor Lossac • In most microstrip designs with high r, conductor losses in the strip and ground plane dominate over dielectric and radiation losses. • It’s a factors related to the metallic material composing the ground plane and walls, among which are conductivity, skin effect, and surface roughness. • Relationships: • Dielectric Lossad • To minimize dielectric losses, high-quality low-loss dielectric substrate like alumina, quartz, and sapphire are typically used in HMICs. • In MMICs, Si or GaAs substrates result in much larger dielectric losses (approximately 0.04 dB/mm).

  37. Radiation Loss ar • Radiation loss is major problem for open microstrip lines with low . Lower  (5) is used when cost reduction is a priority, but it lead to radiation loss increased. • The use of top cover and side walls can reduce radiation losses. Higher substrate can also reduce the radiation losses, and has a benefit in that the package size decreases by approximately the square root of . This benefit is an advantage at low frequency, but may be a problem at higher frequencies due to tolerances.

  38. Formulations of Attenuation Constant a However, the dielectric loss should occur in the substrate region only, not the whole region. Therefore, ad should be modified as

  39. How to evaluate attenuation constant  • Method 1 : in Chapter 2.14 ;  is calculated from RLCG values of material. • Method 2: Perturbation method where Plis power loss per unit length of line, P0is the power on line at z=0 plane. • Method 3: is calculated from material parameters. where ac is attenuation due to conductor loss ad is attenuation due to dielectric loss ar is attenuation due to radiation loss • Combined Loss Effect : linearly combined quality factors (Q)

  40. Recommendations • Use a specific dimension ratio to achieve the desired characteristic impedance. Following that, the strip width should be minimized to decrease the overall dimension, as well as to suppress higher-order modes. However, a smaller strip width leads to higher losses. • Power-handling capability in microstrip line is relatively low. To increase peak power, the thickness of the substrate should be maximized, and the edges of strip should be rounded ( EM fields concentrate at the sharp edges of the strip). • The positive effects of decreasing substrate thickness are : • Compact circuit • Ease of integration • Less tendency to launch higher-order modes or radiation • The via holes drilled through dielectric substrate contributing smaller parasitic inductances However, thin substrate while maintaining a constant Z0 must narrow the conductor width w, and it consequently lead to higher conductor losses, lower Q-factor and the problem of fabrication tolerances.

  41. Using higher  substrate can decrease microstrip circuit dimensions, but increase losses due to higher loss tangent. Besides, narrowing conductor line have higher ohmic losses. Therefore, it is a conflict between the requirements of small dimensions and low loss. For many applications, lower dielectric constant is preferred since losses are reduced, conductor geometries are larger ( more producible), and the cutoff frequency of the circuit increases. For microwave device applications, microstrip generally offers the smallest sizes and the easiest fabrication, but not offer the highest electrical performance.

  42. Design a microstrip line by the method of • “Approximate Graphically-Based Synthesis”

  43. Example1: Design a 50 microstrip line on a FR4 substrate( r=4.5). Solution Assume eff= r=4.5 From Zo1 curve  w/h=1.5 From q-curve  q=0.66 eff= 1+q(r+1)=1+0.66(4.5-1)=3.31 • 2nd iteration From Zo1 curve  w/h=1.7 From q-curve  q=0.68 eff= 1+q(r+1)=1+0.68(4.5-1)=3.38 • 3rd iteration • Stable result w/h=1.88; eff=3.39

  44. Formulas for Quasi-TEM Design Calculations • Analysis procedure: Give w / h to find eeff and Z0. (provided by I.J. Bahl, et. al.) • Synthesis procedure: Give Z0 to find w / h.

  45. Example2: Calculate the width and length of a microstrip line for a 50  Characteristic impedance and a 90° phase shift at 2.5 GHz. The substrate thickness is h=0.127 cm, with eff=2.20. Solution Guess w/h>2 Matched with guess Then w=3.081h=0.391 (cm) The line length, l, for a 90° phase shift is found as

  46. Microstrip on an Dielectrically Anisotropic Substrate Empirical formula

  47. Curve  : i=10.6 ; Curve : used reqformula

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