Contents This discusses the major types of transmission lines involved in fabrication procedures across the world- microstrip, CPW, stripline and Suspended substrate Stripline(SSSL) • Pl. Trans. Lines • Substrate Materials • Dist. Ct. Elements • Solid State Dev. • Mixers • Others
Contents This discusses the variety of options any engineer has when he fabricates the device of his choice. It talks about the conventional quartz, alumina and sapphire and also on the latest composites that are being used. • Pl. Trans. Lines • Substrate Materials • Dist. Ckt. Elements • Solid State Dev. • Mixers • Others
Contents This provides a bird’s eye view of the the variety of distributed circuit elements one has at his disposal, the related uses, equations etc. • Pl. Trans. Lines • Substrate Materials • Dist. Ckt. Elements • Solid State Dev. • Mixers • Others
Contents This builds a grounding in the various solid state devices that are being used, gives their frequency ranges, uses, characteristics and also the visualization diagrams showing their cross sections. • Pl. Trans. Lines • Substrate Materials • Dist. Ckt. Elements • Solid State Dev. • Mixers • Others
Contents Discusses the common types of mixers available, their characteristics and the various applications they are being used for. It also gives diagrammatical representations of the same. • Pl. Trans. Lines • Substrate Materials • Dist. Ckt. Elements • Solid State Dev. • Mixers • Others
Planar transmission Lines MICROSTRIP: The great majority of planar circuits are realized in microstrip. Microstrip is a practical medium for a wide variety of components and is a natural choice for large, integrated systems. Microstrip, like most planar circuits, is a "quasi- TEM" transmission line. This means that it is usually treated as a TEM line at frequencies low enough for dispersion to be negligible. At higher frequencies, dispersion corrections are usually necessary. Again, a number of methods exist. One of the most popular and most accurate is that of Kirschning and Jansen. Another good one is by Wells and Pramanick. A simple approximate expression for the cutoff frequency of the lowest non- TEM mode is 75/h(k-1)^0.5. where this is got in GHz and h is in mm.
Planar transmission Lines CPW: For many purposes CPW is a good alternative to microstrip. In CPW the ground surfaces are alongside the strip conductor instead of underneath it. This configuration causes many characteristics to differ from those of microstrip. First, the fields are not as fully contained in the dielectric and extend farther into the air above the substrate. This causes dispersion and radiation to be worse in CPW than in microstrip. Second, the currents are more strongly concentrated in the edges of the conductors. Because the edges are likely to be much rougher than the surfaces, losses are higher.
Planar transmission Lines Nevertheless, CPW has significant advantages over microstrip for monolithic circuits. The most important is that ground connections can be made on the surface of the substrate; there is no need for "via" holes, which are used to make ground connections in microstrip circuits. CPW grounds usually have much less inductance than microstrip, an important consideration for many types of high-frequency circuits. Another important advantage is size. CPW conductors can be very narrow, even with low characteristic impedances. Low-impedance microstrip lines often are impractically wide. Finally, CPW is much less sensitive to substrate thickness than microstrip, so the thinning of the monolithic substrate is much less critical. CPW monolithic circuits often are not thinned at all)
Planar transmission Lines STRIPLINE: Strip line is one of the oldest types of planar transmission media, developed in the late 1950s and originally called triplate. Of the lines listed in Table 1.1,stripline is the only true TEM transmission line. As such, it is non-dispersive, but it is not immune to moding, especially if the strip conductor is not centered evenly between the ground planes. Strip line components invariably use composite substrates. One technique is to create a sandwich of two substrates, one having a ground plane and a strip conductor, the other having only the ground plane. These two substrates are clamped firmly together to prevent the formation of an air gap, which would create variations in the dielectric constant of the medium between the ground planes.
Planar transmission Lines Stripline is a great medium for directional couplers. This is virtually impossible in microstrip or CPW, which can use only edge coupling. The homogeneous dielectric of stripline makes its even-mode and odd-mode phase velocities equal, resulting in high directivity. Broadside coupling is also possible in suspended-substrate stripline. Stripline is not a favored transmission medium these days, probably because it is not really suitable for components that include chip diodes, transistors, or other discrete circuit elements, and it does not integrate well with the media that do.
Planar transmission Lines One possibility is suspended-substrate stripline (SSSL). It has many of the properties of stripline but can be realized with either a hard or a soft substrate. The non homogeneous dielectric gives SSSL a very low effective dielectric constant, close to LO, and slightly lower loss than stripline. It is, however, slightly dispersive. The enclosure also is subject to waveguide-like modes, so its cross-sectional dimensions must be kept comfortably less than one-half wavelength in both width and height. An approximate expression for the lowest cutoff frequency fc of such modes, in GHz, is 150/a*(1-(h*(k-1)/bk)^0.5 where a and b are the width and the height of the channel in millimeters, h is the substrate thickness, and k is the dielectric constant.
Substrate Materials Commonly used substrate materials are shown:
Substrate Materials Silica Loosely called quartz, its single-crystal form, fused silica has a number of very good and very bad properties. It is one of the few high-quality materials that have a low dielectric constant. Its dielectric constant is 3.78, much lower than other hard substrates but not as low as the composite materials. This low dielectric constant, combined with low loss and good smoothness, makes fused silica seemingly ideal for millimeter-wave circuits. Unfortunately, fused silica is also very brittle, making it difficult to handle and to fabricate, and its smoothness makes good metal adhesion difficult to obtain. Fused silica has a low thermal expansion coefficient; it is matched only to Invar or Kovar, metal alloys that are expensive and difficult to machine. Metallizations consist of a very thin sputtered adhesion layer with a top layer of plated gold.
Substrate Materials Aluminais the ceramic form of sapphire (see below). It is a moderately expensive substrate but still the least expensive of the "hard" substrates. It is very hard, temperature-stable, and has good thermal conductivity. Although its thermal expansion coefficient is not well matched to brass or aluminum, alumina is so strong that it does not crack easily when bonded to a thermally mismatched surface, even at extreme temperatures. Alumina can be polished to high smoothness, if necessary, and metal adhesion is good. Although hard, alumina can be cut easily with a diamond substrate saw or a laser; holes can be made with a laser or a carbide tool. Alumina has a high dielectric constant, usually 9.5 to 10.0The most common metallization is gold. A very thin adhesion layer is used between the gold and the substrate.
Substrate Materials Sapphire Sapphire is the crystalline form of aluminum oxide (Al2O4). It is relatively expensive. Its only advantage over alumina is its extreme smoothness, which minimizes conductor loss, and slightly lower dielectric loss. Sapphire is electrically anisotropic: its dielectric constant depends on the direction of the electric field in the material. It is 8.6 in a plane and 10.55 in the direction parallel to that plane. Sapphire usually is cut so that the k = 8.6 plane is parallel to the ground plane. This makes the characteristics of microstrip lines independent of their orientation, but it causes the difference between even- and odd-mode phase velocities in coupled lines to be Worse than in an isotropic material. The metallization is invariably gold with an adhesion layer.
Substrate Materials Composite Materials: Composite materials often are called "soft substrates," because they are usually made from flexible plastics. The most common form is poly-tetra-fluoro-ethylene (better known by its trade name, Teflon), loaded with glass fibers or ceramic powder. This is both an advantage and disadvantage; the soft material is easy to handle and inexpensive to fabricate, but the mechanical and thermal properties are not as' good as those of "hard" substrates. The thermal conductivity may be very low.
Substrate Materials • The following are some concerns: • Tolerance of the dielectric constant • Variation of the dielectric constant and loss tangent with frequency and temperature • Electrical anisotropy • Thermal expansion coefficient and Moisture absorption • Volume and surface resistivity.
Distributed Circuit Elements • A stubis a length of straight transmission line that is short- or open-circuited at one end and connected to a circuit at the opposite end. Stubs can approximate inductors, capacitors, or resonators. High- or low-impedance series lines also approximate series inductors or shunt capacitors, respectively • . Stubs are used almost exclusively as shunt elements. Although they could, in theory, be used to realize series elements, there are a couple of problems in doing so. First, the stub would have to be realized by a parallel-coupled line. The even mode on such a line would introduce shunt capacitance, so the stub would not be a series element. Second, such structures often are difficult to realize both mechanically and electrically. Usually they just don't work. • Shortcircuit stub: Zin = jZo tan(ßl) • Opencircuit stub: Zin = jZo cot(ßl)
Distributed Circuit Elements Aradial stubis an open-circuit stub realized in radial transmission line instead of straight transmission line. It is a very useful element, primarily for providing a clean (no spurious resonances) broadband short circuit, much broader than a simple open-circuit stub. It is especially useful on bias lines in high-frequency amplifiers and similar components. Radial stubs are used almost exclusively in microstrip circuits; they could be used in stripline as well. Although radial stubs are shorter than uniform stubs, they cannot be folded or bent; therefore they take up a lot of substrate area. For this reason radial stubs are used primarily at high frequencies, where they are relatively small.
Distributed Circuit Elements A radial stub commonly used in microstrip.
Distributed Circuit Elements Series Lines. The expressions are valid when mod(ß) « n/4, and under these conditions tan(mod(ß)) = mod(ß). We should also quantify what we mean by high and low impedances: we mean that they are high or low compared to the impedances locally in the circuit. For example, a filter designed for SOQ terminations requires Zo » SOQ or Zo « SOQ. Series lines do not provide very good approximations of shunt capacitors or series inductors unless the capacitance or inductance is fairly low. Even then, the discontinuities introduced by cascading low- and high-impedance sections, as would exist in a low-pass filter, for example, can be difficult to characterize accurately.