529_microwave Circuit Design by Prashanth

8/7/2019 529_microwave Circuit Design by Prashanth

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ContentsContents

This discusses themajor 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


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ContentsContents

This discusses the variety

of options any engineer

has when he fabricates

the device of his choice.

It talks about theconventional quartz,

alumina and sapphire and

also on the latest

composites that are beingused.

Pl. Trans. Lines

Substrate Materials

Dist. Ckt. Elements

Solid State Dev.

Mixers

Others


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ContentsContents

This provides a birds 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


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ContentsContents

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


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Planar transmission LinesPlanar transmission Lines


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MICROSTRIP:

The great majority of planar circuits are realized in microstrip.

Microstrip is apractical medium for a wide variety of components

and is a natural choice for large, integrated systems. Microstrip, likemost 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 ofKirschning 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.


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CPW:

For many purposes CPW is a good alternative to microstrip. In CPW

the ground surfaces are alongside the strip conductorinstead of

underneath it. This configuration causes many characteristics todiffer 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 arelikely to be much rougher than the surfaces, losses are higher.


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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 inductancethan microstrip, an important consideration for many types ofhigh-

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)


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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 otherhavingonly 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.


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Stripline is a great medium fordirectional 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 forcomponents

that include chip diodes, transistors, or other discrete circuit

elements, and it does not integrate well with the media that do.


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One possibility is suspended-substrate stripline (SSSL). It has

many of theproperties of striplinebut 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. Theenclosure 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 frequencyfc 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 kis the dielectric

constant.


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Substrate MaterialsSubstrate Materials

Commonly used substrate materials are shown:


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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 is3.78, 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 makesgood 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.


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Alumina is 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 itdoes 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.


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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 lowerdielectric 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 linesindependent 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.


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Composite Materials:

Composite materials often are called "soft substrates," because they

are usually made from flexible plastics. The most common form ispoly-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.


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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.


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Distributed Circuit ElementsDistributed Circuit Elements


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A stub is 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

ofproblems 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.

o Short-circuit stub: Zin =jZo tan(l)

o Open-circuit stub: Zin =jZo cot(l)


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A radial stub is 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 andsimilar 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 usedprimarily at high frequencies, where they are relatively small.


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A radial stub commonly used in microstrip.


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Series Lines. The expressions are valid when mod() n/4, and under

these conditions tan(mod()) = mod(). 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, afilter designed for SOQ terminations requires Zo SOQ or Zo

SOQ. Series lines do not provide very good approximations ofshunt

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.


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SolidStateDevicesSolidStateDevices


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AStudy of MixersAStudy of Mixers

sss


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529_microwave Circuit Design by Prashanth