Transcript General Microwave Circuit Design

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
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 highfrequency 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 suspendedsubstrate 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 crosssectional 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
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 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 (Al O ). 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.
2
4
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
Distributed Circuit Elements
A stub is a length of straight transmission line that is short- or opencircuited 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.
o Shortcircuit stub: Zin = jZo tan(ßl)
o Opencircuit stub: Zin = jZo cot(ßl)
Distributed Circuit Elements
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 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.
Solid State Devices
Solid State Devices
Solid State Devices
Solid State Devices
Solid State Devices
Solid State Devices
Solid State Devices
Solid State Devices
Solid State Devices
A Study of Mixers
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A Study of Mixers
A Study of Mixers
A Study of Mixers
A Study of Mixers
A Study of Mixers