HOME

TheInfoList



OR:

Microstrip is a type of electrical
transmission line In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmis ...
which can be fabricated with any technology where a conductor is separated from a
ground plane In electrical engineering, a ground plane is an electrically conductive surface, usually connected to electrical ground. The term has two different meanings in separate areas of electrical engineering. *In antenna theory, a ground plane is a ...
by a
dielectric In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the mate ...
layer known as the substrate. Microstrip lines are used to convey
microwave Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ran ...
-frequency signals. Typical realisation technologies are
printed circuit board A printed circuit board (PCB; also printed wiring board or PWB) is a medium used in Electrical engineering, electrical and electronic engineering to connect electronic components to one another in a controlled manner. It takes the form of a L ...
, alumina coated with a dielectric layer or sometimes silicon or some other similar technologies. Microwave components such as
antennas In radio engineering, an antenna or aerial is the interface between radio waves propagating through space and electric currents moving in metal conductors, used with a transmitter or receiver. In transmission, a radio transmitter supplies a ...
, couplers,
filter Filter, filtering or filters may refer to: Science and technology Computing * Filter (higher-order function), in functional programming * Filter (software), a computer program to process a data stream * Filter (video), a software component tha ...
s, power dividers etc. can be formed from microstrip, with the entire device existing as the pattern of metallization on the substrate. Microstrip is thus much less expensive than traditional
waveguide A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting the transmission of energy to one direction. Without the physical constraint of a waveguide, wave intensities de ...
technology, as well as being far lighter and more compact. Microstrip was developed by ITT laboratories as a competitor to
stripline Stripline is a transverse electromagnetic (TEM) transmission line medium invented by Robert M. Barrett of the Air Force Cambridge Research Centre in the 1950s. Stripline is the earliest form of planar transmission line. Description A strip ...
(first published by Grieg and Engelmann in the December 1952 IRE proceedings). The disadvantages of microstrip compared with waveguide are the generally lower power handling capacity, and higher losses. Also, unlike waveguide, microstrip is typically not enclosed, and is therefore susceptible to cross-talk and unintentional radiation. For lowest cost, microstrip devices may be built on an ordinary
FR-4 FR-4 (or FR4) is a NEMA grade designation for glass-reinforced epoxy laminate material. FR-4 is a composite material composed of woven fiberglass cloth with an epoxy resin binder that is flame resistant (''self-extinguishing''). "FR" stands fo ...
(standard PCB) substrate. However it is often found that the dielectric losses in FR4 are too high at microwave frequencies, and that the
dielectric constant The relative permittivity (in older texts, dielectric constant) is the permittivity of a material expressed as a ratio with the electric permittivity of a vacuum. A dielectric is an insulating material, and the dielectric constant of an insulat ...
is not sufficiently tightly controlled. For these reasons, an alumina substrate is commonly used. From monolithic integration perspective microtrips with integrated circuit/
monolithic microwave integrated circuit Monolithic microwave integrated circuit, or MMIC (sometimes pronounced "mimic"), is a type of integrated circuit (IC) device that operates at microwave frequencies (300 MHz to 300 GHz). These devices typically perform functions such as ...
technologies might be feasible however their performance might be limited by the dielectric layer(s) and conductor thickness available. Microstrip lines are also used in high-speed digital PCB designs, where signals need to be routed from one part of the assembly to another with minimal distortion, and avoiding high cross-talk and radiation. Microstrip is one of many forms of
planar transmission line Planar transmission lines are transmission lines with conductors, or in some cases dielectric (insulating) strips, that are flat, ribbon-shaped lines. They are used to interconnect components on printed circuits and integrated circuits working ...
, others include
stripline Stripline is a transverse electromagnetic (TEM) transmission line medium invented by Robert M. Barrett of the Air Force Cambridge Research Centre in the 1950s. Stripline is the earliest form of planar transmission line. Description A strip ...
and
coplanar waveguide Coplanar waveguide is a type of electrical planar transmission line which can be fabricated using printed circuit board technology, and is used to convey microwave-frequency signals. On a smaller scale, coplanar waveguide transmission lines are ...
, and it is possible to integrate all of these on the same substrate. A differential microstrip—a balanced signal pair of microstrip lines—is often used for high-speed signals such as
DDR2 SDRAM Double Data Rate 2 Synchronous Dynamic Random-Access Memory (DDR2 SDRAM) is a double data rate (DDR) synchronous dynamic random-access memory (SDRAM) interface. It superseded the original DDR SDRAM specification, and was itself superseded by DDR3 ...
clocks,
USB Hi-Speed Universal Serial Bus (USB) is an industry standard that establishes specifications for cables, connectors and protocols for connection, communication and power supply (interfacing) between computers, peripherals and other computers. A broad ...
data lines,
PCI Express PCI Express (Peripheral Component Interconnect Express), officially abbreviated as PCIe or PCI-e, is a high-speed serial computer expansion bus standard, designed to replace the older PCI, PCI-X and AGP bus standards. It is the common ...
data lines,
LVDS Low-voltage differential signaling (LVDS), also known as TIA/EIA-644, is a technical standard that specifies electrical characteristics of a differential, serial signaling standard. LVDS operates at low power and can run at very high speeds ...
data lines, etc., often all on the same PCB. Most PCB design tools support such differential pairs.


Inhomogeneity

The electromagnetic wave carried by a microstrip line exists partly in the
dielectric In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the mate ...
substrate, and partly in the air above it. In general, the
dielectric constant The relative permittivity (in older texts, dielectric constant) is the permittivity of a material expressed as a ratio with the electric permittivity of a vacuum. A dielectric is an insulating material, and the dielectric constant of an insulat ...
of the substrate will be different (and greater) than that of the air, so that the wave is travelling in an inhomogeneous medium. In consequence, the propagation velocity is somewhere between the speed of radio waves in the substrate, and the speed of radio waves in air. This behaviour is commonly described by stating the effective dielectric constant (or effective relative permittivity) of the microstrip; this being the dielectric constant of an equivalent homogeneous medium (i.e., one resulting in the same propagation velocity). Further consequences of an inhomogeneous medium include: * The line will not support a true
TEM Tem or TEM may refer to: Acronyms * Threat and error management, an aviation safety management model. * Telecom Expense Management * Telecom Equipment Manufacturer * TEM (currency), local to Volos, Greece * TEM (nuclear propulsion), a Russian ...
wave; at non-zero frequencies, both the E and H fields will have longitudinal components (a
hybrid mode A hybrid electric vehicle (HEV) is a type of hybrid vehicle that combines a conventional internal combustion engine (ICE) system with an electric propulsion system (hybrid vehicle drivetrain). The presence of the electric powertrain is intended ...
). The longitudinal components are small however, and so the dominant mode is referred to as quasi-TEM. * The line is dispersive. With increasing frequency, the effective dielectric constant gradually climbs towards that of the substrate, so that the
phase velocity The phase velocity of a wave is the rate at which the wave propagates in any medium. This is the velocity at which the phase of any one frequency component of the wave travels. For such a component, any given phase of the wave (for example, ...
gradually decreases. This is true even with a non-dispersive substrate material (the substrate dielectric constant will usually fall with increasing frequency). * The
characteristic impedance The characteristic impedance or surge impedance (usually written Z0) of a uniform transmission line is the ratio of the amplitudes of voltage and current of a single wave propagating along the line; that is, a wave travelling in one direction in ...
of the line changes slightly with frequency (again, even with a non-dispersive substrate material). The characteristic impedance of non-TEM modes is not uniquely defined, and depending on the precise definition used, the impedance of microstrip either rises, falls, or falls then rises with increasing frequency. The low-frequency limit of the characteristic impedance is referred to as the quasi-static characteristic impedance, and is the same for all definitions of characteristic impedance. * The
wave impedance The wave impedance of an electromagnetic wave is the ratio of the transverse components of the electric and magnetic fields (the transverse components being those at right angles to the direction of propagation). For a transverse-electric-magnetic ...
varies over the cross-section of the line. * Microstrip lines radiate and discontinuity elements such as stubs and posts, which would be pure reactances in stripline, have a small resistive component due to the radiation from them.


Characteristic impedance

A closed-form approximate expression for the quasi-static
characteristic impedance The characteristic impedance or surge impedance (usually written Z0) of a uniform transmission line is the ratio of the amplitudes of voltage and current of a single wave propagating along the line; that is, a wave travelling in one direction in ...
of a microstrip line was developed by
Wheeler Wheeler may refer to: Places United States * Wheeler, Alabama, an unincorporated community * Wheeler, Arkansas, an unincorporated community * Wheeler, California, an unincorporated community * Wheeler, Illinois, a village * Wheeler, Indiana, a ...
: :Z_\textrm = \frac \mathrm\left( 1 + \frac \left( \frac \frac + \sqrt\right)\right), where is the ''effective width'', which is the actual width of the strip, plus a correction to account for the non-zero thickness of the metallization: :w_\textrm = w + t \frac \ln\left( \frac\right). Here is the
impedance of free space The impedance of free space, , is a physical constant relating the magnitudes of the electric and magnetic fields of electromagnetic radiation travelling through free space. That is, , where is the electric field strength and is the magnetic fiel ...
, is the
relative permittivity The relative permittivity (in older texts, dielectric constant) is the permittivity of a material expressed as a ratio with the electric permittivity of a vacuum. A dielectric is an insulating material, and the dielectric constant of an insulat ...
of substrate, is the width of the strip, is the thickness ("height") of substrate, and is the thickness of the strip metallization. This formula is asymptotic to an exact solution in three different cases: #, any (parallel plate transmission line), #, (wire above a ground-plane), and #, . It is claimed that for most other cases, the error in impedance is less than 1%, and is always less than 2%. By covering all aspect-ratios in one formula, Wheeler 1977 improves on Wheeler 1965 which gives one formula for and another for (thus introducing a discontinuity in the result at ). Curiously, Harold Wheeler disliked both the terms 'microstrip' and 'characteristic impedance', and avoided using them in his papers. A number of other approximate formulae for the characteristic impedance have been advanced by other authors. However, most of these are applicable to only a limited range of aspect-ratios, or else cover the entire range piecewise. In particular, the set of equations proposed by Hammerstad, who modifies on Wheeler, are perhaps the most often cited: : Z_\textrm = \begin \dfrac \ln\left( 8 \dfrac + \dfrac \right), & \text\dfrac \leq 1 \\ \dfrac, & \text\dfrac \geq 1 \end where is the effective dielectric constant, approximated as: : \varepsilon_\textrm = \frac + \frac \left( \frac \right).


Bends

In order to build a complete circuit in microstrip, it is often necessary for the path of a strip to turn through a large angle. An abrupt 90° bend in a microstrip will cause a significant portion of the signal on the strip to be reflected back towards its source, with only part of the signal transmitted on around the bend. One means of effecting a low-reflection bend, is to curve the path of the strip in an arc of radius at least 3 times the strip-width. However, a far more common technique, and one which consumes a smaller area of substrate, is to use a mitred bend. To a first approximation, an abrupt un-mitred bend behaves as a shunt capacitance placed between the ground plane and the bend in the strip. Mitring the bend reduces the area of metallization, and so removes the excess capacitance. The percentage mitre is the cut-away fraction of the diagonal between the inner and outer corners of the un-mitred bend. The optimum mitre for a wide range of microstrip geometries has been determined experimentally by Douville and James. They find that a good fit for the optimum percentage mitre is given by :M = 100 \frac \% = (52 + 65 e^) \% subject to and with the substrate dielectric constant . This formula is entirely independent of . The actual range of parameters for which Douville and James present evidence is and . They report a
VSWR In radio engineering and telecommunications, standing wave ratio (SWR) is a measure of impedance matching of loads to the characteristic impedance of a transmission line or waveguide. Impedance mismatches result in standing waves along the transmis ...
of better than 1.1 (i.e., a return loss better than −26 dB) for any percentage mitre within 4% (of the original ) of that given by the formula. At the minimum of 0.25, the percentage mitre is 98.4%, so that the strip is very nearly cut through. For both the curved and mitred bends, the electrical length is somewhat shorter than the physical path-length of the strip.


See also

*
Distributed element filter A distributed-element filter is an electronic filter in which capacitance, inductance, and electrical resistance, resistance (the electrical element, elements of the circuit) are not localised in discrete capacitors, inductors, and resistors as th ...
* Slow-wave coupler *
Spurline The spurline is a type of radio-frequency and microwave distributed element filter with band-stop (notch) characteristics, most commonly used with microstrip transmission lines. Spurlines usually exhibit moderate to narrow-band rejection, at about ...
, a microstrip notch-filter


References

{{Reflist


External links


Microstrip in Microwave Encyclopedia

Microstrip Analysis/Synthesis Calculator
Microwave technology Planar transmission lines Printed circuit board manufacturing