What is Embedded Microstrip?
Embedded, or buried, microstrip is similar to the surface version, however, the signal line is embedded between two dielectrics and located a known distance from the reference plane.
Because of the increase in the effective dielectric constant, the capacitance per unit length of the embedded microstrip structure is greater than the corresponding non-embedded microstrip structure.
Microstrip Impedance
The following content is the discussion about the formula for regular microstrip of IPC, and all numerical references herein are from IPC-D-317A.
In the development of the formula for the impedance of a microstrip trace, IPC, in section 5.5.1.1 starts with the impedance of a single bare wire near the ground:
Equation 5.29:
where er is the relative permittivity of the medium (dielectric) surrounding the wire, H is the height above the ground, and d is the diameter of the wire.
In order to get to the microstrip equation, two adjustments to this formula are required. First is the adjustment for the fact that the wire is not surrounded by a homogeneous medium. In fact, it has a dielectric below it and the air above it. The adjustment for this is to change er to e’r where:
Eq. 5.30
The second adjustment accounts for the fact that a trace is flat and rectangular, not round. This adjustment substitutes
Equation DGB1
5.98H/(.8W+T) for (4H/d)
in Equation 5.29, where W and T are the width and thickness, respectively, of the rectangular trace. These two adjustments lead to:
Equation DGB2:
This equation is the same as, and reduces to, IPC’s Equation 5.32 and the equation shown in 5.34 (Hint: divide numerator and denominator by the square root of .475):
Equation 5.32 and 5.34
Equation 5.32 is the equation we usually rely on for the impedance of a microstrip trace.
Applications
The embedded microstrip has a similar construction to the microstrip except for the additional dielectric substrate on top of the conductor. Microwave antennas and couplers as well as some filters can be created using the embedded microstrip. These transmission lines are not as easy to manufacture as microstrips, but nevertheless are still far cheaper than the traditional waveguide, as well as being more compact and lighter. However, microstrips cannot handle power levels as high as waveguides can. Microstrips also have issues in power loss, cross-talk and unintentional radiation because they are not enclosed like the waveguide.
Frequently Asked Questions
1. What is microstrip line and stripline?
Stripline is a transmission line trace surrounded by dielectric material suspended between two ground planes on internal layers of a PCB. Microstrip routing is a transmission line trace routed on an external layer of the board. Because of this, it is separated from a single ground plane by a dielectric material.
2. What is the characteristic impedance of a transmission line?
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 traveling in one direction in the absence of reflections in the other direction.
3. What is quasi TEM mode?
TEM wave referred to as Transverse electromagnetic wave. In this mode, the electric field and magnetic field are perpendicular to each other and perpendicular to the direction of propagation.
4. What does cable impedance mean?
Impedance is a term expressing the ratio of voltage to current in a cable of infinite length. In the case of coaxial cables, impedance is expressed in terms of "ohms impedance". The coaxial cables generally fall into three main classes; 50 ohms, 75 ohms, and 95 ohms.
5. Is impedance good or bad?
Increasingly, people tend to use high-quality 'professional'-impedance headphones with portable equipment, and this is rarely a problem, except that the maximum volume will be reduced compared to a lower-impedance design — which is no bad thing in most cases and could potentially increase the battery life of the player
6. What does 75 ohms impedance mean?
The 75 Ohm impedance is a worldwide accepted value for all kinds of coaxial high-frequency signal connections. Impedance is another word for the value of electrical resistance for alternating current: An alternating voltage applied at one cable end results in an alternating current flowing into the cable.
7. How do you calculate characteristic impedance?
The characteristic impedance is the ratio of the effective sound pressure at a given point to the effective particle velocity at that point in a free, plane, progressive sound wave. It is equal to the product of the density of the medium times the speed of sound in the medium (ρ0c).
8. What is impedance matching?
Impedance matching is defined as the process of designing the input impedance and output impedance of an electrical load to minimize the signal reflection or maximize the power transfer of the load.
9. How is impedance matching done?
Impedance matching to minimize reflections is achieved by making the load impedance equal to the source impedance. If the source impedance, load impedance and transmission line characteristic impedance are purely resistive, then reflection-less matching is the same as maximum power transfer matching.
10. Why is impedance matching needed?
Matching the impedances throughout the circuit yields a desired low voltage standing wave ratio (VSWR). Low VSWR circuits transfer the maximum amount of power from the source to the load. There's more. Digital circuits deliver desired performance because of short transition times and high clock rates.
11. Is characteristic impedance a function of frequency?
Although it can be represented in terms of inductors, capacitors and resistors, the characteristic impedance is a complex number that is highly dependent on the frequency of the applied signal. Zo is not a function of the cable length. At high frequencies (> 100kHz), the characteristic impedance is almost purely resistive.