The question here is: what is a microstrip line? A microstrip line carries microwave frequency signals or electromagnetic waves. This line features 3 layers. These are the dielectric, ground plane, and the conducting strip. Engineers use it to design microwave and RF components like directional couplers. A microstrip line will feature radiation that could be low or high. Also, it will support within 20 to 120 ohm impedance.
Microstrip line is available in different types. The stripline is a type of microstrip line with low radiation. The suspended stripline is another type of microstrip line with low radiation. The Q factor is up to 500. You can mount a chip on a suspended stripline.
The slotline is a microstrip line type that features medium radiation. Also, it supports a Q factor of almost 100 impedance. It is quite difficult to mount the chip in series mode. The finline is a microstrip line that features no radiation. It is ideal to use this transmission line to mount the chip.
A microstrip features a conductor designed on a dielectric substrate. It can be easily integrated with microwave devices. This makes it a great option for a transmission line. A microstrip has a conductor of width, permittivity (εr), and a dielectric substrate of thickness (d). It is a transmission line fabricated with the use of PCB technology. A microstrip can help to transmit Radio Frequency Signals.
How Does a Microstrip Line Work?
The microstrip features a thin layer installed on a dielectric surface. There is a thick ground plane beneath the dielectric substrate in which the conducting layer is routed. The manufacturer then etches the feed lines and radiating element on the dielectric.
What is a Microstrip Line Calculator?
The microstrip line calculator helps to determine the length and width of a microstrip line for characteristic impedance and electrical length. To calculate the width of a microstrip, you will need some parameters. These include dielectric thickness (h), target impedance (Zo), and relative dielectric constant (εr).
What is a Microstrip Impedance Calculator?
This is a calculator used for determining the characteristic impedance of a microstrip. To calculate this, you need some values. These values are trace width, substrate heights, substrate dielectric, and trace thickness. These values are usually represented in mil.
What is a Microstrip Width Calculator?
The microstrip width calculator determines the width of a microstrip. The width of the conducting layer is crucial in determining the radiation intensity. The microstrip width calculator uses the following parameters to calculate:
- Dielectric thickness (h)
- Target impedance (Zo)
- Relative dielectric constant (εr)
- Trace thickness (t)
Functions of Microstrip Transmission Lines in RF PCB Design
Microstrip is a widely used transmission line in the microwave and radiofrequency designs. Generally, microstrip features a wider trace than stripline. Also, microstrip offers increased radiation. Microstrip needs more space for routing and more clearance to nearby traces. This is not a problem in a microwave or RF design.
Advantages of a Microstrip Transmission Line
Microstrip has the ability to utilize a 2 layer PCB with components on one side of the board. This simplifies the assembly and fabrication processes. In a microstrip line, all components and connections stay on the same surface. Therefore, vias are not needed when creating connections. Also, the microstrip transmission line features no added inductance and capacitance.
The trace of a microstrip is usually wider than that of a stripline for the same impedance. Therefore, you can maintain better control over the trace’s characteristic impedance. This is because the etching tolerances during manufacturing are a complete value. Therefore, if there is a 20mil wide trace that is over etched in a way that the width drops by 1 mil. This is better than a 5 mil stripline trace being over etched.
For instance, a microstrip trace width is 20 mils and 11.5 mils above ground. If the dielectric constant of the microstrip is 3.8, it will yield about 50.8 ohms. If you reduce this trace width to 19 mils, the characteristic impedance will result in about 52.6 ohms. Therefore, there is a rise in characteristic impedance of 3.6%.
However, a 5mil stripline trace featuring ground 6 mils below and above will produce about 50.35 ohms. When you reduce the stripline trace to 4mil, the characteristic impedance will result in 56.1 ohms. Therefore, the characteristic impedance increases by 11.5%.
Some designs don’t indicate the characteristic impedance of the traces. However, it would indicate the finished width. For instance, if a 5 mil trace reduces to 4mil, there is a 20% decrease in finished trace width. If a trace width of 20mil reduces by 1mil, there is a 5% decrease in width.
Disadvantages
There is not enough surface area for placing components. This is because microstrip transmission lines are a bit wide and route on the board surface. Therefore, microstrip isn’t a perfect option for higher-density technology designs.
Crosstalk is a big problem here. This is because microstrip has increased radiation. Therefore, it is important to provide more spacing to other circuit elements. This will help to minimize the routing density available.
Microstrip transmission lines produce more radiation than other types of transmission lines. Also, there is a need for external shielding for microstrip design. This is becoming a significant problem in designing portable devices. Many devices are becoming smaller and thinner. Therefore, the shielding will get closer to the surface of the board. In the long run, the transmission line’s capacitance per unit length will increase.
Ensure you consider this problem when selecting microstrip transmission lines. You may need to change the transmission line width for a small distance. The use of solder mask will affect the microstrip characteristic impedance. The use of solder mask may vary from one manufacturer to another. Therefore, the effects of these coatings on the surface microstrip trace are unknown.
What is a Differential Microstrip Impedance Calculator?
This calculator determines the differential microstrip impedance of a transmission line. On a reference plane, there is an edge coupled differential microstrip line with two traces. A transmission line always features a differential microstrip impedance. The differential micro strip calculator requires some parameters. These parameters include:
- Trace width (w)
- Dielectric thickness (h)
- Trace separation (d)
- Dielectric constant (εr)
- Trace thickness (t)
Each of these parameters is in mils.
More Facts About Microstrip Line
One of the greatest benefits of microstrip is that it allows all components to be on the circuit board. Microstrip circuits have the ability to radiate. This results in an unintended circuit response. Microstrip is dispersive. This means that signals of various frequencies travel at different speeds.
Microstrip can use a 2 layer board with electronic components on one side. Also, microstrip is commonly used in RF and microwave applications. It is usually routed on the surface of a PCB. There are two materials that surround the microstrip line. These include the PCB material and air. Microstrip radiates when there is an increase in the spacing between the strip and the ground plane.
Printed circuit boards use two major types of transmission lines. These are microstrips and striplines. There are ways to avoid signal reflections in PCB transmission lines. One of these ways is matching the impedance of a PCB trace to the source. However, it is crucial to understand the appropriate impedance matching technique. Quasi-TEM is the ideal mode of propagation in microstrip. The “TEM” refers to Transverse Electromagnetic.
The Effective Dielectric Constant in a Microstrip
The electric energy in a microstrip arrangement depends on the air and dielectric. Therefore, the effective dielectric constant will be between the dielectric and the air. The phase velocity of electromagnetic waves on the microstrip line depends on the effective dielectric constant (ϵeff).
The ϵeff increases towards the substrate as frequency rises. This leads to a decrease in the phase velocity. Also, the effective dielectric constant differs based on the free-space wavelength. This difference is more with the decline in the ratio of substrate thickness to strip width.
There is much dispersion as there is an increase in strip width. Therefore, the microstrip can be a parallel plate capacitor. ϵeff should be less than the dielectric constant of the substrate. Also, it should be more than the substrate’s dielectric constant.
Reduction of Losses in a Microstrip Line
Microstrip has more losses than a waveguide or stripline. Therefore, how can we reduce these losses? There are several ways to reduce losses. One of these ways is to suspend the substrate in the air. This means that there is an increase in the separation between the ground paths and signal. Also, this increases the tendency of the microstrip to radiate.
There is an electromagnetic field between the ground plane and the substrate’s bottom. There is a decrease in insertion loss of the microstrip. This is because air features no dielectric loss. Also, there is an increase in the microstrip line width. This is because of the reduced effective dielectric constant.
Difference between a Microstrip Line and a Stripline in a Circuit Board
EM field distribution of stripline and microstrip transmission lines varies. The functions of microstrip and stripline differ in PCB. A stripline routes on the inner layers of a circuit board. Therefore, it has just one environment surrounding it. There are two different environments surrounding a microstrip.
The PCB material is the only environment that surrounds a stripline. The current path for a signal trace with a high frequency is above and below. In a stripline, several factors determine the characteristic impedance. This factor includes the cross-sectional geometry of the ground planes and strip center-conductor. There is a decrease in the characteristic impedance as the strip width increases.
A stripline comprises three layers of conductors. The sandwiched conductor is the internal conductor. The two other conductors are the ground conductors. Stripline offers shielding to the signal traces. Also, it offers fewer crosstalk and emissions. Stripline features lower impedance and improved EMI characteristics.
Engineers fabricate stripline circuits by bonding a dielectric substrate with a metal layer. The dielectric constants of the insulator materials affect the electrical characteristics of conductors. There are two ground planes in the stripline while there is one ground plane in mircrostrip. It is much easier to fabricate microstrip circuits than stripline circuits.
Also, stripline circuits are ideal for designing multilayer circuits. In this case, the engineer interconnects the layers by plated through holes.
Routing Techniques Using Stripline and Microstrip
In terms of routing, stripline and microstrip designs have their differences.
Routing
Microstrip lines route on the external layers. The trace width and thickness determine their modeling. Also, the dielectric type and substrate height determine their modeling. In a stripline routing, the traces route on internal layers. Here, the modeling depends on the trace width and thickness. Also, the type of dielectric determines the stripline modeling.
Coplanar microstrip routing
The signal trace routes in parallel to two ground planes in microstrip. The ground planes protect the signal against interference from the traces on the board. The signal trace internally routes in parallel to two ground planes.
Differential pair routing
In microstrip, this technique is ideal for routing differential pairs. Also, it has got similar arrangement as regular microstrip routing. For stripline routing, this technique helps to route internal layer differential pairs.
Applications of Microstrip Lines
Microstrip lines help to convey microwave and radio-frequency signals. This type of transmission line is widely used in several applications like:
- Printed circuit boards
- Power dividers
- Filters
- Antennas
- Directional couplers
- Microwave amplifiers
Frequently Asked Questions
Which mode does the microstrip line support?
The microstrip line is in support of a pure TEM wave. This is because this transmission line features a very thin dielectric substrate.
What is the function of a microstrip line in microwave amplifiers?
Microstrip line conveys electromagnetic waves in microwave amplifiers. This transmission line serves as the backbone of microwave amplifiers.
Conclusion
Microstrip lines play a crucial role in PCB design. This transmission line is widely used in radiofrequency and microwave applications. Microstrip lines feature a reference plane and a signal trace. Also, this transmission line has its EMI field distributions that feature its properties.
