All circuits use a sequence of components to send signals to achieve a particular outcome. These are made of copper traces, which give a specific amount of power, resistance, and speed. 

Transmission lines are an effective way to increase board performance. In PCB designs, two types of transmission lines are commonly used: microstrips and striplines. Striplines are incorporated within the board's layer stack up whereas microstrips are used for routing signals on the circuit board's outside layers. 

Learn more about transmission lines and how a PCB assembly business can use them to increase the power of PCBs. 

Key Highlights

  • Striplines are incorporated within the board's layer stack up whereas, microstrips are used for routing signals on the circuit board's outside layers. 
  • Stripline can defend against unknown invading frequencies, improve performance, reduce emissions, and improve EMI characteristics.
  • Microstrips facilitate simple connections between electrical components, reduce crosstalk, speed up propagation, and provides mechanical support.
  • Striplines are complicated and costly to produce, but they provide good signal integrity and isolation. 
  • Although simpler and less expensive than microstrips, they may cause signal distortion at higher frequencies.

Introduction to Stripline Technology

What is a Stripline?

Stripline is a transverse electromagnetic (TEM) transmission line medium in electronics. It was the first type of planar transmission line. A stripline is also known as a PCB transmission line. This kind of transmission line is routed between two ground planes on the inner layers of a printed circuit board design. A stripline circuit comprises a flat strip of metal placed between two parallel ground planes. 

The substrate's insulating substance functions as a dielectric. The characteristic impedance of a transmission line is determined by the strip's width, substrate thickness, and relative permittivity. 

In general, the dielectric material may differ above and below the center conductor. Air striplines employ air as their dielectric substance. Microstrip is frequently used in high-speed computing systems and is the most common microwave transmission line for microwave integrated circuits. It is easier to construct than stripline and provides for better system integration. 

However, microstrip has certain drawbacks, including weaker isolation and higher crosstalk effects than stripline. Software like Agilent ADS LineCalc can be used to analyse microstrip's characteristic impedance and other features.

The Structure of Stripline

A stripline's physical configuration plays a vital role in its operation. It is made up of two parallel ground planes and a flat conductor. The stripline's ability to function depends on the dielectric substance. It controls the pace at which signals go through the stripline. It also preserves the distance between the conductor and the ground planes. 

The material's dielectric constant, or 𝜺ᵗ. PTFE, also known as Teflon and FR-4. It has dielectric constants of roughly 4.5 and 2.0, respectively, which are common dielectric materials. The stripline's ground plane is the metal case, which is often composed of conductive materials like copper or aluminum.

The specifications of the stripline, including the conductor and dielectric thickness, as well as the trace width, are carefully determined based on the desired impedance and frequency characteristics. For example, a thicker dielectric internal layer will cause slower signal transmission, while a wider stripline will have a lower characteristic impedance.

A stripline in which the two conductors are positioned side by side on the edge of a dielectric substrate is known as an edge-coupled stripline. The two conductors are positioned adjacent to each other on the same dielectric substrate layer and are known as broadside-coupled strip lines.

Two conductors convey inverted signals known as a differential pair, which is sometimes utilized. However, a symmetric stripline is a kind of stripline in which the two ground planes are equally spaced apart from the central conductor.

 A stripline's structure is a collection of components and measurements that all work together to enhance the machine's overall functionality and performance. It is essential to understand this structure to utilize striplines' benefits in a variety of applications.

Working principle of Stripline

Electromagnetic waves travel along a conductor that is positioned between two ground planes, and then the stripline operates. In this operation, the dielectric substance that surrounds the conductor is very important.

An electromagnetic field is created around the conductor of the stripline when an electrical signal is applied. As a result of interactions between this field and the dielectric substance, the signal spreads over the whole stripline. The dielectric constant of the material controls the propagation speed, or the pace at which the signal moves.

 In a stripline, signals travel in the Transverse Electromagnetic (TEM) mode. The magnetic and electric fields in this mode are both perpendicular to the propagation direction. Low signal distortion and attenuation are the result of this feature, which makes striplines ideal for high-frequency applications.

The ground planes on either side of the conductor serve two tasks.  First, they finish the electrical circuit by giving the current a passage back.  They shield the conductor from unwanted electromagnetic radiation.

One important factor in signal transmission is the stripline's impedance. It is dictated by the dielectric material's characteristics and the stripline's geometry. Given by the following formula:

In this formula, Z₀ represents the characteristic impedance, 𝜺ᵣ denotes the dielectric's relative permittivity, 𝒉 denotes the dielectric's height, and 𝒕 is the thickness of the substrate. 

This equation demonstrates the stripline's dimensions and the dielectric material selection. It can be used to influence the impedance.The operation of a stripline is based on the propagation of electromagnetic waves along a conductor, where ground planes and dielectric material are important components.

Advantages and Disadvantages of Stripline

Advantages of Stripline

  • Striplines have the ability to defend against unknown invading frequencies that could harm signal traces.
  • This routing technique will improve performance by up to 50MHz. 
  • Low impedance, which reduces emissions and crosstalk 

Moreover, this particular routing technique also has improved EMI characteristics.

Disadvantages of Stripline

  • The layout and building of striplines are more complicated than those of other transmission line types, including microstrip, which might end up in increased production costs and longer lead times.
  • The stripline is built embedded in the dielectric substance, it might be challenging to tune and troubleshoot.
  • A microstrip line with the same impedance and height has a larger trace width than a stripline. The strip conductor may come out of the dielectric ground plane if not handled correctly.
  •  Striplines are widely used in microwave and radio frequency circuits. They are used in radar systems, wireless communication systems, and other high-frequency electrical gadgets.

The common parameters for a strip line directional coupler are TRM Microwave, ARRA Inc., and Marki Microwave.

• Operational frequency band

• VSWR

• Mean Coupling in dB

• Flatness of amplitude (dB) p

• dB directivity

Comparative Analysis: Stripline vs. Microstrip 

Structural Differences

Based on where they are in a PCB stack up, traces in a PCB might be referred to as strip lines or microstrips. A PCB layout can be used for two different transmission line structures: microstrip or stripline. Coplanar and differential Microstrips and straplines are also available; each of these has advantages when used for high-speed or high-frequency communications. 

Prior to beginning, it's important to understand that the main difference between striplines and microstrips is where they are located within the PCB stackup.

Stripline and microstrip are two forms of low-impedance, planar transmission lines that are widely utilized for high-frequency signal transmission on printed circuit boards (PCBs).  

Their functioning principles, advantages, and weaknesses, as well as their structures, are similar and different. Selecting the appropriate type for an application might be helped by comparing them.

 Striplines and microstrips have basically different line structures, which has an effect on how well they perform and are used.

In a microstrip or stripline PCB, the dielectric substrate is an insulator. A conductor embedded in a dielectric substance is positioned between two ground planes to form a stripline, a particular kind of transmission line. While it offers superior protection against external electromagnetic interference, its fabrication is more complex and thicker than that of a microstrip.

A microstrip is a kind of transmission line that has one conducting strip and a dielectric layer separating it from the ground plane. Compared to a stripline, it is less expensive and easier to build, but it is more vulnerable to outside electromagnetic interference. 

Careful dimension selection must be made for both striplines and microstrips to achieve the correct impedance and frequency characteristics. However, because striplines have two ground planes, they are usually larger than microstrips. Compared to microstrips, striplines are less vulnerable to radiation losses because of their waveguide.

In summary, the structure of microstrips and striplines has a big impact on their suitability for various applications, fabrication difficulty, and performance. Selecting between these two kinds of transmission lines requires an understanding of these distinctions.

Working Principle Comparison

A stripline, electromagnetic waves propagate in a (TEM) mode. Striplines are perfect for applications where good signal integrity is required, especially at higher frequencies.

One kind of transmission line that displays quasi-TEM mode propagation is the microstrip. The way electromagnetic fields are contained in striplines and microstrips, two different kinds of transmission lines, varies. Because the field in a microstrip is not completely contained, there is some radiation and a higher vulnerability to outside interference. 

The range of GHz frequencies that a transmission line can broadcast without significant attenuation is referred to as its bandwidth. Microstrip has a lower bandwidth than stripline.

The characteristics of the dielectric material and the geometry of the transmission line dictate the impedance of both striplines and microstrips. The impedance ranges of striplines and microstrips differ because of their dissimilar architectures. To find a transmission line's typical impedance, use an impedance calculator.

Comparably, impedance matching refers to the process of matching a transmission line's impedance to that of the load and source. This is done to reduce signal reflections. Because striplines often have a larger impedance range than microstrips, they are more adaptable for uses where a particular impedance value is needed.

 In the process of constructing a high-frequency circuit, transmission line tolerances must be taken into account. Stripline is better suited for applications where performance and EMI are crucial since it has finer tolerances than microstrip. 

Striplines are more complicated and expensive to produce, though. For low-cost applications where performance and EMI are not as crucial, microstrip is a good option.

 Advantages and Disadvantages

Advantages

  • Microstrip transmission lines are a popular option for many applications, especially in high-frequency electronics. Microstrips require less labor to construct than strip lines. They are made up of a single conducting strip that is divided from a ground plane by a dielectric layer.
  •  As compared to a stripline, a microstrip has a faster rate of signal propagation. This is because the electromagnetic field in a microstrip is not completely contained within the dielectric material, resulting in a lower effective dielectric constant than in a stripline.
  •  The benefits of microstrips, such as their ease of use, adaptability, radiating properties, high power handling capability, and quicker signal propagation speed, make them a good choice for a range of applications.
  • Striplines have also a number of benefits, including the ability to incorporate active and passive components within the same layer, support for pure TEM mode propagation, superior isolation between neighboring signal traces, immunity to external electromagnetic interference, and uniform impedance.
  • Stripline and microstrip are both utilized in high-frequency applications, stripline is more frequently employed in GHz-frequency applications.

Disadvantages

  • Striplines do, however, have certain drawbacks, including a more complicated and large structure, more expensive and complex fabrication, trouble accessing the conductor for testing and troubleshooting, possible heat dissipation problems, and slower high-speed signal propagation than microstrips.On the other hand, Microstrips are a flexible and efficient solution for a range of applications because of these benefits.
  • Certain drawbacks of microstrips include their smaller characteristic impedance range compared to striplines, higher amounts of cross-talk, quasi-TEM mode propagation, vulnerability to external electromagnetic interference, and radiating characteristics.

Depending on the particular needs of the application, the PCB manufacturing technique with stripline or microstrip transmission lines will be used. Before choosing one, engineers should carefully analyze the needs of their project as each has pros and pitfalls of its own.

Which Is Better, Stripline or Microstrip?

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Every PCB transmission line has its own benefits and drawbacks. High circuit density is necessary for the majority of contemporary high-frequency PCBs with high-speed designs. Therefore, a combination of the two traces is required.

Simplifying the microstrip and stripline routing configurations can be assisted by modern PCB tools. Stack-up editors help with entering in the correct board widths and materials, and impedance calculators can help you figure out the ideal trace width and layer layouts.

You can configure your routing layers with the best stripline or microstrip routing method and layer by combining these tools.

 Conclusion

A minor difference between microstrips and strip lines greatly impacts their nature and properties. The decision between stripline and microstrip transmission lines is based on the particular needs of the application. It includes operating frequency, signal isolation and integrity, available space, and cost. 

Striplines are complicated and costly to produce, but they provide good signal integrity and isolation. Although simpler and less expensive than microstrips, they may cause signal distortion at higher frequencies.

In the end, choosing between stripline and microstrip should be determined by carefully examining the unique requirements and limitations of the application. Understanding the distinct advantages and disadvantages of each type of transmission line might aid in decision-making.