PCB Tech - Choosing high-frequency microwave radio frequency circuit board materials

With the increasing demand for mobility and portability of electronic equipment, the miniaturization of high-frequency microwave and radio frequency circuits has become more and more important. Before starting to design electronic products, choosing an appropriate high-frequency microwave radio frequency circuit board material can help design smaller RF and microwave circuits. For a given frequency range, the use of high-frequency circuit board materials with a higher dielectric constant (Dk) usually makes the design size and structure of the circuit smaller. However, the use of plates with a higher Dk value will increase the insertion loss of the circuit and may also reduce the performance of other aspects of the circuit. At the same time, the Dk value of the circuit board material will also affect the index parameters of the circuit, such as: radiation loss, dispersion, coupling, etc.

For a given frequency, the wavelength in the medium will decrease with the increase of the circuit board material Dk, resulting in a circuit size designed on a circuit board material with a higher Dk value than a circuit with a lower Dk value The circuit size designed on the material of high frequency microwave radio frequency circuit board is smaller. In addition, high-frequency microwave radio frequency circuit board materials with a higher Dk value will also reduce the phase velocity of electromagnetic waves (EM) passing through these materials. The Dk of the high frequency microwave radio frequency circuit board material usually adopts the value measured through the z-axis direction of the material (that is, the thickness direction) at 10 GHz. The z-axis Dk value of commercial circuit board materials can be as high as 10 (or higher) or as low as 2 (compared to air with Dk equal to 1). But objectively speaking, but usually with a Dk value of 6 or higher, it can be considered as a high dielectric constant sheet.

The transmission line made of high frequency microwave radio frequency circuit board material with lower Dk value has a faster phase velocity. For the miniaturization of phase-sensitive circuits (such as phased array antennas), the influence of Dk must be considered. In addition, the high-frequency microwave radio frequency circuit board material with a higher Dk value exhibits greater dispersion than the circuit board material with a lower Dk value. High-frequency microwave radio frequency circuit board materials with a higher Dk value are usually used in directional couplers and other circuits that require higher coupling coefficients.

As far as Dk is concerned, high-frequency microwave radio frequency circuit board materials are usually anisotropic. Although the Dk values of the materials on the three axes are different, people are usually used to according to the Dk value of the material in the z-axis direction. Compare with each other. For materials with higher Dk values, the difference in Dk between the z-axis and the x-y plane of the circuit is often greater than that of materials with lower Dk values. The Dk values in all three dimensions of the high frequency microwave radio frequency circuit board material will jointly determine the performance of the transmission line (such as the microstrip line) made on the material. For many high-frequency circuits, there is usually no need to consider the anisotropy characteristics of high-frequency microwave radio frequency circuit board materials Dk, but anisotropy does bring some potential unknown problems, especially when the xy plane Dk value and z When the Dk values on the axis are very different. This difference may cause unexpected problems in the edge-parallel coupling circuit, because the coupling is highly dependent on the Dk value on the x-y plane.

When trying to miniaturize the circuit, the easiest way to think of is to minimize the thickness of the high-frequency microwave radio frequency circuit board material, but the thickness of the high-frequency microwave radio frequency circuit board material will affect the performance of multiple indicators of the high-frequency circuit. Although the radiation loss of high-frequency circuits increases with frequency, thicker circuit board materials will also exhibit higher radiation losses than thinner circuit board materials with the same Dk value. For a given circuit layout and design, the choice of Dk will also affect the size of the radiation loss, because the radiation loss of the circuit board material with a higher Dk value is lower than that of the circuit board material with a lower Dk value.

For circuits that may cause resonance or stray interference (for example, between circuits in multi-layer PCB high-frequency microwave radio frequency), it is beneficial to use thinner circuit board materials. The degree of resonance spurs usually depends on the type of transmission line in the circuit. For example, microstrip transmission lines are often more susceptible to resonance and propagation problems than other types of RF/microwave transmission lines (such as strip lines, coplanar waveguide CPW transmission lines). Thinner circuit board materials can help reduce the size of PCB high-frequency microwave radio frequency boards, while limiting radiation loss and transmission line propagation problems, such as resonance and intermodulation. Common engineering experience is to use high-frequency microwave radio frequency circuit board materials that are thinner than a quarter-wavelength of the highest operating frequency of the circuit. But a safer method is to choose a high-frequency microwave radio frequency circuit board material that is thinner than one-eighth wavelength of the highest operating frequency in terms of thickness.

The line width of a transmission line (such as a microstrip line) depends on the thickness of a high-frequency microwave radio frequency circuit board material (such as a circuit laminate or a prepreg material). Circuits with thicker substrates will widen the conductor width, which can reduce the conductor loss and insertion loss of the circuit. However, in this case, some electromagnetic wave propagation problems may occur. In order to select the thickness of the circuit board material suitable for high-frequency design, usually the conductor width should also be less than one-eighth wavelength of the highest operating frequency.

The Dk of the high-frequency microwave and radio frequency circuit board material plays an important role in determining the conductor width of the transmission line, because the conductor of the same size designed on the high-Dk high-frequency microwave radio frequency circuit board material has a lower value than the same circuit on the low-Dk material的impedance. Therefore, in order to keep the circuit with a characteristic impedance of 50Ω, the circuit designed on the circuit board material with a higher Dk value will be narrower.

When designing circuits using high frequency microwave radio frequency circuit board materials with different Dk values, many trade-offs need to be considered. The use of high-Dk high-frequency microwave radio frequency circuit board materials can not only reduce the circuit size, but also realize high-performance miniaturized circuits by combining high-Dk and low-Dk circuit board materials. For example, a band-pass filter composed of a resonant unit, its size depends on the Dk of the circuit board material. Because of the spacing between each filter unit, the coupling strength in the circuit affected by the circuit board material Dk is determined. The high-frequency microwave radio frequency circuit board material with high Dk provides stronger coupling and allows more space between the filter resonant units.

In order to verify the advantages of using high-frequency microwave and radio frequency circuit board materials with different Dk values (combining materials with different Dk values into a composite component), a composite material of high-Dk and low-Dk high-frequency microwave radio-frequency circuit boards was designed Band pass filter. The high-Dk material used in this filter is RT/duroid®6010.2LM circuit laminate with a Dk value of 10.7; and the low-Dk material used is a 2929 prepreg with a Dk value of 2.9, both of which are from Rogers Corporation. Because different Dk values of high-frequency microwave radio frequency circuit board materials will bring differences in circuit performance, it is necessary to use a computer for simulation and modeling to determine the required ratio of two different material thicknesses. This modeling method can help us design a perfect composite filter. The experimental results show that the size of the filter designed by composite materials not only maintains the size on a single high-Dk material, but also has improved electrical performance. . For example, the high-order harmonic resonance is significantly reduced, and the stop-band characteristics of the filter have also been significantly improved. Studies have shown that by using more than one circuit board material in the circuit, miniaturization of the circuit is often possible without sacrificing performance.