High-frequency high-speed PCB Dielectric Constant (DK) at Different Frequencies Under the Change of Characteristics
When selecting substrates for high-frequency circuit PCBs, special study should be made of the characteristics of the material's change in dielectric constant (DK) at different frequencies. For requirements that focus on high-speed signal transmission or characteristic impedance control, focus on dielectric loss (DF) and its performance under frequency, temperature and humidity conditions. Today, Xinchenger Electronics brings you a little knowledge of high frequency and high speed PCB sheet materials!
In the case of frequency changes, the general substrate material dielectric constant (DK) and dielectric loss (DF) values vary greatly. Especially in the frequency range of 1MHz to 1GHz, their dielectric constant (DK) and DF values change more significantly. For example, the dielectric constant (DK) value of ordinary epoxy glass fiber cloth-based substrate (ordinary FR-4) is 4.7 at lmhz frequency, while the dielectric constant (DK) value becomes 4.19 at lghz frequency.Above lghz, the trend of change in the dielectric constant (DK) value is flat. The trend is to decrease with the increase of frequency (but the change is small).
For example, at l0HGz, the dielectric constant (DK) value of FR-4 is usually 4.15. For substrate materials with high-speed and high frequency characteristics, the dielectric constant (DK) value changes less with frequency. At varying frequencies from lmhz to lghz, the dielectric constant (DK) remains in the range of 0.02. At varying frequencies from low to high, the value of dielectric constant (DK) tends to decrease slightly.
Under the influence of frequency change (especially in the high frequency range), the dielectric loss factor (DF) of the general substrate material changes more than the dielectric constant (DK). The change pattern tends to increase. Therefore, when evaluating the high-frequency characteristics of the substrate material, the study focuses on the variation of its dielectric loss (DF) value.
For substrate materials with high-speed and high-frequency characteristics, as far as high-frequency variation characteristics are concerned, there are two distinctly different types of general substrate materials: one is that their dielectric loss (DF) values change very little with frequency. The other is that its dielectric loss (DF) value is lower, although the range of variation is similar to that of general substrate materials.
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