The effect whereby the dielectric constant of a material changes with frequency is called 'dispersion'.
To be useful in RF design, any substrate material needs to have low dispersion over the RF band of interest, as well as low loss, and will have been measured over those frequencies. If it's also been measured at optical, then maybe people expect it might be integrated with LEDs at some point, or the physicists were curious. Either way, the constants specified at your operating frequency are the right ones to use for your operating frequency.
There is no universal relation between dielectric constant at RF and optical frequencies. Pure water (for example) is 2(ish) at optical and 80(ish) at RF.
manpreet![Tuteehub forum best answer]()
Best Answer
2 years ago
OK, so when working in a technology, say Bipolar, to build a high frequency amplifier, can I trust that the dielectric constant info for the various films contained in the electrical design rules, are measured at the frequency of the transistor Ft(6GHz)?
What do I mean? So the dielectric constant formula has a frequency component, but it is often meaured on optical measurement tools operating in the 500nm, which would be great if my design operated at 600THz.
Do I need to do a conversion to capture the correct dielectric constant of the films related to the frequency?