We always enjoy [FesZ’s] videos, and his latest about FREQ function in LTSpice is no exception. In fact, LTSpice doesn’t document it, but it is part of the underlying Spice system. So, of course, you can figure it out or just watch the video below. The FREQ keyword allows you to change component attributes in a frequency-depended way.
Of course, capacitors and inductors are frequency dependent by design. But the FREQ technique allows you to adjust things like voltage sources or resistance in arbitrary ways. By default, you must specify the frequency response data in decibels, which isn’t always convenient. However, [FesZ] shows you how to use other methods to express them using modifiers to the command.
When you have experimental frequency response data, you probably have many samples. About midway through the video, you’ll see how to manage data with more points using a subcircuit that you can put in an external file.
The end of the video looks at some practical applications. He shows an inductor characterized by a vector network analyzer. With the S1 parameters, he can produce a high-fidelity model of the actual inductor instead of using a generic model of an inductor. A little spreadsheet gymnastics allows him to convert the file into Spice format quickly.