Hey, Frank. It is not normally possible to do a PWL (piecewise linear) source in the frequency domain. There is a quirky way to rig up such a setup into LTspice and PSpice, but I am unable to find a syntax which ngspice accepts without crashing or complaining. The trick is to use a behavioral voltage source with the FREQ keyword, which allows you to enter in the transfer function in a PWL fashion. Then you simply run the 1V AC source into it and plot the output. Maybe @holger might know more about if it’s possible to get this to work in ngspice, so I’ll attach the the .net that I got working in LTspice along with a screenshot of the plot I was able to acheive doing this.
freq_pwl_lt.net (26.8 KB)
Even if there’s a way to get it to work in ngspice, it still requires a little bit of massaging of the original text file along with sufficient knowledge of subcircuits to get it working within Eeschema. Therefore, may I suggest a different approach? It sounds like you want to overlay a plot of the ngspice simulation output with an output generated elsewhere (i.e. your FRD file). Perhaps you would be better served generating a 2nd text file (in a similar format) using ngspice, and then using a single plotting program (like a Microsoft Excel or something) to plot both files on the same plot pane for comparison. A good discussion on how to export simulation data to text files can be found here: