Interpolation and parametric sweep

Hi,

i have a problem in my project. I use v.4.2 and AC/DC module in frequency domain.

I have a model built by some materials, that they depend of the frequency (electrical conductivity, relative permittivity) .
I would like to built a function to inteporlate the values in frequency and then to make a parametric sweep on frequency values.

For example:
In material --> new material --> functions --> interpolation
Function name: a
t f(t)
1 10
2 20
3 30
....

In study --> Parametric sweep: f= range (10,10,100)

It is corrects?
I have an error: 'Failed to evaluate variable' .
How can I define the function?

Thank you so much for your help!
Chiara