1. The problem statement, all variables and given/known data I am simply simulation using a small circuit consisting of a square wave generator, a resistor and a GND. The Voltage varies from 0 to 0.5V and the frequency is 100kHz for the sig-gen. I can simulate and output the transient response, a square wave with the above parameter. The next step I am doing is to apply a fft, that was achieved by using the function of the simulation window: Trace-->Fourier. Then I set the x-axis(to Log scale, ranged 50k to 30MHz) and everything goes as it should, until I start to apply some formula on the variable V(IN) in order to take log of the amplitude of the harmonic. 2. Relevant equations That is, modifying V(IN) to LOG10(V(IN)). But the obtained values are not correct at all. On investigating, it seems the log is taken on the transient signal and it is no longer a logarithm after fourier transforming the signal. 3. The attempt at a solution I tried to search for the solution of how should the formula be in time domain so that I am taking a log in frequency domain but no luck. Would you please sharing with me some ideas on this? Thank you.