# Flipping of x axis values when the Fourier Transform is compared to the FFT

EDITED 3-Jun-20

I have a lorentzian lineshape $$f(z) = \frac{1+iz}{R(1+z^2)} \qquad (1)$$ where $$z=\frac{-2\pi(f - F0)}{R} \qquad (2)$$ and $$R$$ is the decay rate, $$f$$ is the frequency, $$F0$$ is the peak frequency.

The time domain function should be $$\exp(2\pi iF0t)\exp(-Rt)/Fs \qquad (3)$$ Where $$Fs$$ is the sampling frequency (and is used as a scaling factor).

This FT pair has been obtained from the response here with a few simplifications, and i'm fairly certain its correct as i've gone through and derived it, and checked it against other derivations of the Lorentzian FT.

I am comparing the analytical Fourier Transform with the Fast Fourier Transform. I want to be able to obtain the original Lorentzian lineshape (equation 1) when i take the FFT of equation 3. I understand that there are differences between the two, and that there will be errors (truncation and/or aliasing), however when i compare them, the analytical FT result appears to be mirrored. This is easy to see when the peak tips are compared. I can write an algorithm to flip the x axis values and shift the peak back to where it should be, however i'm wondering why this flip happens. What is the theoretical basis? Is there a way to solve this without reversing and shifting each x axis value?

Please find the script showing the mirroring below.

library(SynchWave)
library(plotly)

# 1) Lineshape parameters
Fs <- 30            # sampling frequency Hz
F0 <-  2            # resonance frequency
f_length <- 27000   # number of samples
A <- 1              # Peak intensity (Amplitude)
R <- 0.03           # Decay rate

# 2) Frequency data ---------------------------------------------
# Creating the frequency axis
f <- seq(0, Fs, length.out = f_length)

# The lorentz frequency lineshape
z <- -2*pi*(f - F0) / R
LL <- complex(r = 1, i = z)/(1+z^2)/R

# 3) Creating Time function ------------------------------------------
# Time axis
t <- seq(0, f_length/Fs, length.out = f_length)

# Ideal lorentz time lineshape
ft <- A*exp(complex(i = 2*pi*F0*t))*exp(-R*t)/Fs

#-------------------------------------------------------------
# 4) Checking for accuracy
x <- list(
# X axis title
title = "Frequency",
titlefont = "f"
)
y <- list(
# Y axis title
title = "Intensity",
titlefont = "f"
)

p <- plot_ly(x = f, y = Re(LL), mode = "lines", type = "scatter", name = "Original Lorentzian") %>%
add_trace(x = f, y = Re(fft(ft)), mode = "lines", name = "Analytical Algorithm", line = list(color = 'rgb(205, 12, 24)')) %>%
layout(xaxis = x, yaxis = y)
show(p)


and the script with the rudimentary flipping

library(SynchWave)
library(plotly)

# 1) Lineshape parameters
Fs <- 30            # sampling frequency Hz
F0 <-  2            # resonance frequency
f_length <- 27000   # number of samples
A <- 1              # Peak intensity (Amplitude)
R <- 0.03           # Decay rate

# 2) Frequency data ---------------------------------------------
# Creating the frequency axis
f <- seq(0, Fs, length.out = f_length)

# The lorentz frequency lineshape
z <- -2*pi*(f - F0) / R
LL <- complex(r = 1, i = z)/(1+z^2)/R

# 3) Creating Time function ------------------------------------------
# Time axis
t <- seq(0, f_length/Fs, length.out = f_length)

# Ideal lorentz time lineshape
ftna <- A*exp(complex(i = 2*pi*(Fs-F0)*t))*exp(-R*t)/Fs
ftnew <- fft(ftna)
bot <- (Fs-F0)/Fs*f_length - F0/Fs*f_length
bot <- round(bot) + 2
ft <- ftnew[bot:(f_length-1)]
ft <- append(ft, ftnew[1:bot] , f_length)- min(Re(ftnew[1:bot]))

#-------------------------------------------------------------
# 4) Checking for accuracy
x <- list(
# X axis title
title = "Frequency",
titlefont = "f"
)
y <- list(
# Y axis title
title = "Intensity",
titlefont = "f"
)

p <- plot_ly(x = f, y = Re(LL), mode = "lines", type = "scatter", name = "Original Lorentzian") %>%
add_trace(x = f, y = Re((ft)), mode = "lines", name = "Analytical Algorithm", line = list(color = 'rgb(205, 12, 24)')) %>%
layout(xaxis = x, yaxis = y)
show(p)


This just seems like an error due to the finite number of samples you have.

You would get better results if you just increased the number of samples (make f_length <- 100000 for example).

Increasing the number of samples will improve your resolution in the frequency domain. Implicitly, you are also observing your signal for a longer time.

You will have to decide the tradeoff between these two.

I ran a Python script to display the same.

f_length is 27000.

f_length is 100000.

You could also reduce your sampling frequency and keep the number of samples the same. Since most of the signal energy is present around $$f_o=2$$, you could make $$f_s=5$$. Keep in mind, even here you are implicity observing the continuous-time signal for longer but at a lower sampling rate.

f_length is 27000, Fs is 5.

Hope this helped.

• Thanks for the help! Increasing the number of samples does indeed slightly improve the fit, but flipping it is appears far more accurate (and potentially computationally less intensive than just increasing the set). I'm really wondering why the flip happens. It appears that the sampling frequency/# of samples ratio impacts whether the curve is flipped or not but i'm not sure how. Thanks again for the suggestion! – TS1 Jun 4 at 3:39
• Just wanted to mention this: In a typical DFT/FFT, the discrete frequencies go from 0 to $2\pi$. So you will have to perform the fftshift operation to properly align the frequency axes. As per my knowledge, there is no reason for the FFT and the CTFT to be flipped and I believe that the reason you are facing the error is due to the inadequate resolution in frequency. – 2vrk1504 Jun 4 at 5:57