Spectral baseline distortion simulation - Bruker smile
My guess is that somewhere in the processing you are using an AR (or ARMA) model that is being over / under fitted and that is what is causing the issue.

If I make a fake signal like this in R:

group_delay &lt;- 84 
T &lt;- 350

omega &lt;- 2*pi*0.252352890
tau &lt;- 0.005
phi &lt;- 2*pi*0.0989038
t &lt;- 1:T

bruker_signal &lt;- rep(0,T)
t_index &lt;- seq(group_delay, T)
bruker_signal[t_index] &lt;- exp(-(t_index-group_delay)*tau)*(
    sin(omega*t_index) + sin(1.5*omega*t_index + phi)) +
     rnorm(length(t_index),0, 0.1)


i.e. it's a noisy, delayed, damped sum of two sinusoids and then do AR modelling of the spectrum with different orders, then I get something like the smiles / frowns you are seeing.

An earlier attempt I made used a low-pass filtered version of the signal added to the same thing, modulated to $f_s/2$.

smile_filter &lt;- butter(3,0.1)
smile_signal &lt;- filter(smile_filter, bruker_signal)
plot(abs(fft(bruker_signal+2*smile_signal + 2*smile_signal*rep(c(1,-1),     lenght.out=T)))[1:175], type='l')
title('Attempt at Smile Spectrum ')


but that seemed way noisier than what you're after.