Hi,

while working on some practice tasks using pyCBC, I encountered something unexpected when trying to generate several FD waveforms by converting waveforms originally in TD into FD using `to_frequencies()`

method.

I generated several SEOBNRv4P waveforms with different fMin settings and converted them into frequency domain. While I expected them to behave in a different manner in the frequency range [10Hz,20Hz], I noticed an unusual difference at a frequency around 1024Hz, and I can’t understand why this should happen, since I have set sampling rate `srate`

to be 16384Hz.

I suppose this is because of some issues that I didn’t noticed during my conversion?

Below is the figure of these waveforms and the code I used to generate the waveforms.

```
>>> from pycbc.waveform import get_td_waveform, fd_approximants
>>> hpe, hce = get_td_waveform(approximant="SEOBNRv4P",
... mass1=10,
... mass2=10,
... spin1x = 0.2,
... spin2y=-0.3,
... delta_t=1.0/16384,
... f_lower=20,
... distance=100,)
>>> hpef =hpe.to_frequencyseries()
>>> plt.loglog(hpef.sample_frequencies, hpef.squared_norm(), label=f'SEOBNRv4P')
>>> hpe, hce = get_td_waveform(approximant="SEOBNRv4P",
... mass1=10,
... mass2=10,
... spin1x = 0.2,
... spin2y=-0.3,
... delta_t=1.0/16384,
... f_lower=10,
...
... distance=100,)
>>> hpef =hpe.to_frequencyseries()
>>> plt.loglog(hpef.sample_frequencies, hpef.squared_norm(), label=f'SEOBNRv4P-2')
>>> hpe, hce = get_td_waveform(approximant="SEOBNRv4PHM",
... mass1=10,
... mass2=10,
... spin1x = 0.2,
... spin2y=-0.3,
... delta_t=1.0/16384,
... f_lower=10,
...
... distance=100,)
>>> hpef =hpe.to_frequencyseries()
>>> plt.loglog(hpef.sample_frequencies, hpef.squared_norm(), label=f'SEOBNRv4P-3')
```

Ps: the waveform labled as “Plus Polarization” refers to SEOBNRE, which should be mostly equivalent to SEOBNRv4PHM when eccentricity e0=0.

Thanks for help!