|
REVISED BOKEH NOV 12 2017 NOISEBUDGET.
I realized that the Nov 12 Spectrum 4 Hz frequency comb an strong spikes at 256 and 512 Hz are not true noise artifacts, but merely poor measurement post processing.
In particular, when spectra are stitched together, there is sometimes some overlap in the frequency domain. When this happens, the data taken at lower resolution is supposed to be thrown out. I neglected to do this, but now have fixed the code in iris to do this. awade calls this the Hierarchical Chucker.
When I measured a spectrum with high span up to 102.4 kHz, the first point is at 0 Hz, and the second is at 256 Hz, and so on. These points are inflated with all low frequency noise, and should be thrown away if we have spectra taken at higher resolution. When one includes these points, we get the blue 'Nov 12 2017 Beat' line below. When the points are properly hierarchically chucked, you get the gold 'Nov 13 2017 Beat' line. These are the same measurement, just with some different post processing, so they ought to lie right on top of one another, which they do except for the line artifacts.
I have also included a plot of all the spectra so people can see why a comb can appear where it shouldn't if you don't throw out the low frequency data of your low resolution measurements. The '4 Hz comb' was actually just the spectra plotting script switching between the light orange and dark orange spectra in Plot 2. The light orange spectrum is taken at 1 Hz resolution, while the dark orange is taken at 4 Hz. The light orange, at higher resolution, should be the only spectrum reported at low frequency. But if you leave in the low frequency dark orange points, the higher noise floor emerges as a 4 Hz frequency comb. This is why hierarchical chucking is a very important process.
|