[Bteam] 240 Hz modulation with/without FFB/FF

[Jay Benesch]

Josh Hoskins has computed FFTs of modulation runs taken with varying 
FFB/FF conditions.  For each FFB/FF case there was a five minute run 
without modulation followed immediately by a run with modulation.  FFTs 
were computed for each five minute period and subtracted.  Modulation 
frequencies 210 Hz and 240 Hz.  Surprising to me, 240 Hz modulation with 
FFB/FF seems to work well.

https://hallcweb.jlab.org/hclog/1012_archive/101219220021.html
has most of the plots but the first set has problems.

https://hallcweb.jlab.org/hclog/1012_archive/101219220917.html
corrects those problems

https://hallcweb.jlab.org/hclog/1012_archive/101219220300.html
has the last set of plots

The plots for runs 8038 in the first URL suggest to me that hall C can 
run with FFB/FF turned on while modulating position at 240 Hz.  YMMV

I do not understand the peaks at 770 Hz with 240 Hz modulation and at 
800 Hz with 210 Hz.  The 30 Hz shift in the higher frequency peak with 
the 30 Hz change in modulation suggests a FFT artifact to me, but what 
do I know?  I've suggested a check with 270 Hz modulation, FFB/FF on, 
predicting that the upper peak will move down to 740 Hz.  Hmm. 
1000-240=760 and 1000-210=790, both consistent with the plots.  So 
perhaps the peak will move to 730 Hz.

I am a bit surprised that the difference in FFTs (with-without) 
modulation were cleaner at a 60 Hz harmonic than halfway between 
harmonics.  It may be that FFB has wide response to 180 Hz.

Comments?  Suggestions for more tests?

Jay