I'm trying to get a spectrometer working based on Tutorial 3 and I'm
running to some problems. The signal I'm measuring has a bandwidth of
about 300 Mhz and I'm clocking my ADC at 800 Mhz. When I plot my
spectrum with the y axis on a log scale and I notice there are "humps"
with an amplitude of about 20db that appear and disappear periodically
in what should be the stop band of my signal. It should be flat in the
stop band. Please see:
ftp://ftp.cv.nrao.edu/NRAO-staff/jcastro/CASPER/Spectrometer/Humps_in_the_stop_band.PNG
ftp://ftp.cv.nrao.edu/NRAO-staff/jcastro/CASPER/Spectrometer/No_Humps_in_the_stop_band.PNG
These humps are not present when the same signal is measured on a
spectrum analyzer, so I'm fairly certain that these humps are not real
and are produced inside the Roach spectrometer. The occurrence of these
humps is very predictable. With acc_len set to 100000 the humps occur
with the pattern of every 6th, 7th, 6th, 7th, 6th, 7th, 6th, 7th, 7th
acc samples. To me, something this predictable points to a problem in
the digital design. A plot of the time between humps can be seen here:
ftp://ftp.cv.nrao.edu/NRAO-staff/jcastro/CASPER/Spectrometer/Humps%20in%20the%20stop%20band%20time%20between%20humps.PNG
Has anyone else seen this behavior? Any ideas of what it is and how to
get rid of it???
Thanks,
Jason Castro
NRAO
