hi jason, do you care about signals that are -80 dB down? these spurs are likely the effects of finite precision arithmetic. to get suppression beyond 80 dB, you'll need to add more bits to the coefficients and data paths, and/or be careful about scaling, rounding and the signal levels at various points in your design.
here's an interesting paper on the number of bits needed for coefficients and data path in FFT and PFB: ftp://data.prao.ru:8021/Astro_archive/USERS/sam/My_doc/Radioastron/Tituls_add_note/RT/LOFAR/DELTA_MITIN/polyphasequant.pdf best wishes, dan On Fri, Sep 14, 2012 at 10:35 AM, Jason Castro <[email protected]> wrote: > 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/Humps_in_the_stop_band.PNG> > ftp://ftp.cv.nrao.edu/NRAO-**staff/jcastro/CASPER/** > Spectrometer/No_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<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 > >
