Hi Jason and Dan, This is really an interesting phenomenon that you picked up. From the two spectra it appears as though the 'interference' may be present over the entire frequency range, but obviously overshadowed by the input signal. Does the level of interference change with input power changes? Can you try and investigate if it is present across all frequencies, e.g. by doing cross-correlations?
Furthermore I'd be interested to know what will cause the interference to be there only every 6th or 7th accumulation if the source was quantization errors? Would you not expect to see it there all of the time, or at least over an accumulation length of 100,000? *Paul Herselman* *System analyst* SKA South Africa Pinelands, Cape Town *Contact Details:* t. 021 506 7353 f. 021 506 7375 c. 083 415 5143 On 14 September 2012 23:21, Dan Werthimer <d...@ssl.berkeley.edu> wrote: > > 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 <jcas...@nrao.edu> 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 >> >> >
