ricardo, i suggest you modify the shifter settings to avoid quantization noise.
for pure noise, downshift every other butterfly stage (shift setting AAAA = 1010101010..._). for pure CW, you need to shift every stage to prevent overflow (shift setting FFFF = 111111111....). for noise with strong CW, the shift settings should be somewhere in between. best wishes, dan On Thu, Jul 26, 2012 at 1:01 PM, Ricardo Finger <[email protected]> wrote: > Hello Casper community, > > I am working with the tutorial 3 spectrometer (slightly modified to work > with the iADC083000 board). > I need to analyze strong CW tones with a dynamic range of at least 50dB. I > am using the standard libraries and running at 500MHz ADC clock. > I started reducing the digital gain of the 'quant' block, and adding noise > to the tone to reduce the digital artifacts/harmonics. > So far so good, I got a strong line 75dB above the noise floor. The > problem is that those 75dBs are not the effective dynamic range because > there is a strange nonlinearity in the response of the spectrometer at the > lower power end. Please see the attachment "200MHz tone with 0 to 40dB > attenuation.pdf" > > When I reduce the tone power in 10dB steps I can see the peak going down > accordingly for the first two steps (20dB) but with the next 10dB of > attenuation, it drops more than 20dB, and with the following 10dB of > attenuation (40dB in total) it does completely disappear, dropping more > than 35 additional dBs. > Have any of you seen this 'low end' nonlinearity before? > Is it part of the ADC response or an artifact of the digital processing? > is it maybe related to the re-quantization before accumulation? > > The nonlinear response also applies to the noise floor as it can bee seen > in the attachment "200MHz tone with 5 steps digital gain.pdf", where for a > gain of '0F 00 00 00' the noise floor has a slope of around 10dB, but for > a gain of ' 00 80 00 00" the slope is more than 50dB. > > Regards, > > Ricardo. > > -- > Ricardo Finger Camus > Electrical Engineer > Astronomy Department > University of Chile > Of: 56(2)9771150 > Casilla 36-D, Santiago. > http://www.das.uchile.cl/lab_**mwl/ <http://www.das.uchile.cl/lab_mwl/> > >
