Hi Ricardo
I think along with the FFT shift, the quant block is a likely culprit.
I'd recommend increasing the quant block from 6 bits to somewhere above
8 bits and seeing if the problem persists. (You'll have to modify the
vacc block too).
Also, you might want to consider digitizers with more bits if dynamic
range is critical. The iADC and 083000 have 8-bits (255 levels), so the
dynamic range is 20*log10(1/255) = 48dBm, a tad lower than the 50dB
figure you mentioned.
Regards
Danny
On 26/07/2012 21:01, Ricardo Finger 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.
