Hello,
I've made a spectrometer that uses a 2^11 point biplex (num inputs = 2^0)
FFT block, and it works great. I then changed it to 2^12 points and it looks
good except that when I put in a CW tone, I get the expected spike plus
another spike 2048 channels away. I've checked everything I can think of, so
I started digging into the blocks. The one thing that caught my mind is that
each of the fft_stage_n building blocks has the "Maximum Coefficient Depth"
parameter set to 2^11. I vaguely recall that this parameter allows
conservation of resources for large FFTs where the twiddle factors do not
change by much, so you can store fewer of them and reuse them. The parameter
is set at 2^11 for all stages, and it seems it would only be important at
the 12th stage, which is exactly the difference between my 2^11 point
spectrometer and my 2^12 point spectrometer. I assume any bugs in this with
the green blocks would have gone unnoticed because large FFTs previously
crashed MATLAB. I will try rebuilding the 2^11 point design with Maximum
Coefficient Depth set to 2^10 to see if I observe similar behavior. In the
mean time, if you know how this parameter should work, please explain.
Thanks,
Glenn
