Hi John
We are building several new pulsar and spectral line personalities, and we
have a weird problem with the FFT block. No matter how many channels we
configure a block for, the last channel in each 1/8th of the spectrum is
getting lost. For instance, if we have a 64 channel personality, channel
0-6 is fine, channel 7 is not fine, channel 8-14 is fine, 15 is not fine,
etc.
Just to ask if this is the fft_wideband_real block?
In my recent adventures into the fft library I did come across a bug
that affects fft_biplex_real_4x (and by extension fft_wideband_real) output.
It was in the init script for mirror_spectrum used in the
bi_real_unscr_4x block. There was a swap in latency settings for delay
blocks for the sync input (sync_delay0 and sync_delay1 specifically).
This caused some of the output channels to be swapped on output (with
the actual fft_bins affected depending on latency settings).
Unfortunately, this bug was introduced in commit 007f55c55 (in 2011) so
has been around affecting our data for a while. This is fixed in the
latest fft changes in the ska-sa repo on github.
You can see this by placing a sine wave into the ADC corresponding to bin
6. Bin 6 shows a healthy spike in the output FFT. Life's good.
Now change the frequency of the sine wave and put it into channel 7. The
line (very nearly) goes away in the output. Move the frequency again to
channel 8, and the line pops back up as it should.
This happens no matter how many channels you build, the last channel in
each 1/8 of the band is corrupted this way.
This is using snap blocks to read the data straight from the FFT block,
btw, so there's no other cruft in the test.
Has anyone seen this? It seems to simulate correctly from what we can
see, which is even more mysterious.
You can see this bug clearly in simulation using an impulse response as
input to the fft. Input a pulse of amplitude 0.5 into the second fft
input one clock cycle after the sync pulse (0s on shift input). You
should get a complex exponential with frequency 1 (f(t-delta) <=>
e^(-delta*fw)F(w)) on the output. It is easy to see channel swapping as
the output function is nice and smooth.
We're using the casper library:
[ptcs@ptcs /export/home/ptcs/scratch/guppi2/libraries/mlib_devel]$ git log -1
commit 6050c725f770f0bdb776fae107dc4fe26f7fce51
Merge: f50084e 0c15bf7
Author: Andrew Martens <[email protected]>
Date: Mon May 20 10:59:32 2013 +0200
Merge branch 'master' of https://github.com/ska-sa/mlib_devel
Copied new bitsnap block into separate module and added it by hand in
casper
Made the change of BRAM to bram in the snapshot block in
casper_library_scop
Conflicts:
casper_library/casper_library_misc.mdl
casper_library/casper_library_scopes.mdl
Try with the latest commit for the same repo and see if that makes a
difference.
Regards
Andrew
