Hi, everyone,
To make troubleshooting this kind of problem easier, you can feed in
all zeros except for a single non-zero (real) sample. Depending on
where this single non-zero sample (i.e. impulse or delta function) is
relative to the sync signal, you should get a complex exponential out
(i.e. cos(wt)+j*sin(wt)). If the non-zero sample is one cycle after
the first cycle of the "window", the complex exponential out will be
exactly N samples long (i.e. one cycle out per window). Doing this
with short to moderate length FFTs can make it easy to see whether
there is a reordering problem.
HTH,
Dave
On Sep 19, 2008, at 0:58 , G Jones wrote:
Attached is a simple 7.1 model that illustrates the problem. The
out0 from each fft is different even though the pol0 input is the
same. The only difference is the pol1 input.
The green block implementation is significantly different from the
pink as far as the reordering, so this may be difficult to track down
Glenn
On Thu, Sep 18, 2008 at 6:14 PM, G Jones <[email protected]>
wrote:
Thanks for clarifying Aaron, I will go through the block carefully
tonight and see if I can trace down the "cross-coupling."
Glenn
On Thu, Sep 18, 2008 at 6:12 PM, Aaron Parsons
<[email protected]> wrote:
Your description of what the FFT block *should* do for n_inputs = 2^0
is correct. Assuming the output has been correctly descrambled into
to separate the spectra of the two inputs, one input should not
influence the other's spectrum in any way. This has been tested
extensively in the library as I left it last summer. If this is not
the behavior that you are seeing, then I suspect an error in one of
the mask scripts has been introduced during this massive code
migration.
As for the PFB FIR, it is correct that in "biplex mode" it simply
makes two copies of the same polyphase filter.
On Thu, Sep 18, 2008 at 8:19 PM, G Jones <[email protected]>
wrote:
> Hello,
> I am trying to use the green FFT block with number of
simultaneous inputs
> set to 0. My understanding was that this makes a biplex fft that
processes
> two independant data streams in parallel, thus the pol0 input
should have x0
> x1 x2... and pol1 should have y0 y1 y2 and then the pol0 output
will just
> have frequencies related to pol0. However, when I simulate this,
I find that
> the two inputs are interacting. If I set one input to a constant,
the output
> is significantly different than if I connect the two inputs
together (in
> which case I should get two identical copies at the output.
> I did notice in the packetized correlator iBOB design (using pink
blocks)
> the pfb_fir block preceeding the fft is set to biplex mode, but I
had
> assumed this didn't change the functionality of the block since a
biplex
> pfb_fir block should be equivalent two single input pfb_fir blocks.
> Can someone more clearly explain the functionality of the FFT block?
> Thank you,
> Glenn
>
--
Aaron Parsons
510-406-4322 (cell)
787-878-2612 x329 (arecibo office)
787-721-3991 (home)
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