Hi everyone,
I've also been having problems with the 10.1 green FFT block this week.
I tried the test Dave suggested, and the results were ok. As an input I used
the sync pulse delayed by two clocks for FFT sizes of 2^{4,5,6,12}, and in
each case result was a complex sinusoid with a period equal to the length of
the FFT. I also put the delta funtion into both pol0 and pol1, as well as
zeros into pol0 and the delta function into pol1. The former case gave
identical complex outputs on out0 and out1, and the second case gave zeros
on out0 and a complex sinusoid on out1.
I did get strange results when my sync pulse was too short, but that is
likely just the block resetting before the reorder finishes.
Let me know if you want me to send out screen shots.
Cheers,
Laura
On Fri, Sep 19, 2008 at 4:08 AM, David MacMahon
<[email protected]>wrote:
> 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)
>>
>>
>> <fftcheck.zi_><fftcheckmdl.PNG><fftcheckplt.PNG>
>>
>
>
>
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