Hi Dan
I totally agree that a FIR filter with N taps (N < fft length) will not
perform as well as a real valued FFT.
I was not precise in my earlier comment about using a Hilbert transform.
Its not important so I won't beat that horse.
Thanks
Gerry
On 1/23/2014 8:35 PM, Dan Werthimer wrote:
hi gerry,
to compute a power spectrum,
instead of doing a hilbert transform to convert real
adc samples to complex, followed by a complex FFT,
it's better to do a real FFT. which is more efficient
computationally, and has better (almost perfect)
side band rejection.
the hilbert transform has a problem that it doesn't convert
the full bandwidth very well to complex - depending
on the number of taps in the hilbert filter,
there are phase and amplitude errors which get worse
near the edge of the band,
which result in poor sideband rejection.
(so one RFI source will appear in two places in the spectrum,
and the SNR of the signal of interest is degraded...)
best wishes,
dan
On Thu, Jan 23, 2014 at 7:06 PM, Gerry Harp <gh...@seti.org
<mailto:gh...@seti.org>> wrote:
FWIW:
We generally pass the real-valued digitized signal through a
Hilbert (?) filter to get half as many complex-valued samples
(hence sample rate ~= frequency bandwidth). But I'm sure there is
a complex FFT in there somewhere.
Thanks everyone, I'm good for now.
Gerry
On 1/20/2014 10:40 PM, Andrew Martens wrote:
Hi Gerry
If you put a real signal through an FFT you get a mirror image
of one
half of the channels in the other half. Our fft_wideband_real
(which I
am assuming that you will be using), removes half the channels
automatically. So a 2^17 bin fft_wideband_real block will give
you 2^16
channels.
Regards
Andrew
Thanks Andrew.
I suppose you suggest 2^16 useable channels from a 2^17
FFT b/c of
aliasing?
Yes, we must give it a try to really know.
Gerry
On 1/20/2014 9:55 PM, Andrew Martens wrote:
Hi Gerry
Dan's idea of ganging together filter banks works well
provided you can
make do with an FFT for the second filter bank i.e you
don't need any of
the benefits of a PFB.
We currently are working on a design that includes 2
2^16 filter banks
(10 bits ADC input data, 8 tap PFB FIR, 18 bit data
path) capable of
processing 880MHz each in a ROACH2. You should be able
to comfortably
fit a 2^17 filter bank (2^16 effective channels) using
a single FFT into
a ROACH2. 2^17 effective channels will be more
difficult but might be
possible I think if you kept the number of taps in the
pfb_fir low and
did the reordering of channels post-FFT in software.
Best would be to
take a tutorial, change the parameters and see.
Regards
Andrew
On 21/01/2014 07:13, Dan Werthimer wrote:
hi gerry,
we haven't tried this, but i think the largest
spectrometer you
could fit on a roach2 is 256M points,
implemented by a 16K point FFT,
followed by DRAM based corner turn and twiddle
factors,
followed by another 16K point FFT.
if you have this many channels in your correlator,
you also be running up near the correlator X
engine memory limits:
for instance, if you cross correlate in a Titan
GPU, then you only have
5 or 6 GB of memory on each GPU card.
let's assume you have a max of 32 GPU's for your X
engine.
then max frequency channels =
32 GPU's x 6GB/GPU x 42^2 baselinepols x
4B/baseline
= 435M channels max for 32 GPU's (round down to
256M max channels)
if you cross correlate in a CPU (eg: DiFX) then
you can have more
memory,
but you'll need a lot more CPU's to keep up with
the data rate, so CPU's
won't help.
be wary of readout rate too - that's a lot of data
to read out :
256M channels x 42^2 baselinepols x 4B = 1 TB
every integration
time
best wishes,
dan
On Mon, Jan 20, 2014 at 7:53 PM, Gerry Harp
<gh...@seti.org <mailto:gh...@seti.org>
<mailto:gh...@seti.org <mailto:gh...@seti.org>>>
wrote:
Hi
Just for fun, how large of an FFT (filter
bank) can fit into one of
the Roach# boards? Has anyone ever
successfully compiled a filter
bank with length 2^17? We're interested in
building a relatively
narrow-band correlator so we need lots of
channels. Any experience
at large lengths or educated guesses are
welcome. Also, how fast
did
it go? Possible to keep up with 100 MSPS?
It is proposal time, once more...
Thanks
Gerry Harp
On 1/17/2014 11:56 AM, Dan Werthimer wrote:
Dear Casper
Collaborators,
We hope you can attend this year's
Casper Worshop
in Berkeley,
California
June 9 throuh June 13, 2014
We'll have more information later about
registration,
travel, abstracts, etc, but for now,
please reserve these
dates.
Hoping you can participate,
Dan and the Scientific and Local
Organizing Committees
--
----------------------
Gerald R. Harp, Ph.D.
Director, Center for SETI Research
SETI Institute
--
----------------------
Gerald R. Harp, Ph.D.
Director, Center for SETI Research
SETI Institute
--
----------------------
Gerald R. Harp, Ph.D.
Director, Center for SETI Research
SETI Institute
