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 <[email protected]> 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 <[email protected] >>>>> <mailto:[email protected]>> 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 > > >
