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Wideband FFT analyzers? SDR?

Discussion in 'Electronic Design' started by Joerg, Dec 18, 2012.

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  1. Joerg

    Joerg Guest

    Folks,

    Got a need for FFT analysis. But it's got to be fast and wide, for
    example a swath from 70MHz to 90MHz would need to be FFT'd in 500usec or
    less. Meaning at a rate of more than 2000 FFTs per second. The
    resolution doesn't have to be great, a 8192-point FFT would be fine.
    Analog RF conversion could be done as well if it absolutely has to be
    done. The frequencies and requirements will change but in essence that's
    one of the jobs at hand.

    The widest-BW SDR I found is this one, at 4MHz BW:

    http://www.srl-llc.com/

    Then there is Ettus but the Windows programming environment for those is
    IMHO absolutely horrid. I have a Signalhound but somehow those no longer
    seem to do super high BW fast scans. At least mine won't go past
    3frames/second.

    The best would be if the whole FFT enchilada could happen inside a DSP
    or FPGA in the radio or analyzer, and only the results are ferried to
    the PC via a USB link.

    Is there something better out there? It can cost a kilobuck or more if
    needed. But ideally not 10 kilobucks. Should run without fancy LabView
    SW or the like if possible. What we need is basically a waterfall FFT
    with adjustable rolling speed.
     
  2. Guest

    buy or make?

    http://www.xilinx.com/support/documentation/ip_documentation/ds808_xfft.pdf

    a XC6SLX150T is ~200$

    -Lasse
     
  3. Joerg

    Joerg Guest

    Most definitely "buy" :)

    I know how it's done, what I am looking for is some company that's
    already dunnit. A brief email chat with an engineer at Signalhound a few
    minutes ago turned out that they should very soon have a glove-fit
    USB-pod for this task. A device that can stream large swaths of RF
    blazingly fast into the PC. Can't wait.
     
  4. Joerg

    Joerg Guest

    Streaming? Yes. At one client we were pumping over 400Bbits/sec into the
    computation machine, via USB. But their system was full custom and I
    really want to avoid that.
     
  5. Nico Coesel

    Nico Coesel Guest

    If it doesn't need to be portable I'd put the digitizer (after a down
    converter) onto a PCI express card and let the CPU do the number
    crunching.
     
  6. Crash? It can't do stuff 200x a second reliably IME (Ethernet comms
    with custom instrumentation).


    Best regards,
    Spehro Pefhany
     
  7. Nico Coesel

    Nico Coesel Guest

    Yes. Even if you add a few zeroes. Need more computational power? No
    problem. By adding (headless) CUDA graphics cards you can get hundreds
    of gigaflops from a standard PC. IOW nothing beats the PC platform
    when it comes to number crunching on a tight budget.
     
  8. Joerg

    Joerg Guest

    You can if it's a waterfall FFT. It updates only one line (or columns if
    running it horizontally) of the waterfall display per finished FFT and
    then scrolls all the previous ones one pixel position up (or to the
    left). So if you have 1920 pixels available you'd see a smidgen more
    than one frame per second scrolling along the screen. On my netbook it
    would be about 2frames/sec because that has 1024 pixels. No a big deal,
    really.
     
  9. Joerg

    Joerg Guest

    Possible if we have to. But ideally it should be able to work with an
    off-the-shelf PC without opening it or even a laptop. A modern processor
    can crunch FFTs really fast.
     
  10. miso

    miso Guest

    When you don't need the full spectrum, I thought chirp-z was the way to
    go. That is how the audio DSA's work. The idea is the algorithm is more
    complex, but you don't need as many points since you can limit the
    analysis to the desired bandwidth.

    I'm sure Vladimir the DSP guru will comment.
     
  11. Fred Bartoli

    Fred Bartoli Guest

    Spehro Pefhany a écrit :
    Not even once a day...
     
  12. Guest

    but it is only 3.2MHz sample rate, that doesn't quite cut it for 20MHz
    BW

    -Lasse
     
  13. Joerg

    Joerg Guest

    The Bluestein method would still require a wideband receiver. I have
    thought about doing it all analog (again ...) and finding the response
    in the demodulated signal. Then I only need the sound card or something
    to display. Considering the SW- and HW-obstacles that still seem to be
    in the way of contemporary SDR I might just do that.
     
  14. Fred Abse

    Fred Abse Guest

    Annoy?
     
  15. josephkk

    josephkk Guest

    Just some numbers to work with. FFT has O=n*log2(n) butterflies. for
    8196 points that is 8196*14 butterflies per FFT and 2000 FFT/s.

    That is about 230 million butterflies per second or about 1 GFLOP.

    Next, I/O bandwidth 8196 points times 2000 times a second is 16,392,000
    samples per second. That will overrun most PC's I/O capability. You may
    have to drive the display yourself.

    Looks like FPGA + a fast ARM is the most reasonable solution, unless you
    can find what you want otherwise.

    I think i saw something once that could receive either half or the whole
    FM band all at once, but that may be a happy dream. It was armature radio
    related.

    ?-)
     
  16. josephkk

    josephkk Guest

    OK i had neglected that option. Plenty of GFLOPS to be had there. Still,
    shoving 16 MBytes/s to 32 MBytes/s into a PC is no joke.

    ?-)
     
  17. Joerg

    Joerg Guest

    Amateur radio :)

    The widest BW direct digital RX I found has 4MHz. Not a whole lot. But
    meantime I have decided to do it all analog in hardware as usual, no
    FFT. I just ordered the stuff at Digikey. SDR just ain't ready yet.
     
  18. Joerg

    Joerg Guest


    Sure. We shipped ultrasound machines in the 90's that digitized a 200MHz
    swath of spectrum. And yeah, those also cost in that range.

    SDR is not ready for a simple reason. I can do the same thing for under
    a hundred bucks in hardware. BTDT. Now if SDR could do it a lot better
    or cheaper I'd be all for it. But it can't (yet).
     
  19. Joerg

    Joerg Guest


    About 0.005 million :)

    Seriously, I don't do this for the first time.


    On this one? None. Goes into a PC and there you do anything you want.
    With or without SDR. A spectrum is a spectrum is a spectrum. It does not
    matter how you acquire it.
     
  20. Owen Roberts

    Owen Roberts Guest

    Jeorg,

    You can do that with optics. A Bragg receiver running into a linear
    CCD. Very simple to build. I have good relations with a company that
    makes the wideband cells.

    Send me a private email at (decode NATO standard phonetics) oscar
    sierra romeo @ oscar hotel india oscar dot net.

    Steve
     
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