Re: [Discuss-gnuradio] HELP! - Problem with radio application deploy

[Ian Buckley]

Paul,

Attached an über simple flow graph, perhaps this is a better way to get you going…25MHz of bandwidth where ever you center the band, which with an LFRX and your application would make most sense to be somewhere in the range 12.5MHz-17.5MHz …all written to file (as single precision complex float) and to GUI FFT. All the filtering in this case is down in the digital down conversion in the USRP.

-Ian

HF_to_file.grc
Description: Binary data

On Nov 19, 2013, at 4:39 PM, "Paul B. Huter" <address@hidden> wrote:

Thanks, Marcus. I'm okay with not doing real-time, as long as I don't lose a ton of data. I'm only going to be running for about 30 seconds at a time.

On Tue, Nov 19, 2013 at 6:36 PM, Marcus D. Leech <address@hidden> wrote:

On Tue, Nov 19, 2013 at 7:19 PM, Paul B. Huter<address@hidden>  wrote:

I recall that that was what you had mentioned yesterday, but could you
explain why setting it to 15M will allow me to grab 30M?

Sure.

We're working at complex baseband. The signal you capture at 50 Msps
will be observable from -25 MHz to +25 MHz. Because it's a complex
signal, we have both sides around 0 Hz; also, it means that for a 50
Msps sampling rate, we can "see" 50 MHz of bandwidth.

So to get the middle 30 MHz, you want to define a filter that extends
from -15 to +15 MHz. A low pass filter is a real-valued signal. We
define it from 0 to 15 MHz, but because it's real, it's symmetric
around 0 and thus extends from -15 to +15 MHz.

Also, be careful about using 1000 Hz as your transition band. That's
still going to generate a gigantic filter. Using a 100 kHz transition
band with a 50 Msps sampling rate produces a filter 909 taps long,
which is very, very long.

Use gr_filter_design (launched from the command line) to design a
filter that's suitable. You can probably get away with a transition
band of 1 MHz.

Tom

You can actually be fairly "sloppy" with the width of the transition bands and still produce a pretty-good filter.  For 50Msps sample
  rate, I'd use a 5-10MHz transition bandwidth.  Also, if you're really sampling at 50Msps, you'll not be able to do much in real-time
  before your computer falls-over.




--
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org


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