Tom, that was a very useful explanation. Thanks!
On Tue, Nov 19, 2013 at 7:29 PM, Tom Rondeau <t...@trondeau.com> wrote: > On Tue, Nov 19, 2013 at 7:19 PM, Paul B. Huter <paul.b.hu...@gmail.com> > 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 > > > > On Tue, Nov 19, 2013 at 6:17 PM, Tom Rondeau <t...@trondeau.com> wrote: > >> > >> On Tue, Nov 19, 2013 at 6:35 PM, Paul B. Huter <paul.b.hu...@gmail.com> > >> wrote: > >> > I am trying to deploy my radio application (using GRC), and I am > running > >> > into a problem. I am sampling at 50MHz and trying to pare things down > to > >> > 30MHz using a Low Pass Filter, defined as follows: > >> > > >> > Decimation = 50 > >> > Gain = 1 > >> > Sample Rate = samp_rate (50M) > >> > Cutoff Freq = 30000000 (30M) > >> > Transition Width = 1000 (1k) > >> > > >> > I get an error telling me cutoff frequency is greater than sample > rate / > >> > 2 > >> > (30 > 50/2). > >> > > >> > How can I get down to 30MHz? I do intend to look at smaller "chunks" > of > >> > data > >> > at a time, so I am perfectly okay with taking data at 50MHz and > cutting > >> > it > >> > down later, I just wanted to avoid gather unnecessary data. > >> > > >> > Thanks! > >> > >> Set the cutoff freq to 15M, not 30M. > >> > >> Tom > > > > > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio > >
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