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
