On 11/17/2010 02:20 AM, Matt Ettus wrote: > > > Decimation is filtering. When you decimate by 512 you are reducing > noise by a factor of 512 (27dB). Since you are using a BasicRX, there > will be very little noise, and 27dB less after decimation. In fact, > there is so little noise that the output of the filters is a constant > 0 once it is rounded to 16 bit ints. That is why the FFT results > essentially show negative infinity. > Well, OK, I'll buy that. But there's a significant change below decimation=256. A non-linear jump from "reasonable-looking data" to "negative infinity".
I'm seeing a jump from a level of around -20dB to "negative infinity" by changing decimation from 256 to 260, which is a noise bandwidth change of something like 0.04dB. So while I'm totally willing to believe that a gross change from 400KHz to 200KHz might cause a bit of weirdness, it seems highly counter-intuitive that a small change as implied by decimation=256 to decimation=260 would cause a huge nonlinear leap in filter output in the decimator. -- Principal Investigator Shirleys Bay Radio Astronomy Consortium http://www.sbrac.org _______________________________________________ Discuss-gnuradio mailing list [email protected] http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
