Marcus, Thanks. I am dsp idiot, I thought decimation is only set the sample rate before. Didn't know it also set up a filter.
On Wed, Dec 29, 2010 at 2:17 PM, Marcus D. Leech <mle...@ripnet.com> wrote: > On 12/28/2010 09:14 PM, James Jordan wrote: > >> Hi Marcus, Thanks for reply. If set the decimation rate exactly the >> receive bandwidth then gnuradio will automaticly >> filter the signal at the receive bandwidth? >> > Decimation *is* filtering (or, more precisely, filtering is an integral > part of decimation). > > On the USRP1, the A/D is sampled at a fixed 64Msps > On the USRP2, the A/D is sampled at a fixed 100Msps > > The bandwidth that is "presented" to the host interface is whatever the > "native" sample rate is, divided by the decimation value, which is > a fixed integer, and must be even. > > So, using the USRP1 example, which samples at 64Msps, if you use a > decimation value of 8, the bandwidth "presented" to the host > is 8MHz (64Msps/8), centered around whatever your center frequency is. If > your desired bandwidth doesn't exactly match whatever > gets presented to the host, you'll have to further filter in software, > which is where Gnu Radio comes in if you're using Gnu Radio, or > what OpenBTS does using its own software. > > An added "wrinkle" is that for OpenBTS, generally the USRP1 is modified to > use a 52MHz sample clock. > > > > > -- > Marcus Leech > Principal Investigator > Shirleys Bay Radio Astronomy Consortium > http://www.sbrac.org > > > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > http://lists.gnu.org/mailman/listinfo/discuss-gnuradio >
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