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Re: [Discuss-gnuradio] Re: [Openbts-discuss] how openbts set channel ban

From: James Jordan
Subject: Re: [Discuss-gnuradio] Re: [Openbts-discuss] how openbts set channel bandwidth?
Date: Wed, 29 Dec 2010 14:40:00 +0800

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 <address@hidden> 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

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