On Tue, Dec 28, 2010 at 11:43 AM, Marcus D. Leech
<address@hidden> wrote:
On 12/27/2010 10:31 PM, James Jordan wrote:
> Thanks. So RFX board always capture a 20MHz bandwidth signal? But
> applications use
> RFX board such as openbts need only 200Khz bandwidth signal, why it
> can directly use
> the signal from RFX board?
>
Actually, the total bandwidth is 40MHz, due to complex (quadrature)
sampling.
Most of the filtering is accomplished digitally, with the work in most
applications divided between
the USRP FPGA (decimation is also filtering), and whatever filtering
is performed in software
on the host.
The set_bw() function exists to improve analog filtering prior to
digitization, but it's usually
superfluous, since the drivers will set the card up with a baseband
filter that is appropriate
to the sampling rate of the motherboard (64Msps for the USRP1, and
100Msps for the USRP2).
In quadrature sampling, the analog baseband filter is generally arranged
so that the low-pass cut-off
is somewhat below the nyquist limit.
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