Hi all, I check gnuradio code found set_bw( ) is not implemented in
db_flexrf_900_rx_mimo_b.
So can I set bandwidth for rfx1800 board?
On 12/27/2010 09:55 PM, James Jordan wrote:
Hi all, I check gnuradio code found set_bw( ) is not implemented in
db_flexrf_900_rx_mimo_b.
So can I set bandwidth for rfx1800 board?
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Only cards that have a variable low-pass hardware filter on their
baseband outputs can process set_bw(). I believe that only the
DBS_RX and DBS_RX2 have this functionality–the MAX2118 (DBS_RX) and
MAX2112 (DBS_RX2) both have programmable switched-capacitor
filters integrated into the quadrature downconverter chip.
The RFX series cards have a fixed hardware filter giving a roughly 20MHz
low-pass characteristic on the mixer output. When this is sampled
at either 64MHz (USRP1) or 100MHz (USRP2), and then digitally
filtered with the CIC decimator, the out-of-band content should be very
far down.
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.
–
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
Thanks.