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Re: [Discuss-gnuradio] How to adjust the Low pass filter bandwidth

From: Jason Abele
Subject: Re: [Discuss-gnuradio] How to adjust the Low pass filter bandwidth
Date: Wed, 13 Oct 2010 09:29:12 -0700

> My incoming signal has a bandwidth of 1MHz while the DBSRX daughter board that
> I am using has a low pass filter bandwidth of 60MHz. Is it possible for me to 
> adjust
> the low pass filter bandwidth in USRP2?

If you are using the UHD, it is fairly easy to adjust the bandwidth
(4M-33M), there is a bandwidth property for it which might even be
exposed in the gr-uhd wrapper in an upcoming release.  In the
meantime, it is fairly easy to change the call to set_bandwidth in the
daughterboard constructor to give your desired bandwidth


If you are using the rawethernet USRP2 software in the GNUradio tree,
you will need to modify the values of common.d_m and common.d_fdac in
the firmware (around line 100 of the file below), and then rebuild the
firmware and write it to your sd card.


The equation for filter bandwidth is on page 10 of the max2118 datasheet:

F_3dB = Fxtal / M * (4 + 0.145 * FDAC)

M = common.d_m
FDAC = common.d_fdac
Fxtal = 4MHz

However, be aware that integrated noise power inside the filter
bandwidth remains constant, regardless of filter bandwidth, so as you
narrow the filter bandwidth, the noise power per hertz goes up.  We
generally leave the filter set fairly wide because the ADC in the USRP
is usually not limiting the linearity, so narrowing the baseband
filter just degrades noise performance.


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