Maximum of bandwidth and negative frequencies


#1

Hi All,

I am using a usrp1, and an Analysis Filterbank with 3 bands. My IF is
1MHz.
The decimation is 64, therefore i get 64MHz/64=1MHz bandwidth.
Therefore i should see signals from 1MHz to 2MHz.

I used a band_pass filter in the Analysis Filterbank, but my signal goes
down by
~1.6MHz.
I am not sure why ?

Is this because the half of the bandwidth is for the negative
frequencies ?

Here are my parameters again:
decim=64
samp_rate=64e6/decim
firdes.band_pass(1.0, samp_rate, samp_rate0.01, samp_rate1/3,
samp_rate*0.01)

Regards Markus


#2

On Fri, May 29, 2009 at 6:50 AM, feldmaus removed_email_address@domain.invalid
wrote:

Is this because the half of the bandwidth is for the negative frequencies ?

Here are my parameters again:
decim=64
samp_rate=64e6/decim
firdes.band_pass(1.0, samp_rate, samp_rate0.01, samp_rate1/3, samp_rate*0.01)

Regards Markus

If you are tuning your usrp to 1 MHz and have a decimation of 64,
won’t you be receiving signals from 0.5MHz to 1.5MHz?

Have you compensated for the cic filter droop in the passband? You can
use the filter design toolbox in matlab to design one. Here is an
example from Matlab
http://www.mathworks.com/products/filterdesign/demos.html?file=/products/demos/shipping/filterdesign/ddcfilterchaindemo.html

Hope that helps.

Karthik


#3

Karthik <karthik1024 gmail.com> writes:

On Fri, May 29, 2009 at 6:50 AM, feldmaus <feldmann_markus gmx.de> wrote:

I used a band_pass filter in the Analysis Filterbank, but my signal goes
down by

~1.6MHz.
I am not sure why ?

If you are tuning your usrp to 1 MHz and have a decimation of 64,
won’t you be receiving signals from 0.5MHz to 1.5MHz?

It seems so. I generated a sinus signal from 0kHz to 2MHz and
my signal passes the following spectrums:
For band1 in my analysis filterbank:
600kHz to 1000kHz
1000kHz to 1300kHz

for band2 in my analysis filterbank:
300kHz to 600kHz
600kHz to 1000kHz
1000kHz to 1300kHz
1300kHz to 1600kHz

for band3 in my analysis filterbank:
300kHz to 600kHz
600kHz to 1000kHz
1300kHz to 1600kHz

so i think the band_pass i not useful for the analysis filterbank at all
?!

Any idea how to set up the analysis filterbank, so that i get for
band1:
1MHz to 1.33MHz

band2:
1.33MHz to 1.66MHz

band3:
1.66MHz to 2MHz
???

Regards Markus