[USRP-users] E310 setting bandwidth issue

olivani s.olivani15 at gmail.com
Thu Jul 20 10:20:33 EDT 2017


Hi ,

I am reopening this issue as I have not still overcome or understood the
problem.

Let me first describe my application


I have to collected data (type short, wirefmt sc16) at a particular center
frequency at a sampling rate of 25 Msps and set the bandwidth to be 20 MHz
and the master clock is set to be 25 MHz. I have a custom fpga to detect
the presence of unwanted signal within the 20 MHz bw of data collected .

I assumed that when I give the option --bw 20 MHz , only that data will be
received and sent to ddc will be within that 20 MHz bw and any tone or
signal beyond that will be discarded.

But when I used the script ./rx_samples_to_file I noticed that the data
collected is equal to the sample rate specified and when plotted using
pwelch function with bin size 512  I still see the tone after 20 MHz bw.

/usr/lib/uhd/examples/rx_samples_to_file --file /dev/test_5bw_20msps_1
--subdev A:A --args="type=e3x0,master_clock_rate=25e6"   --bw 20e6 --freq
110e6 --rate 25e6 --duration 1  --stats

as per input parameter specified above I set the centre frequency at 110
MHz and have injected a tone at 122.5 MHz and set the BW to be 20 MHz. I
assumed the tone should not be visible , but unfortunately it does.

This may lead to faulty detection in my application.

1. I added debug statements to see if at all the bw setting  code is
executed and it does.
2. I varied the bandwidth to be 5 MHz and I am able to see the roll off and
also the tone.

Please find the plot below for the above input command

[image: Inline image 1]

Now setting the bandwidth to 5 MHz for verification purpose[image: Inline
image 2]


The code snippet used to plot the data in matlab as follows

binary_file = '\\fargo\projects\wls\staging\subbuku-o\test_5bw_20msps_1';%./rx_samples_to_file
--file /dev/test_5bw_20msps_2 --subdev A:B
--args="type=e3x0,master_clock_rate=50e6" --bw 20e6 --freq 110e6 --rate
25e6 --duration 1 --stats

if ~exist('plotOn', 'var') || isempty(plotOn)

plotOn = 1;

end

if ~exist('power_dbm', 'var') || isempty(power_dbm)

power_dbm = -100;

end

fid = fopen(binary_file,'r');

%A = fread(fid,inf,'bit16','ieee-le');

if ~exist('wordWidth','var') || isempty(wordWidth)

% default to 16 bits...

A = fread(fid,inf,'bit16','ieee-le');

else

% extract according to word desired word width

width = ['bit',num2str(wordWidth)];

A = fread(fid,inf,width,'ieee-le');

end

%A = fread(fid,inf,'bit16','ieee-le');

%A = fread(fid,inf,'float32');

%parse the entire binary file

%A = fread(fid);

fclose(fid);

%A = A(1:2*4096);

I = A(1:2:end);

%figure; plot(I,'b-'); %-->counter test

Q = A(2:2:end);

if 0

figure;

mag = 20*log10(abs(I));

plot(faxis,mag(1:N));

end

complex_sig = I + 1i*Q;

%complex_sig = transpose(complex_sig);

%fs = 1024e3;%512e3; %1024kHz for 4x and 8x; 768kHz, 512kHz and 256kHz for
6x, 2x and 1x

%fs = 512e3;

%fs = 256e3;

%fs = 7.68e6;

%fs = 15.36e6;

%fs = 7.68e6;

fs =25e6;

%[Pxx,F] = pwelch(complex_sig(1:4096*15),4096,[],[],fs,'centered');%%4x and
8x

%[Pxx,F] = pwelch(complex_sig(1:end),[],[],fs,'centered');%%4x and 8x

[Pxx,F] = pwelch(complex_sig(1:end),512,[],[],fs,'centered');%%4x and 8x

if plotOn

figure; plot(F,10*log10((Pxx)),'b-'); grid on

title(binary_file)

if exist('title_str','var')

if ~isempty(title_str)

title(title_str)

end

end

end


Please let me know if it is possible to resolve the issue to eliminating
the tone beyond 20 MHz bandwidth for 25 Msps sampling rate





Thanks and Regards,
Olivani Subbukutty
571-331-2481
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