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 From: Firas abbas Subject: Re: [Discuss-gnuradio] Signal reconstruction Date: Thu, 8 Nov 2007 21:24:45 -0800 (PST)

Hi,

I think your problem is simple, you are wrong in two thinks,

1) You chooses to input 50 KHz signal to your USRP, and I think you are using the Basic RX daughter board, which have an RF transformer that needs at least 200KHz input signal to pass it without distortion.

2) You used a DDC default center frequency of 50KHz, which means that your input 50 KHz signal will be centered around the 0 Hz!!!!!.how you will plot it ?

To have thinks work, input 200 KHz signal, and use 0 Hz as the default frequency
[ parser.add_option("-f", "--freq", type="eng_float", default=0, help="set frequency to FREQ", metavar="FREQ")]

Firas,

Hi,

I've been bashing my head for a couple of days on what should be a trivial process.
I would really appreciate it if somebody could please come to my rescue.

I'd like to digitise a signal, intented to be off air FM signal, but for the moment, i'm capturing a 50KHz sinusiod and doing the reconstruction in MATLAB.... to test that im capturing the samples correctly.

on plotting the FFT in MATLAB, it's clear that something's a miss...I don't know where :/ i've been at this for ages!!!

here's my code: I really hope somebody can help, I know this should be a simple matter. The Matlab code follows below the Python stuff.

=============================

#!/usr/bin/env python

"""
Read samples from the USRP and write to file formatted as binary
outputs single precision complex float values or complex short values (interleaved 16 bit signed short integers).

"""

from optparse import OptionParser

class my_graph(gr.flow_graph):

def __init__(self):
gr.flow_graph.__init__(self)

usage="%prog: [options] output_filename"
parser = OptionParser(option_class=eng_option, usage=usage)
help="select USRP Rx side A or B (default=A)")
help="set fgpa decimation rate to DECIM [default=%default]")
help="set frequency to FREQ", metavar="FREQ")
help="set gain in dB (default is midpoint)")
help="number of samples to collect [default=+inf]")

(options, args) = parser.parse_args ()
if len(args) != 1:
parser.print_help()
raise SystemExit, 1

filename = args[0]

if options.freq is None:
parser.print_help()
sys.stderr.write('You must specify the frequency with -f FREQ\n');
raise SystemExit, 1

# build the graph

self.u = usrp.source_c(decim_rate=options.decim)

self.dst = gr.file_sink(gr.sizeof_gr_complex, filename)

rx_subdev_spec = usrp.pick_rx_subdevice(self.u)
self.u.set_mux(usrp.determine_rx_mux_value(self.u, options.rx_subdev_spec))

# determine the daughterboard subdevice we're using
self.subdev = usrp.selected_subdev(self.u, options.rx_subdev_spec)
print "Using RX d'board %s" % (self.subdev.side_and_name (),)
print "USB sample rate %s" % (eng_notation.num_to_str(input_rate))
print "Freq is set to: %s" % (options.freq)

if options.gain is None:
# if no gain was specified, use the mid-point in dB
g = self.subdev.gain_range()
options.gain = float(g[0]+g[1])/2

self.subdev.set_gain (options.gain)

r = self.u.set_rx_freq (0, options.freq) #self.u.tune(0, self.subdev, options.freq)
if not r:
sys.stderr.write('Failed to set frequency\n')
raise SystemExit, 1

if __name__ == '__main__':
try:
my_graph().run()
except KeyboardInterrupt:
pass
==========================================

MATLAB

ms = 1e-3;
kHz = 1e3;

count = 2000;
decim_rate = 8;
Fsamp = 64e6;
Fsamp_real = Fsamp./decim_rate;

%set the time axis
dt = 1./Fsamp_real;
T_end = count./Fsamp_real;
t = 0:dt:T_end-dt;
%t = dt:dt:T_end;

df = fopen('signal_samples.dat');
y = fread (df,[2, count], 'float');
fclose(df);

%plot I and Q
figure (1)
subplot(211)
plot(t/ms,y(1,:));
grid on;
xlabel('ms')
title ('I data')
subplot(212)
plot(t/ms,y(2,:));
grid on;
xlabel('ms')
title ('Q data')

%remove the artefact
z = y(:,601:end);
t0 = t(1:end-600); %new time!

%plot I and Q again
figure (2)
subplot(211)
plot(t0/ms,z(1,:));
grid on;
xlabel('ms')
title ('I data')

subplot(212)
plot(t0/ms,z(2,:));
grid on;
xlabel('ms')
title ('Q data')

%compute fft
z_spectrum = fftshift(fft(z));

%compute frequency axis
df = 1/(t0(end)-t0(1));
freq_axis = -Fsamp_real/2 :df :Fsamp_real/2;

%FFT module
figure (3)
subplot(211)
plot(freq_axis/kHz,abs(z_spectrum(1,:)))
xlabel('kHz')
title ('I data spectrum')

subplot(212)
plot(freq_axis/kHz,abs(z_spectrum(2,:)))
xlabel('kHz')
title ('Q data spectrum')

%FFT phase
figure (4);hold on;
plot(freq_axis/kHz,(angle(z_spectrum(1,:))*180/pi))
plot(freq_axis/kHz,(angle(z_spectrum(2,:))*180/pi),'r')
xlabel('kHz')
ylabel('deg')
title ('I and Q phases')

==========================================

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