Re: [Discuss-gnuradio] Signal reconstruction

[Martin Dvh]

Aadil Volkwin wrote:
> 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!!!
Can you describe what is miss with what you see.
It would help if you could make some screenshots of yout matlab plots available 
on some website.
(Please don't send them as attachments to the list)

If you run usrp_oscope.py or usrp_fft.py does your signal look OK?
If it does then probably matlab does not read in your samples the right way.
I can imagine that matlab gets the order of I and Q wrong  (I=Q and Q=I)
Or do you have the axis right
I don't know if it should be
> y = fread (df,[2, count], 'float');
or maybe
> y = fread (df,[count, 2], 'float');

Greetings,
Martin

> 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 gnuradio import gr, eng_notation
> from gnuradio import audio
> from gnuradio import usrp
> from gnuradio.eng_option import eng_option
> 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)
>         parser.add_option("-R", "--rx-subdev-spec", type="subdev",
> default=(0, 0),
>                           help="select USRP Rx side A or B (default=A)")
>         parser.add_option("-d", "--decim", type="int", default=8,
>                           help="set fgpa decimation rate to DECIM
> [default=%default]")
>         parser.add_option("-f", "--freq", type="eng_float", default=50e3,
>                           help="set frequency to FREQ", metavar="FREQ")
>         parser.add_option("-g", "--gain", type="eng_float", default=None,
>                           help="set gain in dB (default is midpoint)")
>         parser.add_option("-N", "--nsamples", type="eng_float",
> default=2000,
>                           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)
> 
>         self.head = gr.head(gr.sizeof_gr_complex, int(options.nsamples))
> 
>         self.connect(self.u, self.head, self.dst)
> 
> 
>         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(),)
>         input_rate = self.u.adc_freq() / self.u.decim_rate()
>         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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