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 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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