Re: [Discuss-gnuradio] 2-channel AM demodulation on USRP2

[ematlis]

Some progress with my USRP2-  I am able to demodulate 1 channel while using
the translating filters which I was unable to do previously.  However, I
don't seem to be capturing from the second channel of my LFRX.  What makes
me think this is that when I introduce a deinterleave block to sequence the
second channel, I am seeing the frequency of the demodulated result increase
by a factor of two.  In other words, it seems to be deinterleaving just one
stream of data so I am affectively doubling the frequency of the signal.

What code do I need to implement to tell the USRP2 to capture two channels,
one from each input of the LFRX?

thanks,
eric


ematlis wrote:
> 
> Hi all-
> 
> I have a USRP2 with a LFRX daughterboard.  I'm trying to acquire two 
> channels each at a separate frequency where Ch0 is amplitude modulated and 
> Ch1 is not.  As per suggestions made to me from this list, to capture two 
> channels at separate frequencies I was advised to tune the USRP2 to an 
> average frequency and then use translating filters to capture the band 
> around each frequency separately.  Typically the AM carrier on Ch0 is at 
> about 1 MHz, and the signal on Ch1 is about 5 kHz. For both channels I'm 
> interested in no more than 5 kHz of bandwidth.
> 
> I'm using svn 10991 with the SD card updated to the latest fpga and 
> firmware images on a Fedora 10 x86_64 machine.  I don't think I'm doing 
> this correctly since my spectra is all wrong; perhaps somebody can help me 
> out.  First let me post my USRP1 code that does this, as this was my 
> starting point:
> 
> USRP1: options.decim = 128
> 
>          usrp_decim=options.decim
>          self.u = usrp.source_c(0, usrp_decim)
>          self.u.set_dc_offset_cl_enable(int(0),int(15)) # dc removal off
>          adc_rate = self.u.adc_rate()                # 64 MS/s
> 
>          # Set the decimation in the FPGA
>          usrp_rate = adc_rate / usrp_decim           # 500 kS/s
> 
>          #set the decimation in software on the host PC
>          sw_decim = 10
>          demod_rate = usrp_rate / sw_decim     # 50 kS/s
> 
>          if not self.u.set_nchannels(nchan):   #nchan = 2
>              sys.stderr.write('set_nchannels(%d) failed\n' % (nchan,))
>              raise SystemExit
> 
>          self.subdev = self.u.db(0) + self.u.db(1)
> 
>          if (len(self.subdev) != 6 or
>              self.u.db(0,0).dbid() != usrp_dbid.LF_RX):
>              sys.stderr.write('This code requires a Basic Rx board on Side
> A\n')
>              sys.exit(1)
> 
>          self.u.set_mux(gru.hexint(0xf0f0f1f0))
> 
>          # deinterleave two channels from FPGA
>          self.di = gr.deinterleave(gr.sizeof_gr_complex)
> 
>          # Channelize the signal of interest.
>          lpf_coeffs = gr.firdes.low_pass (1,           # gain
>                                              usrp_rate,   # sampling rate
>                                              demod_rate/2, # passband
> cutoff
>                                              500,       # width of
> transition band
>                                              gr.firdes.WIN_HANN)
> 
>          self.lpf_0 =  gr.fir_filter_fff (sw_decim,lpf_coeffs)
>          self.lpf_1 =  gr.fir_filter_fff (sw_decim,lpf_coeffs)
> 
>          # Demodulate with classic sqrt (I*I + Q*Q)
>          self.magblock_0 = gr.complex_to_mag()
> 
>          # Get real part of Ch1
>       self.splitter_1 = gr.complex_to_float()
> 
>          # now wire it all together
>          self.connect (self.u, self.di)
> 
>       # Ch 0
>       self.connect ((self.di,0), self.magblock_0)
>       self.connect (self.magblock_0, self.lpf_0)
> 
>       # Ch 1
>       self.connect ((self.di,1), self.splitter_1)
>       self.connect ((self.splitter_1,0),self.lpf_1)
> etc
> 
> Then I define two set_freq functions, one for each subdevice.  Ch0 is 
> tuned to the carrier frequency of the AM signal, Ch1 is tuned to 0 Hz.
> 
> Now, on the USRP2- since I have only one ddc, I can only tune to one 
> frequency:
> 
>          options.decim = 128
>          freq = 1.e6
> 
>          self.u = usrp2.source_32fc(options.interface, options.mac_addr)
>          self.u.set_decim(options.decim)
> 
>          adc_rate = self.u.adc_rate()    # 100 MS/s
> 
>       #set the decimation in the FPGA
>          input_rate = self.u.adc_rate() / self.u.decim()    # 781.25 kS/s
> 
>       #set the decimation in software on the host PC
>          sw_decim = 16
>          demod_rate = input_rate / sw_decim     # 48.828 kS/s
> 
>          # deinterleave four channels from FPGA
>          self.di = gr.deinterleave(gr.sizeof_gr_complex)
> 
>          # Channelize the signal of interest.
>          lpf_coeffs = gr.firdes.low_pass (1,           # gain
>                                              input_rate,   # sampling rate
>                                              demod_rate/2, # passband
> cutoff
>                                              500,  # width of transition
> band
>                                              gr.firdes.WIN_HANN)
> 
>       self.freq_xlating_lpf_0 = gr.freq_xlating_fir_filter_ccf
> (sw_decim,lpf_coeffs, freq/2., input_rate)
>       self.freq_xlating_lpf_1 = gr.freq_xlating_fir_filter_ccf
> (sw_decim,lpf_coeffs, -freq/2., input_rate)
> 
>          self.magblock_0 = gr.complex_to_mag()
>       self.splitter_1 = gr.complex_to_float()
> 
>       # Ch 0
>       self.connect ((self.di,0),
> self.freq_xlating_lpf_0,self.magblock_0,self.gain_correction_0)
> 
>       # Ch 1
>       self.connect ((self.di,1), self.freq_xlating_lpf_1,
> (self.splitter_1,0),self.gain_correction_1)
> etc
> 
> I then tune the USRP2 to half of my desired frequency, ie 500 kHz.  The 
> other half of the shifting is down in software.
> 
> I would suspect any problem to be in my definition of the 
> gr.freq_xlating_fir_filter_ccf blocks.  I assume the sampling frequency is 
> set to the sampling rate going in to the filter block before decimation. 
> I am setting the center frequency to be again at half the desired 
> frequency to shift by that amount.
> 
> Any thoughts?
> 
> Thanks much,
> eric
> 
> 
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> 
> 

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