[Discuss-gnuradio] usb and lsb transmission

[Matteo Campanella]

Today i dedicated my spare time to the tx basci board, experimenting with 
usb, lsb and am signal synth.
The idea was tp get voice from pc microphone and usb or lsb modulate it. I 
used a phasing modulator, based on the code excerpt reported below; 
basically I apply a pass band to the float source, then an hilbert filter 
followed by a software interpolation, to finish with the usrp interpolating 
the final jump to 128MS.

To my surprise, I got USB with this graph, and i had to conjugate the output 
of the hilbert filter to get the LSB.
That is because in my mind i had modelized the tx path of the usrp as the 
usual NCO (sine and cosine) multiplied by the real and imaginary stream 
arriving from usb and an ADDER in the end. Now, from what I recall, if I sum 
I should get LSB and if I subtract i should get USB; here it looks like it 
behaves the other way around. Any explanation for that?

ciao
Matteo iz2eeq

   audio_rate = 32000
   sw_interp = 8
   usrp_interp = 512

   fg = gr.flow_graph ()

   ampl = 20000

   src0 = gr.sig_source_f (audio_rate, gr.GR_SIN_WAVE, 700 , ampl)
   src2 = audio.source(audio_rate)
   moni = audio.sink(audio_rate)

   gain = gr.multiply_const_ff (20000)

   interp_taps =  gr.firdes.band_pass (sw_interp,
       audio_rate*sw_interp,
       300,
       3300,
       1000,
       gr.firdes.WIN_HANN)

   print len(interp_taps)

   hilb_taps = gr.firdes.hilbert(199)
   hilb_filt = gr.filter_delay_fc(hilb_taps)

   interp = gr.interp_fir_filter_ccf (sw_interp, interp_taps)

   conj = gr.conjugate_cc()

   u = usrp.sink_c (0, usrp_interp)

   tx_subdev_spec = usrp.pick_tx_subdevice(u)
   m = usrp.determine_tx_mux_value(u, tx_subdev_spec)
   print "mux = %#04x" % (m,)
   u.set_mux(m)
   subdev = usrp.selected_subdev(u, tx_subdev_spec)
   print "Using TX d'board %s" % (subdev.side_and_name(),)
   print "Gain Range %s" % (subdev.gain_range(),)
   u.set_tx_freq (0, cordic_freq)
   u.set_pga(0,0)

   fg.connect (src2, gain)
   fg.connect (gain, hilb_filt)
   #fg.connect (src1,hilb_filt)
   fg.connect (hilb_filt,interp)
   fg.connect (interp,conj)
   fg.connect (conj,u)             #LSB
   #fg.connect (interp, u)        #USB