[Discuss-gnuradio] Help with Simple Amplitude Modulation Exercise

[Thomas]

Hi all,

I have been trying to get a simple DSB AM exercise working without success. I'm sure I'm doing something fairly dumb, and I would really appreciate any help you can offer.

I have two USRP's, each connected to a separate computer in the same room. Each USRP has an RFX1800 d'card installed. Using one USRP, I want to modulate a 1.5 GHz carrier with a 150 Hz sinewave and transmit it as DSB AM. I want to receive it with the other USRP, demodulate it, and hear the 150 Hz tone. I am using the usrp_am_mw_rcv.py script to receive the signal.

The following is my transmitter code. I've tried to keep it as simple as possible.

#!/usr/bin/env python

from gnuradio import gr
from gnuradio import usrp

def build_graph ():

    dst = usrp.sink_c(0, 64)
   
    # set up the USRP
    subdev_spec = usrp.pick_tx_subdevice(dst)
    subdev = usrp.selected_subdev(dst, subdev_spec)
    dst.tune(subdev.which(), subdev, 1.5e9)
    subdev.set_gain(1000)
   
    # sampling frequency
    fs = 3000
   
    # 150 Hz tone source
    src1 = gr.sig_source_f (fs, gr.GR_SINE_WAVE, 150, 200, 1000)     

    f2c = gr.float_to_complex()

    fg = gr.top_block ()
   
    fg.connect (src1, f2c, dst)

    subdev.set_enable(True)    
   
    return fg

if __name__ == '__main__':
    fg = build_graph ()
    fg.start ()
    raw_input ('Press Enter to quit: ')
    fg.stop ()

The following is a link to a screenshot of the receiver.

http://img21.imageshack.us/img21/7355/rxpic.png

The audio spectrum doesn't look right, and it doesn't sound right either. I get a continuous stream of "aUaUaU" at the terminal from which I started the receiver script.

How am I messig it up?

Thank you!

Thomas