Re: [Discuss-gnuradio] Help with Simple Amplitude Modulation Exercise

[Tom Rondeau]

Thomas,

Thanks for your clearly presented question. Answers embedded below.

Thomas wrote:
> 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)

This device doesn't handle a 1000 dB gain. You can always ask the device 
for its min/max gain values.

>   
>     # sampling frequency
>     fs = 3000

The USRP is using an interpolation of 64, which means the transmit rate 
is fs*64 = 128 Msps (the speed of the USRP DAC), so your fs should be 
2e6 in this example. I'd recommend using a much higher interpolation 
rate (it can go up to 512, so the fs is 250e3).

Make sure match the decimation rate at the receiver.

>     # 150 Hz tone source
>     src1 = gr.sig_source_f (fs, gr.GR_SINE_WAVE, 150, 200, 1000)     
>
>     f2c = gr.float_to_complex()

Why not just use the gr.sig_source_c instead of having both of these blocks?

>     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.

You're ALSA card is not handling the resampling rate that you have set 
for it. I'm assuming the rate is 32 ksps and your card is probably 
wanting to do 44.1 ksps. You will want to pass the script "-O plughw:0,0"

> How am I messig it up?
>
> Thank you!
>
> Thomas
>
>     

Tom