— On Sun, 3/29/09, Tom R. [email protected] wrote:
From: Tom R. [email protected]
Subject: Re: [Discuss-gnuradio] Help with Simple Amplitude Modulation
Exercise
To: “Thomas” [email protected]
Cc: [email protected]
Date: Sunday, March 29, 2009, 5:48 PM
Thomas,
Thanks for your clearly presented question. Answers embedded below.
Thomas wrote:
from gnuradio import gr
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)   Â
    f2c = gr.float_to_complex()
Why not just use the gr.sig_source_c instead of having both of these
blocks?
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
   Â
Tom
Thank you so much, Tom, for your help. Your answers were very useful,
and I was able to get it working.
You mentioned using a complex source. I tried that, but I got additional
spikes in the audio spectrum at multiples of the 3 kHz tone I was
transmitting. The following image shows what I mean.
http://img12.imageshack.us/img12/7129/screenshotusrpbroadcast.png
The next images shows the audio spectrum using the non-complex source.
http://img12.imageshack.us/img12/5867/screenshotusrpbroadcastj.png
I’m not sure why the spikes are only there when using the complex
source. I’d like to know if anyone here knows.
Thanks!