Re: [Discuss-gnuradio] Transmit and Received Signals are Different

[Frederick Lee]

On Thu, Jul 26, 2012 at 12:06 PM, Ben Hilburn <address@hidden> wrote:
> You said you replaced the channel model block with a USRP sink and source?
> Are you trying to TX / RX using a loopback?  If so, remember to use
> attenuators so that you don't blow out your RX chain.

When I said "replaced the channel model block with a USRP sink and
source", I meant in two separate flow graphs. In other words, I have
the transmit portion of the graph on one computer and the receive
portion on my laptop, and I am trying to send the signal from one USRP
to another. The transmit flow graph has exactly the same blocks up to
the channel model block, but instead I used a USRP_sink instead of the
channel model.The receive flow graph has the portion from the channel
model onward, except the channel model is replaced with the
USRP_source. Also, the throttle was taken out. Sorry if that wasn't
clear.

>
> Also, the channel model block cannot simply be replaced with a source and
> sink.  There is a lot of other stuff going on in there - look into the
> documentation for the block, as you will need to make other adjustments so
> that your samples come out as you expect.

The channel model block contains a timing offset, and noise adder
which I didn't use in my simulations. The only things I changed were
the frequency offsets and the multipath variable (from what I found
online, it's used to change amplitude and phase shift of the signal).
The only difference that I can see between the channel model and using
actual USRPs is that  USRPs up converts the signal into IF ( I believe
), converts it to analog, and the daughter board converts it to RF.
Then the reverse happens in the other USRP. When the two USRPs are
transmitting, any of the four things that the channel model does can
happen, but the main ones I'm concerned about is the frequency shift.,
and the DPSK block seemed to have taken care of that ( at least in the
simulation ).
I was using the */gr-uhd/examples/grc/uhd_dpsk_mod.grc and other
similar examples as somewhat of a guide, and I don't see anything that
looks very different from what I used. What kind of other adjustments
must I make?

Frederick