Baseband vs. Carrier freq

GNU Radio
1

I’m wondering whether my flow graph is working correctly.

I think that my flow graph just send a baseband signal

Sender
flow graph http://dl.dropbox.com/u/11382666/02_RXTX/TX.grc.png / scope
plot http://dl.dropbox.com/u/11382666/02_RXTX/ScopePlotTX.png

http://dl.dropbox.com/u/11382666/02_RXTX/ScopePlotTX.pngRecevier
flow graph http://dl.dropbox.com/u/11382666/02_RXTX/RX.grc.png / scope
plot http://dl.dropbox.com/u/11382666/02_RXTX/ScopePlotRX.png

http://dl.dropbox.com/u/11382666/02_RXTX/ScopePlotRX.pngYes, it
receives
correctly. But my question is

The sender seems to send just a baseband signal, not a signal whose
center
freq. is not 450 MHz

I think that it need to be multiplied by sinusoidal signal…

2

Songsong,

The baseband signal you are transmitting will be upconverted by the
center frequency you specify in the USRP sink. In this case, the
signal’s center frequency will be 450MHz. Likewise, the USRP source
center frequency you specify for the receiver will convert that
frequency to baseband for the rest of your receive flowgraph.

–n

3

Thank you for your answer :slight_smile:

Can you tell me more detail?

With my understanding, a rectangular pulse from the file source will be
an
envelope

and by up-conversion, a sinusoidal signal will be in the rectangular
pulse?

I want to know more detail on the up-conversion,

Especially, a waveform on the air.

2011/1/12 Nick F. [email protected]

4

I suggest you to first look into the USRP documentation available on
gnuradio website. In it you will learn how a USRP upcoverts and
downconverts
a signal. In gnuradio we always deal with baseband signals i.e. the
signal
reaching your computer from the usrp or the signal entering the usrp
from
the computer is always a baseband signal.

To know more about the signal that is in the air you have to find
yourself a
good book on communication systems. If you are new to Signals, RF
communications etc then Communication Systems by Simon Haykin is a very
good
start. To understand more about upconversion/downcoversion and other
digital
signal processing then get yourself “Understanding Digital Signal
Processing” by Rick Lyons but first download the pdf of the USRP
documentation and read the first 15 pages. In that the author Firas
explains
how the USRP works in a simple way.

Before, you start playing around with GRC by making your own flowgraph
start
with benchmark_tx.py/benchmark_rx.py examples. These are like the “hello
world” of “digital communications using gnuradio/usrp”.

There is a GRC example for bpsk in the GRC examples folder. Check that
out
too.

John