# Signal coming from the USRP to the computer

Hi All,
I want to know what is the signal coming from the USRP onto the USB bus.
I
know that the received signal is a baseband signal and assuming complex
sampling (I have RFX2400 daughterboards) each complex sample that enters
the
USB bus is the following,

x[i] = (inphase_component) + j (quadrature_component), and
x[i] = m(t)cos( 2piFREQ_OFFSETt + PHI ) + jm(t)sin(
2
piFREQ_OFFSETt +
PHI ), where m(t), is the actual message signal, FREQ_OFFSET is the
frequency offset, and PHI is the phase.

Is that correct?

Thanks

On 05/29/2011 04:50 PM, John A. wrote:

Is that correct?

Thanks

More or less, yes.

There will be random noise components as well, since this is “real
world”, rather than purely a Matlab simulation

Marcus L.
Principal Investigator

Of course that is true. I just wanted to make sure my understanding of
the
whole signal flow in GNU Radio/USRP system was right. The IF, down
coversion
etc stages confused me a bit. I was only interested in what I am getting
on
the USB bus to work with so, I wanted to make sure my understanding of
this
system was right.

On 05/29/2011 05:17 PM, John A. wrote:

Of course that is true. I just wanted to make sure my understanding of
the whole signal flow in GNU Radio/USRP system was right. The IF, down
coversion etc stages confused me a bit. I was only interested in what
I am getting on the USB bus to work with so, I wanted to make sure my
understanding of this system was right.

You should perhaps do a literature search on “complex downconverter” or
get more understanding.

With one exception, all of the daughtercards use a direct-conversion
scheme to directly convert from RF to a complex baseband, which
is described mathematically as you’ve shown.

I’m always nervous when I see a purely-mathematical description, because
if you think in terms purely mathematical, without also
considering the very-real, very-practical, you can be in for some
surprises.

Marcus L.
Principal Investigator