# Re: sending single tone Am modulated using usrp1+wbx

Ok, I understand your example, but before work with complex examples I
want
to experiment with a simple circuit as a simple DSB AM modulator based
on
the theory that I wrote in the past mail, Im aware about realistic
situations like noise and other factors and it was the reason I
connected
directly a wbx output to wbx input (different wbx) in the same usrp
trying
to avoid this realistic situations.

Now I know that is necessary an attenuator (in this moment I dont have
an
attenuator) and I use two antennas, but I need to know if the usrp
mathematically works like Im believing.

On 08 Jan 2013 08:48, José María Valencia wrote:

Ok, I
understand your example, but before work with complex examples I want to
“experiment” with a simple circuit as a simple DSB AM modulator based on
the “theory” that I wrote in the past mail, I’m aware about realistic
situations like noise and other factors and it was the reason I
connected directly a wbx output to wbx input (different wbx) in the same
usrp trying to avoid this realistic situations.

Now I know that is
necessary an attenuator (in this moment I don’t have an attenuator) and
I use two antennas, but I need to know if the usrp mathematically works
like I’m believing….

I previously observed that direct connection
between TX and RX has a high probability of damaging the LNA in the
RX.

Further, a direct connection will not remove all the noise.
Analog electronics don’t work that way. There is always noise in an
electronic circuit at ordinary temperatures. 1/f noise, shot noise,
various quantum effects. The only reason we don’t have to think about
that too much in the digital domain is that we quantize, so that there
are voltage “zones” that represent 0s and 1s, and we choose those
“zones” such that the various circuit noise “features” never (or almost
never, statistically) cause us to make a wrong decision with respect to
1 and 0. In the analog domain, the noise matters and is always present.