Re: [Discuss-gnuradio] Failure of sending square wave over USRPs (back-to-back)

[Activecat]

Dear Marcus,

On Sat, Mar 15, 2014 at 11:14 AM, Marcus D. Leech <address@hidden> wrote:
> The SBX does analog downconversion, nothing more.  It knows nothing about
> the incoming signal, and doesn't demodulate it in any way.

Please be clarified what do you mean by "analog downconversion".
At the transmitter side, complex-based (I/Q) signal is fed into USRP.
Says, the signal is x(t) = I(t) + j.Q(t)

I think this is performed at transmitter SBX:
  y(t) = I(t).cos(wt) - Q(t).sin(wt)      where w = central frequency, t = time
here the y(t) is the output of SBX to the antenna.
Is this what you meant by "analog upconversion" ?

Whereas at the receiver side, the received signal from antenna is real
signal, says,
  z(t) = c.y(t) =  c.I(t).cos(wt) - c.Q(t).sin(wt)    where c =
channel attenuation
The receiver SBX performs this:
  Retrieved I(t) = LPS( z(t).cos(wt) )    where LPS = low pass filter
  Retrieved Q(t) = LPS( z(t). sin(wt) )
With this process the receiver SBX is able to produce complex-based
(I/Q) signal from the real signal from antenna.
Is this what you meant by "analog downconversion"?

If not, please describe what you mean by "analog upconversion" and
"analog downconversion", because I have no idea of what you are trying
to explain.  It is impossible that SBX only mix the incoming
complex-based signal with a central frequency and then send directly
to antenna for transmission, because the simple mixing (analog
upconversion) produces another complex-based signal which cannot be
transmitted through antenna.  A physical antenna can only transmit
real signal, not complex-based signal.

Please clarify, thanks.
Regards,
Activecat



> That is what SDR is all about--the signals are represented as
> complex-baseband (i/Q) format for processing by computer algorithms.
>
> The SBX (or any other daughtercard) is simply doing downconversion (or,
> upconversion for TX).
> Frequency offset in a digital demodulator implemented in software generally
> drives a local correction--not the hardware.  You achieve this by
>   bringing in a bit more bandwidth than you actually need, and then applying
> frequency corrections in software.