On 03/15/2014 12:10 AM, Activecat wrote:
Dear Marcus,
On Sat, Mar 15, 2014 at 11:14 AM, Marcus D. Leech <mle...@ripnet.com> 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
The term "upconversion" and "downconversion" are common terms used in
the radio engineering industry, but may not be common among
other technical folks.
The complex upconverter simply sums the two components after mixing, to
produce a valid real-valued analog signal.
But, again, none of the daughtercards that Ettus produces are designed
to extract *information* from the signal--they merely manipulate it
so that it is represented in a complex baseband form that can easily
be digitized by the ADCs.
One impresses *information* on an RF carrier by modulating it--doing
"stuff" to that carrier so that the information can be recognized by
a suitable receiver. There are hundreds of different ways of doing
this, depending on the required attributes of the resulting signal, etc.
You square-wave example is roughly similar to so-called OOK--on/off
keying. Such techniques are generally only used for very-low-speed
data transmission, owing to the unpleasant spectral properties of
signals with sharp edges.
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