[USRP-users] Trouble on loopback for square waves

Marcus Müller marcus.mueller at ettus.com
Tue Apr 7 05:25:10 EDT 2015


Hi Soumen,

> 1) The data(both the I and Q channels) is broken into bits - this gives us
> 2 bitstreams (I and Q).
Not really - the I and Q channels are sent to the B2x0 over USB in
packets containing integer representations of the numbers you feed in.
So, in essence, you don't really juggle bits, but integers.
> 2) The bitstream modulates the intermediate carrier using QPSK(Or, is the
> modulation scheme something else?)
There is no modulation scheme that the USRP uses. You might be
misunderstanding what the USRP does:
It really just takes your samples, and brings them to the frequency you
want. So the signal you see when you plot the samples you send to the
USRP should be equal to what you see in your RF band.
> 3) We upconvert the intermediate carrier to the transmit frequency and
> this
> is the signal that the USRP finally transmits
There's no intermediate carrier. The USRPs are direct conversion
transmitters.
There's a minor caveat here, which is the two-stage tuning process
explained in [1], but I personally wouldn't interpret that as
intermediate carrier here -- it's just a frequency offset correction in
your case.

Best regards,
Marcus

[1] http://files.ettus.com/manual/page_general.html#general_tuning_process





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