Basic RX/TX


#1

Hi,

I have a question about the basic RX/TX boards. What should I see if I
hook the basic TX board to an oscilloscope if I send out a QPSK
signal? Here is what I do:

  • I generate a QPSK signal in GnuRadio
  • I send the signal to a complex file source and the signal looks right
  • I send the signal to the complex USRP source and have a BasicTX on
    it. The outputs TXA and TXB are hooked up to a oscilloscope on channel
    1 and 2 respectively.

Now, what should I see on the oscilloscope? Is it the baseband signal?
How does the I and Q phases get mixed together? Does that depend on
the mux values? Unfortunately I couldn’t find a good block diagram of
the TX path on the wiki, i.e., the link to [A system block diagram of
the USRP receiver and transmitter path] on the wiki page
http://www.comsec.com/wiki?UniversalSoftwareRadioPeripheral” is dead.

Cheers,

Thomas


#2

On Sat, 2006-05-27 at 12:16 -0700, Thomas S. wrote:

Now, what should I see on the oscilloscope? Is it the baseband signal?
How does the I and Q phases get mixed together? Does that depend on
the mux values? Unfortunately I couldn’t find a good block diagram of
the TX path on the wiki, i.e., the link to [A system block diagram of
the USRP receiver and transmitter path] on the wiki page
http://www.comsec.com/wiki?UniversalSoftwareRadioPeripheral” is dead.

If you set the carrier frequency to 0 you will see the I signal on the I
channel out and the Q signal on the Q channel out. This will only be
useful with the LFTX board since the BasicTX will block DC.

If you set the carrier frequency to something other than 0, then the I
and Q signals will be upconverted and sent to the outputs. As long as
the carrier frequency is greater than your signal bandwidth, then you
only need either the I output or the Q output since they are now
double-sided real signals. You can still use the I and Q connectors
with an image-reject mixer if you need.

Matt


#3

On Tue, 2006-05-30 at 00:05 -0700, Angilberto Muniz Sb wrote:

Sorry if it sounds silly, but why one is supposed to
get DC out of the DAC if Carrier Freq is set to zero?

What if one generates a 1Mhz QPSK in GNURadio ? Its
not zero hertz, its baseband…
Im puzzled…

It isn’t DC exactly, but it will have components below 1 MHz, which will
be attenuated by the transformers in the Basic boards.

Matt


#4

Got it – Thanks Matt.

Angilberto.

— Matt E. removed_email_address@domain.invalid wrote:

Im puzzled…

It isn’t DC exactly, but it will have components
below 1 MHz, which will
be attenuated by the transformers in the Basic
boards.

Matt


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#5

Sorry if it sounds silly, but why one is supposed to
get DC out of the DAC if Carrier Freq is set to zero?

What if one generates a 1Mhz QPSK in GNURadio ? Its
not zero hertz, its baseband…
Im puzzled…

Angilberto.

— Matt E. removed_email_address@domain.invalid wrote:

  • I send the signal to a complex file source and
    Does that depend on
    the mux values? Unfortunately I couldn’t find a
    good block diagram of
    the TX path on the wiki, i.e., the link to [A
    system block diagram of
    the USRP receiver and transmitter path] on the
    wiki page

http://www.comsec.com/wiki?UniversalSoftwareRadioPeripheral

than 0, then the I
Matt


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