Re: [Discuss-gnuradio] Carrier Frequency

[Johnathan Corgan]

On Tue, 2009-03-17 at 11:03 -0500, William Harding wrote:

> I cannot find in any of the transmission examples for digital
> communication where the carrier frequency comes into play.  I have
> tried to track it down, but cannot figure out where the actual
> "carrier modulation" is taking place.  Any help?

That's because there aren't any.

The USRP is usually treated as a "complex baseband, direct conversion"
system.  The modulation is created in its baseband representation, with
no carrier component, and sent to the USRP and converted to analog I and
Q signals by the DAC.  The analog daughterboards then upconvert these
using a quadrature mixer to a passband frequency.

Receiving works exactly the same way, in the opposite order.

This a slightly simplified description, and there are some additional
considerations on the receive side due to frequency and timing offset
between the transmitter and receiver, but that's the gist of it.

Johnathan