Re: [Discuss-gnuradio] New to gnuradio - Complex sampling question - urgent

[Peng Huo]

hi,,

To convert this complex signal to a real, one has to do
this according to the following formula, isn't it?

x(t) = Y(t)Cos(2*pi*fc*t) - Y(t)Sin(2*pi*fc*t) where
Y(t) -> Complex signal,fc -> Carrier frequency, x(t) -> real signal.

I looked at the block "complex_to_real" and it outputs the Inphase component
as the real signal. Is it because, "fc" is 0Hz as the signal being at
baseband? Won't we lose essential information by dropping the Quadrature component. What about the 
reverse operation?

I want to mulitply this real signal with a PN sequence and then pass the result
on to a costas loop which takes a complex input. I want to know how can we
get a complex signal from a real one. Is there a block that does that?

The book "Communication Systems" by Simon Haykin 2nd edition says,
- The complex envelope g_complex(t) equals a frequency shifted version
of the pre-envelope g+(t) as shown
                  g_complex(t) = g+(t)*exp(-j2*pi*fc*t), where fc->
carrier frequency.
  and the pre-envelop is defined as, g+(t) = g_real(t) + j*g_hilbert(t).

My problem is that we are working at OHz. How do I go ahead with this?

If this what I need to do to get what I want then is there a
block that does it all?

Thanks,
Peng.

P.S. - I am sorry but I thought to make sure before I go ahead so that
I don't spend time doing things that may be unnecessary.

On Sat, Jun 13, 2009 at 5:23 PM, Eric Blossom <address@hidden> wrote:

On Sat, Jun 13, 2009 at 02:31:59AM +0200, Sebastiaan Heunis wrote:
> Jason
>
> From what I've picked up, you need two external sampled signals to get
> an I-Q signal.  With the TV tuner daughterboard, you automatically get
> two signals.  You just need to set the mux accordingly.  When you use
> a BasicRX, you need to feed the same signal to both inputs and use a
> mux value of 0x32103210.

If you use the standard interfaces, you don't have to do anything. You
always get a valid complex baseband signal.

The daughterboard code knows which boards have a single real output
and which have quadrature inputs.  In the single real case (e.g.,
TVRX), the library configures the mux such that it feeds the single
input into the I side of the DDC and connects a constant zero into the
Q side.  The output of the DDC is always complex.  In the case of
daughterboards with quadrature output, one input is connected to the
I input of the DDC and the other is connected to the Q input.

See http://gnuradio.org/trac/wiki/UsrpRfxDiagrams

Eric

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