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Re: [Discuss-gnuradio] Question about LFTX and RFTX
From: |
Brian Padalino |
Subject: |
Re: [Discuss-gnuradio] Question about LFTX and RFTX |
Date: |
Fri, 31 Aug 2007 00:37:10 -0400 |
On 8/30/07, Ismail Mohamed <address@hidden> wrote:
> Hi again,
>
> Recently there has been talk about the I and Q components of the
> signal. I understand the analytic signal concept, also the
> implementation as shown on:
>
> http://gnuradio.org/trac/wiki/UsrpRfxDiagrams
>
> where it says,
>
> "Right idea, except that there is effectively a single complex
> multipler. I.e., I2+jQ2 = (I1+jQ1) * exp(jwt)."
>
> So the analytic signal is generated on the motherboard and mixed up to a
> higher carrier on the daughterboard, i.e. to a new 'w' but same I and Q.
>
> My question is that, with the LFTX and LFRX, if I tune the daughterboard
> to DC, this would make w=0. So there would be no carrier. How are the
> I and Q components separated? Or is it simply that each alternate
> sample is I and Q, and in essence we are sending a purely real signal,
> but saying each second sample represents the imaginary?
>From my limited DSP understanding, at baseband, I and Q are simply
phase relationships and amplitude information around and within the
unit circle.
With that being said, I don't think I and Q can be combined into a
single real signal while preserving information simply because DC has
no phase component.
On the other hand, if the LF cards have both I and Q inputs/outputs,
then you should be able to see essentially two signals come out and
maintain their proper information. If it's only real - you'll
probably have to mix up to some IF to maintain all information.
I believe the real question here is if you can do complex sampling on
a real signal centered strictly around DC - which I don't think you
can.
Anyone wish to comment? I am most likely wrong, but I'd definitely
like to know the real answer to this as well.
Brian