Re: [Discuss-gnuradio] Question about LFTX and RFTX

[Ismail Mohamed]

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?

I hope the question makes sense.

Regards,
- Ismail

Johnathan Corgan wrote:
> Ismail Mohamed wrote:
>
>   
> > My question is, with the LFTX/RX boards, is the communication truly
> > baseband such that frequency synchronisation can be totally ignored?
> >     
>
> Not quite.
>
> With the LFTX/RX boards, DC does indeed remain DC at the other end.
> However, the crystals in each USRP still govern the sampling rate, and
> each can be up to 50 PPM off (new USRP hardware has 20 PPM spec'd
> crystals now.)
>
> So the same issues with respect to symbol timing recovery exist.
>
> In practice I've never encountered a USRP that had more than a few PPM
> crystal offset--but you can only rely on the spec range, especially in
> variable environments.
>
>