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

From: Ismail Mohamed
Subject: Re: [Discuss-gnuradio] Question about LFTX and RFTX
Date: Fri, 31 Aug 2007 13:07:30 +1000
User-agent: Thunderbird (X11/20070403)

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:


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.

- 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.

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