I/Q samples and Analytic Signals

GNU Radio
1

Hello to all,

I am using GNU Radio to study digital communications systems.
I have been working on the basic FM receiver with RTL-SDR for a while.

I was trying to find out what is the theory behind the complex I/Q
samples.
What I discovered is that I/Q samples are first related to analytic
signals (the pre envelope) and then to the complex envelope of the
signal
(the complex signal in base band).
In fact, what RTL-SDR outputs to the flow graph is the complex base
band signal, in other words, the signal translated to base band with
just
the positive or negative portion of the spectrum.

Please, can someone with more experience confirm to me if I am correct?
Can someone please recommend me some reference books about the theory of
I/Q samples?

Thanks very much in advance,
Lucas Lorenzi Ingles

2

Perhaps, Chapter 2 of “Digital Communications” by John Proakis?

best,
aditya

3

Hi Lucas,

On Mon, Sep 30, 2013 at 11:26:16AM -0300, Lucas Ingles wrote:

I am using GNU Radio to study digital communications systems.

good choice!

Can someone please recommend me some reference books about the theory of I/Q
samples?

There is actually a page on this topic on the web site:
http://gnuradio.org/redmine/projects/gnuradio/wiki/SuggestedReading

Perhaps this helps.

Complex baseband is in fact one of the most important topics to
understand.

MB

–
Karlsruhe Institute of Technology (KIT)
Communications Engineering Lab (CEL)

Dipl.-Ing. Martin B.
Research Associate

Kaiserstraße 12
Building 05.01
76131 Karlsruhe

Phone: +49 721 608-43790
Fax: +49 721 608-46071
www.cel.kit.edu

KIT – University of the State of Baden-Württemberg and
National Laboratory of the Helmholtz Association

4

Thanks very much for the help!
I obtained the book (fifth edition) and already read whole section 2.1
(Band pass and Low pass Signals).
That was exactly what I was looking for.

I have one more doubt. Figure 2.1-6 in page 25 shows a real demodulator
(I
attached the figure here).
This is not the same block diagram of the demodulador that we have
inside
Elonics E4000 Tuner (zero-IF demodulador).
We don’t have a Hilbert transformer inside the zero-IF demodulator. We
have
just two paths, one multiplying by cos() and the other by sin().
Perhaps the Hilbert transform is implemented by handling the I and Q
components as a complex number I+jQ inside GNU Radio?
Can someone please explain to me how the zero-IF demodulator is related
to
the real demodulator shown in the book?

Thanks very much again,
Lucas Lorenzi Ingles

2013/9/30 Aditya D. [email protected]