Re: explaining i/q

[Jeff Long]

This is a great thing to try to figure out. If we can come up with an answer that gives someone a feel for why I/Q is used in SDR in 10 minutes, and does not include phasors, exponentials to a complex power, a derivation of any equation, the concept of orthogonality, etc. ... it will win a Nobel prize in education.

On Tue, Nov 3, 2020 at 4:56 AM gilles rubin <rubingilles@yahoo.com> wrote:

Hello,

You can have a look here

The Concept of Frequency | Wireless Pi

The Concept of Frequency | Wireless Pi

Qasim Chaudhari CEO of Wireless Pi is great ! You will find plenty of information on his website.

Gil.

Le mardi 3 novembre 2020 à 00:06:02 UTC+1, Kristoff <kristoff@skypro.be> a écrit :

Hi all,

I was watching the webinar of Heather on GNU Radio today, when it came

to me that one of the most difficult part doing a presentation of GNU

Radio is the data-types, .. and especially these 'complex numbers'.

The problem, or at least for me, is that when you mention 'GNU Radio

complex numbers', you also have to mention iq-signals, which is a topic

that is very difficult to explain in 10 seconds to an audience who has

never seen anything about i/q sampling before.

I have been thinking on how to explain the concept of I/Q signalling in

just a few lines, e.g. in the context of -say- a workshop on GR.

I have this idea in my head:

Statement:

The main difference between sampling with reals ('floats') and i/q

sampling with complex numbers, is that the latter does not only provide

the  instantaneous value (voltage) of a signal at a certain point of

time, but also includes phase information (i.e. the slope of the signal

at that point).

To make this visual:

Take half a sine-wave and plot it out for every 45 degrees.

This gives you 5 points: 0 (0 degrees), sqrt(2)/2 (45 degrees), 1 (90

degrees), sqrt(2)/2 (135 degrees) and 0 (180 degrees).

Now look at the 2nd and the 4th point (45 degrees and 135 degrees), if

you sample this using only real/float values, these two points are

exactly the same (sqrt(2)/2). Just based on these values by themselves

(i.e. remove all other points from the graph), there is no way you can

tell that at the first point (45 degrees) the graph was going up, while

at the 135-degrees point the graph was going down.

So, ... what i/q sampling does, is for every point "x", it not only

provide the value of the graph at that point of time, but also

information of the slope of the graphs at that time.

This also explains while i/q sampling is done by not just taking the

value of a signal at point "t", but also at 1/4 period later (which is

the information you need to determine the 'slope' of that graph at point

't')

So, ... is this statement correct?

If it is more-or-less correct and it can help provide a basic mental

image of the concept of i/q sampling, I would be more then happy! :-)

73

kristoff - ON1ARF