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Re: explaining i/q

From: Kristoff
Subject: Re: explaining i/q
Date: Wed, 4 Nov 2020 17:14:00 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0

David, all,

Well, I had also been thinking about this.

I do like the idea of the spinning doll. It provides a model for positive and negative frequencies: if it spins one way, the frequency is positive, if it spins in the other direction, it is a negative frequency.

And, it does also give a 'hint' at the issue that a frequency in the "real" domain is both a positive and a negative frequency in the complex frequency domain>

However, the problem is that people associate "spinning" with movement, i.e. a change of the location of an object in time, or -in this case- a rotation around an axis -which also includes an element of time-.

Now, unless I am completely wrong, the model you use captures both the I and the Q samples at the same time. This means that there is no element of 'time'. In electronics, this works fine, due to the nature of mixing and a difference of phase of the two Local-Oscillators. But that's not how people see the spinning doll. Our eyes do not see a difference in phase of light. For us humans, the object is spinning due to the element of "time": multiple observations.

For us, "even if we would be able to look at a rotating object up-front and from a 90 degrees angle at the same time, if the object would be frozen in time we would still not be able to determine if the doll rotates left of right". (except perhaps that we will probably make a assumptions as the doll leans slightly backwards). (*)

(*) https://en.wikipedia.org/wiki/Spinning_dancer

Now, I completely agree that the model I used (taking two samples, 1/4 sample-period apart), is indeed based on only one of the architectures of a SDR-receiver.  And, yes, other SDR receiver architectures exist that are not based on that model. But at least it provides something that is not that far from our daily experience: detecting the direction of a movement by two observations at difference times.

Small sidenote:
That particular receiver-architecture does happen to be one that is used in most amateur-radio DIY receiver-kits. So if a student of the workshop gets to see a schematics of an SDR receiver- it IQ mixing-part should at least ring a bell.

Anycase, don't get me wrong. I really appreciate your feedback.

What you say is completely correct.
The question however is how to "package" your model into something that is easy to understand.

kristoff - ON1ARF

On 4/11/2020 02:58, David Hagood wrote:
Like I said previously:

Think of the spinning dancer illusion. It works because you only see from one vantage point. If you saw a real doll spinning, and assuming you have two eyes and normal binocular vision, you will have parallax, and that will allow you to determine in reality which way the figure is spinning, because each eye will have a different view of what is going on, and so can work out what direction the figure is spinning in.

I/Q is like that - it allows the SDR to see have "parallax" - to have 2 points of view on the signal at the same time, and so it can "see" which way the signal is rotating - whether the signal is spinning clockwise (negative frequencies, cause math) or counter-clockwise (positive frequencies).

There - no advanced math, short, and yet accurate.

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