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.
73
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.
