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Re: explaining i/q

From: Christophe Seguinot
Subject: Re: explaining i/q
Date: Wed, 4 Nov 2020 14:29:51 +0100
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I do agree with Jeff Long.

You will have to distinguish Sampling and IQ:

Regards, Christophe

On 04/11/2020 13:50, Kristoff wrote:

Concerning the term "slope". Well, I also have my doubts about it. I think that for a lot of people, this would create the assumption that the signal then goes from the 'i' value to the 'q' value in a straight line, which is -as we know- not the case.

Sometimes it helps to -at first- give a very basic mental image of something, and -at the end, when people understand the topic- "correct" that image with a more correct one, or just point them to some youtube video that explains the topic in more detail.

Anycase,this is indeed all an interesting exercise in braking down concepts into very small steps.

The amateur-radio community is a bit strange as most people do have a technical background, but for a large number of hams, that is mainly based on assumptions or "that's what they said in the ham-radio courses", without understanding the full technical details, especially topics that are highly based on math.
For most hams, "SDR" is just "that piece of software you install on your computer to look at  waterfall graphs".

So we have a very long way to go. :-)

kristoff - ON1ARF

On 4/11/2020 02:21, Jeff Long wrote:
It's more important to give people some mental picture than to make sure it's completely correct. But, I would not use the "slope" terminology. The important things are, as you've said, (1) with the complex type, you can have a signal at baseband that is not symmetric, and (2) the price for this is doubling the amount of data needed. The signal you deal with at baseband is the same signal that is seen centered on the RF carrier.

I don't see a great way to talk about "phase" without going into the math. It is important to get into "phase" when you talk about any modulation fancier than slow FSK.

Good luck. Hope you find the right balance between useful, digestible, and correct.

On Tue, Nov 3, 2020 at 7:20 PM David Hagood <david.hagood@gmail.com <mailto:david.hagood@gmail.com>> wrote:

    I am sorrowful that you have decided you are going to stick with an
    explanation that is fundamentally incorrect. I know how direct
    conversion systems work - I design the software for them for a
    What you are basing your mental model on is an optimization for
    the case
    where the system is both sub-sampling the signal and going digital in
    the same operation. However, in many extremely high sample rate
    the signal is brought down to baseband by mixing it with analog
    quadrature signals - that's the place where I and Q come from - and I
    assure you the only "delay by 90 degrees" is in the creation of the
    quadrature LO signals, not in the sampling of the actual data. But
    been around the Sun enough times to know that since you have decided
    upon this course and don't seem to want to change, there's no
    point in
    continuing to try to help.

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