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Re: explaining i/q

From: Jeff Long
Subject: Re: explaining i/q
Date: Tue, 3 Nov 2020 20:21:09 -0500

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> 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 living.
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 systems,
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 I've
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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