[Elecraft] I/Q

[Walter Underwood]

I/Q can be continuous frequency, not just sampled, though the digital form is the most common these days.

Don’t be worried if this takes a while to absorb. It is a shock to pretty much all EE students, right up there with Dr. Burrus’s lecture on negative frequency (with the omega belt buckle). I/Q is a natural fallout of the complex Fourier transform, but that does not make it intuitive.

wunder
Walter Underwood
wunder at wunderwood.org
http://observer.wunderwood.org/  (my blog)


On Mar 3, 2015, at 4:18 PM, Al Lorona <alorona at sbcglobal.net> wrote:

> Hi, Richard,
> 
> These are very good questions. You have already been given links to tutorials on the subject, but here are some short answers to your questions to get you thinking.
> 
> 1) The I and Q signals do have a "frequency", but it's called 'sample rate'. I and Q are constantly changing, but they are being sampled, or measured, at a regular rate which is the sample rate. By the time I and Q appear at the input of your soundcard they are considered 'baseband' or 'audio' and no longer have a carrier frequency associated with them because they've been demodulated. Think about this: does CW coming out of your speaker have a frequency? Well, not a carrier frequency, because it's been removed in the detector or demodulator, and besides we can't hear at the carrier freq, but the CW definitely has a 'words per minute' rate which your ear locks on to when it copies the CW. This is kinda like sample rate. You could call it 'data rate'.
> 
> 2) I and Q are always 90 degrees out of phase. But their absolute phase is unknown. So once you see I and Q, if they aren't squared up you can rotate them artificially so that they line up on the X and Y axes that you see in all the math books. This is easily done with a phase shifter, which is just adding a delay to I and Q. If you take a picture of a football field, and your camera wasn't perfectly parallel to the chalk lines, your mind automatically 'adds phase' so that the lines are nice and square in your mind. That's kinda how it works in a demodulator. I hope I answered the question you had.
> 
> Al  W6LX
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