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 [email protected] http://observer.wunderwood.org/ (my blog) On Mar 3, 2015, at 4:18 PM, Al Lorona <[email protected]> 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 > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[email protected] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to [email protected] ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected]
