Two Birds with One Tone: I/Q Signals and Fourier Transform
wirelesspi.com

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103 points

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teleforce

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3 days ago

27 comments

KeplerBoy ・ 3 days ago

The liberal use of AI generated images really cheapens the entire article. Please don't do it. At that point I suspect most of the text is also AI generated.

  • dsp_man ・ 2 days ago

    Only two images are AI generated where horses as carriers needed to be shown. Can you please explain why this is a problem? Thank you (Also, all text is written by me :)

    • KeplerBoy ・ 2 days ago

      I guess I just don't like the style of the images, they just scream "AI slop ahead!". Also I don't think they add anything of value to your article, which is indeed well written.

      So kudos from someone working on radar signal processing!

    • CamperBob2 ・ 2 days ago

      Safe to say that 150 years ago, his great-great-grandpa was ranting about photography putting painters out of work.

MrBuddyCasino ・ 3 days ago

This reads like someone proficient in signal processing is explaining the core concepts to another person who is already proficient in signal processing.

  • sevensor ・ 2 days ago

    Exactly right! As somebody who’s spent a great deal of time with the discrete Fourier transform, I thought, “this article reads like it was written specifically for me.” I/Q modulation is new to me though.

yodon ・ 3 days ago

Pro tip: If you're writing an article on the significance of something called I/Q, it's cool to somewhere in the first couple pages say something about what I/Q is.

  • wucke13 ・ 3 days ago

    https://www.pe0sat.vgnet.nl/sdr/iq-data-explained/

    This is an excellent introduction to the concept and also to the why complex numbers are used to represent signal samples.

    • msravi ・ 3 days ago

      I prefer a more "physical" explanation - you have two carriers: sin(wt) and cos(wt), and you're modulating bits I and Q onto the two carriers and adding them up before transmitting. Now, mathematically, that's the same as representing the two bits as I+jQ and multiplying it with cos(wt)+jsin(wt). Demodulation is simply multiplying that output with the complex conjugate cos(wt)-jsin(wt), which in physical terms translates to mixing with a local oscillator output and low pass filtering.

      • exe34 ・ 3 days ago
        ・ 2 more

        Why would you want two carriers?

        • Sesse__ ・ 3 days ago

          Twice as much information.

          My go-to for I/Q is: Having two allows you to represent negative frequencies. With a normal, real signal, this is of course impossible (negative frequencies will automatically mirror the positive ones), but if you have a signal centered around e.g. 1 MHz, there's room for above-1MHz and below-1MHz to be meaningfully different. And _that_ allows you to get a complex signal (I/Q), once you pull the center down to 0 Hz for convenience of calculation.

  • dsp_man ・ 2 days ago

    Thank you for the suggestion. That's the point. I/Q introduced early gets too complicated. This foundation needs to be built up.

  • ykonstant ・ 3 days ago

    Only people with a low I/Q would misunderstand this notation!

mikewarot ・ 3 days ago

Yikes - why even mention the E and B fields? They aren't relevant to the rest of the article.

A few hours playing with Sine and Cosine generators in GNU radio can take you from book knowledge of I/Q complex signals into fully grokking it. You don't even need a radio, just your existing audio I/O.

  • dsp_man ・ 2 days ago

    I mentioned E and B fields so that the reader knows why we focus exclusively on sinusoids. Plus, linking the sinusoid to something we see in physics makes it more real.

  • ygritte ・ 2 days ago

    > existing audio I/O

    I never knew there even is such a thing. Where can I find it?

    • galangalalgol ・ 2 days ago

      There is a source block for your mic and or audio in. That was one of the first things I played with to understand sdr. I remember seeing a strong tone on a waterfall plot that I could not hear, thinking it was an articfact, then looking at the frequency and realizing I wouldn't be able to hear it. Turned out it was a crt TV. That kind of dates the story. Fun to be had.

      • ygritte ・ 2 days ago

        Dang, I just realized that I misread it. I was seeing "I/Q" where it says "I/O". My bad.

myahio ・ 2 days ago

Are there any other high-quality sources for articles about signal processing and its actual application in hardware/software? I've taken signal processing classes at my college and while I have a good grasp of the theory I struggle with actual use case ideas, beyond implementing a simple fir filter on a stm32.

esafak ・ 3 days ago

Yet another thing from school I've never used in the software world.

By the way, QAM is (still) used in 4G and 5G.

  • pythonguython ・ 3 days ago

    Come be a DSP engineer. I take FFTs of IQ data almost every single day

  • cycomanic ・ 3 days ago

    Some variation of QAM will always be used in communication. As soon as you deal in with EM-waves, be it physics, engineering or even biomedical stuff you will have to deal with complex numbers, which by extension is dealing with I/Q signals. You probably don't need this for programming a server or a website, but it's indispensable for signal processing.

  • userbinator ・ 3 days ago

    Work on low-level software for communications, especially RF, and you will see plenty of this stuff.

    • cycomanic ・ 3 days ago

      Not just RF, also optical communications. Really, the only domain left where PAM transmission is used is baseband communication for electronics, and datacom for optics.