Two Birds with One Tone: I/Q Signals and Fourier Transform
24 comments
·July 29, 2025KeplerBoy
dsp_man
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 :)
yodon
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.
gsf_emergency_2
Q=Quadrature, I=In-phase
(As you point out not in the first couple pages, but waaay down)
he "explains" those
https://wirelesspi.com/two-birds-with-one-tone-i-q-signals-a...
Not trying to be charitable like furgot ... The wikipedia page is the first time I've seen authors go pro on the topic
wucke13
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
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
Why would you want two carriers?
dsp_man
Thank you for the suggestion. That's the point. I/Q introduced early gets too complicated. This foundation needs to be built up.
ykonstant
Only people with a low I/Q would misunderstand this notation!
furgot
Most technical writing is going to assume some familiarity with the discipline. If a reader encounters unfamiliar vocabulary in a technical article, they'd be well advised to look it up.
https://en.wikipedia.org/wiki/In-phase_and_quadrature_compon...
mikewarot
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
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
> existing audio I/O
I never knew there even is such a thing. Where can I find it?
galangalalgol
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.
myahio
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.
MrBuddyCasino
This reads like someone proficient in signal processing is explaining the core concepts to another person who is already proficient in signal processing.
sevensor
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.
null
esafak
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
Come be a DSP engineer. I take FFTs of IQ data almost every single day
cycomanic
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
Work on low-level software for communications, especially RF, and you will see plenty of this stuff.
cycomanic
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.
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.