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General Radio Discussion / Re: I could use some help understanding SSB bandwidth

« on: December 09, 2024, 2359 UTC »

I think you've got a good sense of understanding with those charts. Now you're running into the limits of what these charts can show you.

The issue with these charts is they are in the time domain. That is time advances as you read them left-to-right while showing the absolute value of a signal at a particular point in time. All's well until the signal becomes more complex than a continuous wave or amplitude modulated audio.

When we start looking at more signals with more nuanced modulation, we reach for the frequency domain. Signals charted in the frequency domain show the amount of energy at a particular frequency. Sometimes frequency domain charts use color to show energy at a frequency, with a new line of colors added every few milliseconds. That kind of frequency domain chart is a waterfall chart, but now with time as a third dimension (power-frequency-time).

Understanding the frequency domain is why skeezix was suggesting using a web based SDR. They'll often have a waterfall that can help get you that intuitive feeling of modulation in the frequency domain.

I have three videos in mind that might get you started. First up is a ham radio related one that directly addresses your struggle:

Dave Casler - Why We don't look at Single Side Band in the Time Domain: https://www.youtube.com/watch?v=F5zuLi19ar4

While not RF related, getting an intuitive understanding of sine waves can be helpful. I think Posy did a great job in visually, and interestingly, breaking it down.

Posy - Every sound is SINE: https://www.youtube.com/watch?v=UrBZsUBibtk

This last one may seem a bit thick, but trust me in that it's completely approachable and that you don't have to do the math yourself. Understanding Fourier series at a very high level is a good way of getting a better grasp of frequency domain charts.

3blue1brown - But what is a Fourier series? From heat flow to drawing with circles | DE4: https://www.youtube.com/watch?v=r6sGWTCMz2k

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General Radio Discussion / Re: I could use some help understanding SSB bandwidth

« on: November 15, 2024, 0748 UTC »

Long time lurker, first time poster here. Hopefully the admins don't mind our noise.

It's a bit hard to understand what you're trying to convey, but I think I'm following what you're pondering about. The first key is defining bandwidth. Let's start there.

> if we were transmitting a 3kHZ signal on a 20kHZ carrier wave, using SSB, I believe the conventional wisdom is that the bandwidth would be 23kHZ

This is wrong in the sense that if you had a 3khz tone on a SSB signal, then the bandwidth would 3khz. If you had a simultaneous 1khz, 2khz, and 3khz tone on a SSB signal, then the bandwidth would still be 3khz. If that SSB was located at 1Mhz, it'd be 3khz wide. If it was located at 5mhz, it'd be 3khz wide. If it were at 7.200mhz, it'd be 3khz wide with wackos talking. So what gives?

The word bandwidth is conveying the width of a signal. That's measured by noting where a transmission's RF energy is above and below a center frequency. Taking the difference between those two points is the bandwidth.

Let's work with your 20khz example. Assume you have an AM transmitter with a 20khz carrier wave modulated with a 3khz sine wave. What you'll see is RF at 20 - 3 and 20 + 3 khz. Mark those two points, being 17 and 23, and take the difference: 23 - 17 = 6. That AM signal has a 6khz bandwidth. Doing the same at 40khz: 43 - 37 khz = 6khz. Taking the difference is why the bandwidth of a signal is independent of the signal's center frequency.

What makes SSB special are the points in which you'll find that RF energy around a center frequency. By removing the carrier and opposing side band, the difference between the lowest and highest frequency in which you measure RF power is going to be half that of AM. For example, a USB signal with 3khz wide audio transmitted at 20khz would have RF energy at 20khz through 23khz, hence 23 - 20 = 3khz bandwidth.