Messages

1

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

« on: December 14, 2024, 0344 UTC »

I'm pretty sure I understand this now.  For one thing, I understood exactly what you were saying in you last post .

I had all the parts in my toolbox already, but I was stuck on the notion that AM didn't mess with the frequencies.  Maybe I was never told, and maybe I missed it, but I wasn't aware that the carrier was re-introduced to the signal at the receiving end.  I probably just missed it, since I couldn't get past the part about sending a signal on a carrier wave that had been removed.

NJQA, I'm going to remember two of your statements from you last post - they're quotable:

1. Teaching is just a process of telling smaller and smaller lies.
2. As engineers we know that we only need to know it well enough so that we can build things.

Great and true statements.


I want to thank everyone on this post for their help.  It's allot to ask people to take the time to type up a response to someone's question who seems to have no idea what he's talking about.  You never know if you're helping to fill in a hole, or throwing dirt into a bottomless pit.  But you guys took the time to push some dirt in the hole until I was able to climb out, and it's a great feeling to resolve the matter of whether I'm not quite smart enough to understand a concept or just looking at things the wrong way.

By the time I finished watching PC486's first link I understood what I was missing.

2

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

« on: December 13, 2024, 0330 UTC »

I think I get it.

I would describe it as follows:

Keeping in mind that any waveform can be broken down into specific set of component sinewaves, the SSB process can be understood as follows:

1.  Perform amplitude modulation upon the carrier frequency with the signal waveform, resulting in a time-domain waveform that looks like the carrier frequency with the shape of the signal waveform represented by its amplitude.

2.  Picture this amplitude modulated signal as a specific set of component sinewaves.

3.  Once the signal is created, remove the carrier frequency and every frequency below it in the frequency domain and transmit the rest.

4.  On the receiving end, provide the same carrier frequency and add it to the received set of component sinewaves to rebuild half of the original signal.

Please don't tell me I'm wrong.  This finally makes sense to me.   

3

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

« on: December 13, 2024, 0209 UTC »

pc486 and NJQA, thank you for the responses.  I'm sorry for not having the time to get back to you sooner.  I just read your posts now.  Give me some time to read the resources you suggested and soak it in, and I'll get back with you.

I did allot with Fourier Transforms when I was in school, but that was a long time ago.  I was pretty good at the math back then, but I'm sure that's not the case today.  However, I remember the gist of it - transferring a signal from the time domain to the frequency domain.  Although I thought I understood the concept, I don't deny that there could be more for me to learn.

3blue1brown is one of my favorite YouTube channels.  I watch it with my daughter from time to time.  That kid amazes me.  I wish I had half the mathematical intuition that he has.

Thanks again - I'll get back to you soon, I hope.

4

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

« on: December 09, 2024, 2216 UTC »

What you're describing is what I picture for FM modulation.  For AM modulation, I understood there to be a carrier wave that is typically much higher frequency than the signal being sent, and a message signal that modulates the amplitude of the carrier so that the shape of the carrier wave has the shape of the message signal.  This way, without the carrier, there is no signal.

The situation where the message signal bandwidth is present above and below the carrier frequency sounds like FM modulation to me.  I found images to share, but I don't know if I can share images here without some kind of repercussions, so I have a link.

https://reviseomatic.org/help/2-modulation/Amplitude%20Modulation.php

if you look at the second image on this page, you'll see how I understand it to be.  And you'll see that without a carrier wave, there's no signal at all.

Please comment back.

Thanks,

5

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

« on: December 07, 2024, 1650 UTC »

Guys, thanks for the help.  What you're describing is the way I've always understood bandwidth and AM signals.  But there MUST be something I'm missing, because I don't understand how there's a signal at all without the carrier signal.  When I read that the carrier signal is removed from SSB transmission, I thought they were mistaken, but I couldn't find anything that said they were wrong.  That sent me into the mind-bend that produced the post above, as I second guessed everything I ever though I knew about AM transmission.

If someone could just explain to me how the carrier signal could possibly be removed from the transmission in AM signals, I would be forever grateful.  It just doesn't compute.  The carrier signal IS the signal.  There's nothing else transmitted.

6

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

« on: November 10, 2024, 1802 UTC »

I thought I understood AM modulation, but now I have to question my understanding, since I've found several sources that explain the bandwidth of SSB transmission  differently than what I think it should be.

Please allow me to present an example that conveys my confusion:

Suppose a carrier signal of 20kHZ is used to transmit a 3kHZ sinewave. The receiver can reconstruct the sinewave from the amplitude of the carrier signal. Therefore, the bandwidth consists of only one frequency, 20kHZ. If we consider the 3kHZ sinewave that's being superimposed upon the carrier wave, we could say that in addition to the 20kHZ carrier wave, there is also a 3kHZ sinewave imposed upon the top of the carrier signal and a mirror image signal that's imposed upon the bottom. However, both the top and the bottom signals that are imposed upon the carrier frequency would be a 3kHZ sine wave, not 23kHZ. This 3kHZ sinewave, if it were transmitted with a higher frequency carrier wave, say 40kHZ, would have greater definition, but the same frequency, 3kHZ. The frequency of the transmitted signal is independent of the carrier frequency.

What I envision is 2 signals that compose the transmission. A 20kHZ carrier frequency, and the 3kHZ signal. There is nothing being transmitted at 10kHZ for instance. However, in the scenario I described above, 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 doesn't make sense to me. We're not using 23kHZ of bandwidth. We're using one frequency only.  If we want to express the bandwidth to represent the entire spectrum of frequencies that include both the signal and the carrier wave, then that would be 20kHZ - 3kHZ = 17kHZ.

Can someone help me make sense of this?