Modulation questions

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GusWroclaw
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Modulation questions

Post by GusWroclaw » 11 Oct 2018, 15:35

Hello everyone. I'm wondering if someone could help me find an answer to these two questions:

Why when I demodulate as FM an AM signal I can still figure out that it's speech and the tone of the voice etc?

My Alan 42 supports 90% AM modulation. Does that mean that if press PTT and don't speak (transmit silence) the transmitter is not transmitting with 4W but instead with whatever power 10%+50%*90%=0.1+0.5*0.9=55% amplitude corresponds to? Or is 4W the power of the carrier during silence and when I talk some of that power is moved from the carrier to the sidebands?

Cheers! And please tell me if this is the proper topic to post such questions.
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ch25
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Re: Modulation questions

Post by ch25 » 11 Oct 2018, 19:36

GusWroclaw wrote:
11 Oct 2018, 15:35
Why when I demodulate as FM an AM signal I can still figure out that it's speech and the tone of the voice etc?
Always you will understand bits and pieces from AM transmission.
GusWroclaw wrote:
11 Oct 2018, 15:35
My Alan 42 supports 90% AM modulation. Does that mean that if press PTT and don't speak (transmit silence) the transmitter is not transmitting with 4W but instead with whatever power 10%+50%*90%=0.1+0.5*0.9=55% amplitude corresponds to? Or is 4W the power of the carrier during silence and when I talk some of that power is moved from the carrier to the sidebands?
4W carrier and 90% modulation gives 5.62W total AM power and 0.81W sideband power.
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GusWroclaw
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Re: Modulation questions

Post by GusWroclaw » 11 Oct 2018, 20:22

ch25 wrote:
11 Oct 2018, 19:36
GusWroclaw wrote:
11 Oct 2018, 15:35
Why when I demodulate as FM an AM signal I can still figure out that it's speech and the tone of the voice etc?
Always you will understand bits and pieces from AM transmission.
Thanks, I am wondering though how is this explainable? Why would the discriminator "see" a peak moving around and a way that mimics the frequencies of the sound that is transmitted, when in AM there is only an non-moving carrier plus two sidebands which simply show the frequencies of the sound transmitted? I assume that through some trigonometric magic the FM frequency spectrum is not completely dissimilar to the AM frequency spectrum but I was wondering if there is some way to understand this intuitively.
ch25 wrote:
11 Oct 2018, 19:36
GusWroclaw wrote:
11 Oct 2018, 15:35
My Alan 42 supports 90% AM modulation. Does that mean that if press PTT and don't speak (transmit silence) the transmitter is not transmitting with 4W but instead with whatever power 10%+50%*90%=0.1+0.5*0.9=55% amplitude corresponds to? Or is 4W the power of the carrier during silence and when I talk some of that power is moved from the carrier to the sidebands?
4W carrier and 90% modulation gives 5.62W total AM power and 0.81W sideband power.
Interesting! When you are transmitting silence, the sidebands obviously need to transmit 0 energy (otherwise you'd be actively modulating the carrier but during silence, you are not). Does that mean that those 0.81W are "lost"? Also why don't those numbers add up? 5.62W-0.81W=4,81W and not 4W like I would expect from what you said. So does that mean that the carrier carries more energy and 4W is the energy it has only when you are transmitting silence?

Also I assume that this 0.81W in the sidebands is the maximum energy that they could carry, when the full amplitude is used, from 10% amplitude up to 190% (of the carrier's "at rest" amplitude).

If you have some article that I could read online I'm fine with a link as well!

Thank you for your answer.
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Re: Modulation questions

Post by Tigersaw » 11 Oct 2018, 23:01

If your radio has a slope detector for FM demodulation the following may help describe better than I can;
https://www.radio-electronics.com/info/ ... inator.php (see the entry under disadvantages)

As for AM power, you would have to measure the carrier power with no modulation to work out what figures they are working to.
at 100% modulation depth each sideband has 25% of the total power.
https://www.radio-electronics.com/info/ ... ciency.php

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Re: Modulation questions

Post by GusWroclaw » 12 Oct 2018, 09:05

Tigersaw wrote:
11 Oct 2018, 23:01
If your radio has a slope detector for FM demodulation the following may help describe better than I can;
https://www.radio-electronics.com/info/ ... inator.php (see the entry under disadvantages)

As for AM power, you would have to measure the carrier power with no modulation to work out what figures they are working to.
at 100% modulation depth each sideband has 25% of the total power.
https://www.radio-electronics.com/info/ ... ciency.php
Thank you for both of the links. The second one covered my questions regarding power. It also mentioned something that I hadn't thought about, the fact that the carrier in AM can be used to find what's the relative strength of the audio encoded in the sidebands compared to maximum modulation. In other words in AM if the power of the signal is going up and down due to propagation, you can always keep the sound at the proper level and only have the noise go up and down relatively to the signal. And I assume that in SSB this is impossible and the only volume control you can do is some kind of AGC on the sound of your speaker. In other words in SSB there is no way to distinguish between a situation where the signal is fading because of propagation worsening and a situation where the speaker is simply lowering the volume of his voice. Again in other words: if you try to transmit a low volume signal in SSB, it will be either amplified by the sound processing on the transmitter and if it's it doesn't get amplified there it will be amplified on the receiving end since it will be "confused" for a weak SSB signal.

As for the FM demod, I doubt a modern CB would be using that kind of demod but nevertheless it's a very interesting way of demoding FM. I still wonder what happened in my case though. Maybe I'll look into the maths that describe the FM spectrum. (are they called sidebands in FM?)
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