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Amplitude Modulation

Jan 24th, 19:30

GrandAdam12

Joined: Jan 18th, 14:06
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I am studying to take the technician exam and had a question I was hoping someone could answer. I am using the ARRL Ham Radio License Manual and I am in Chapter 5. In figure 5.2, the book describes how voice/data is added to the signal with an unmodulated carrier at 800kHz and two side bands (where the voice/data is transmitted), a USB and LSB so there is an effective frequency range of 797kHz to 803kHz. My question is this, how is that actually transmitted? Is it a single sine wave whose frequency 'changes' as it is generated or is it multiple parallel waves transmitted simultaneously that need to be in 'phase'?

I appreciate any direction or whack over the head for asking such a newbie question...LOL

Thank you,

Tom
Jan 25th, 09:14

W1VT

Super Moderator

Joined: Apr 4th 1998, 00:00
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Multiplying the 800kHz carrier with a 3kHz tone automatically creates upper and lower sidebands. This can be done with large amounts of power. A 375 watt carrier mixed with enough audio power can generate the legal limit of 1500 watts. Nothing more than low pass filtering may need to be done to the signal.

Zak W1VT
ARRL Senior Lab Engineer
Jan 25th, 09:39

GrandAdam12

Joined: Jan 18th, 14:06
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Thanks W1VT but are these sidebands part of the same 'wave' or signal that is generated just at different frequencies?
Jan 25th, 10:55

WA0CBW

Joined: Apr 4th 1998, 00:00
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Think of each sideband as a signal. So when modulating with a single tone there are three signals, the lower, the carrier, and the upper. If using voice there will be many lower and upper (and carrier) sidebands each corresponding with the frequencies of your voice.
Bill
Jan 25th, 13:51

GrandAdam12

Joined: Jan 18th, 14:06
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Great, thank you. Just helps me to understand better when I can visualize what is happening.
Jan 25th, 14:17

GrandAdam12

Joined: Jan 18th, 14:06
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Just a follow-up that popped into my head. Being separate signals at different frequencies above and/or below the carrier, how does the receiver know that the sideband belongs to the carrier?
Jan 26th, 08:31

W1VT

Super Moderator

Joined: Apr 4th 1998, 00:00
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The simplest technique is to use the carrier to help demodulate the signal. A simple diode detector can recover the modulation envelope if both sidebands are present, but will distort if one of them fades as in "selective fading."

Zak W1VT
ARRL Senior Lab Engineer
Jan 26th, 11:08

GrandAdam12

Joined: Jan 18th, 14:06
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Total Posts: 0
Interesting. I think I am putting this together. I work in IT and the term in networking we use is 'encapsulation' for essentially moving packets with different headers. In radio, it sounds like the term 'envelope' does the same with signals...Thank you,

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