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Author Topic: Yes/No question--should be easy.  (Read 3715 times)
KF6GUB
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Posts: 29




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« on: January 23, 2013, 09:18:22 AM »

When an RF carrier is frequency-modulated (FM) by an audio signal, multiple sidebands are produced.  Do each of the sidebands have their own frequency-modulated RF carriers?   thx   Jim
« Last Edit: January 23, 2013, 09:28:54 AM by KF6GUB » Logged
K0BT
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Posts: 196




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« Reply #1 on: January 23, 2013, 09:55:27 AM »

If I understand this correctly, there is still a single carrier.  The sideband distribution is equal on both sides of the carrier, with the upper sidebands 180 degrees out of phase with the lower.

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KF6GUB
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« Reply #2 on: January 23, 2013, 10:01:08 AM »

If I understand this correctly, there is still a single carrier.  The sideband distribution is equal on both sides of the carrier, with the upper sidebands 180 degrees out of phase with the lower.



Is that a single carrier for each sideband?  thx  Jim
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WB6BYU
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« Reply #3 on: January 23, 2013, 10:26:48 AM »

No, one carrier for all sidebands.

As the amplitude of the modulation signal swings from positive to negative, for
example, the instantaneous frequency of the output signal also changes from
above to below the carrier frequency.  But without modulation the signal returns
to the original frequency - that is the carrier.
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KF6GUB
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« Reply #4 on: January 23, 2013, 10:33:54 AM »

Let me summarize:  There is one frequency-modulated RF carrier, the only carrier.   From that carrier emanates the sidebands.  If the carrier is not modulated, there are no sidebands.   Do I have it right?  thx  Jim   Grin
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KB4QAA
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« Reply #5 on: January 23, 2013, 10:47:10 AM »

a carrier has no sidebands.
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WB6BYU
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« Reply #6 on: January 23, 2013, 10:55:15 AM »

Well, pretty much.

You start with a carrier on a particular frequency.  With no modulation (pauses in speech, etc.)
it is a steady carrier.

When you modulate it, the frequency of the carrier shifts with the instantaneous amplitude
of the modulating signal (that's why it is called Frequency Modulation.)

If you modulate the signal with a continuous tone, you can detect sidebands at specific
offsets from the original carrier frequency based on the tone frequency.  But the human
voice is a complex combination of simultaneous frequencies, so in practice you don't detect
specific sidebands on an FM voice signal, you just have an occupied bandwidth.
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KF6GUB
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Posts: 29




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« Reply #7 on: January 23, 2013, 11:03:21 AM »

Well, pretty much.

You start with a carrier on a particular frequency.  With no modulation (pauses in speech, etc.)
it is a steady carrier.

When you modulate it, the frequency of the carrier shifts with the instantaneous amplitude
of the modulating signal (that's why it is called Frequency Modulation.)

If you modulate the signal with a continuous tone, you can detect sidebands at specific
offsets from the original carrier frequency based on the tone frequency.  But the human
voice is a complex combination of simultaneous frequencies, so in practice you don't detect
specific sidebands on an FM voice signal, you just have an occupied bandwidth.

Thank you,  Jim (kf6gub)
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K5LXP
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« Reply #8 on: January 23, 2013, 11:55:41 AM »

There are certain modulating frequencies and indexes where the carrier disappears entirely called a bessel null.  So I guess you could say in those instances you would have all sidebands and no carrier.


Mark K5LXP
Albuquerque, NM

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KF6GUB
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« Reply #9 on: January 23, 2013, 02:00:27 PM »

a carrier has no sidebands.

What does that mean?  Jim
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KB3HG
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Posts: 404




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« Reply #10 on: January 23, 2013, 08:24:36 PM »

Quote from: KB4QAA on Today at 10:47:10 AM
a carrier has no sidebands.

What does that mean?  Jim

No audio , just a dead carrier (CW) one single frequency with out frequency or phase shift in a perfect model.

Mark now you have me thinking, now where is that old communications text book of mine.
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G3RZP
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« Reply #11 on: January 24, 2013, 08:40:03 AM »

The amplitude of the carrier in FM is dependent on the modulating frequency and the deviation. There is only one carrier: its amplitude varies because as the sidebands need power from somewhere, they effectively it take from the carrier. At a modulation index of 2.4, the carrier disappears and as the mod indesx increases, it comes back up and then disappears again.

Read an older ARRL handbook for a good non-mathematical explanation.
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KF6GUB
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« Reply #12 on: January 24, 2013, 11:33:14 AM »

The amplitude of the carrier in FM is dependent on the modulating frequency and the deviation. There is only one carrier: its amplitude varies because as the sidebands need power from somewhere, they effectively it take from the carrier. At a modulation index of 2.4, the carrier disappears and as the mod indesx increases, it comes back up and then disappears again.

Read an older ARRL handbook for a good non-mathematical explanation.

G3RZP:  You're right; that's what I need:  an explanation, non-mathematical !!!  (LOL)   Yet yours is a fairly complete one.  Thanks,  Jim
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G8HQP
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« Reply #13 on: January 24, 2013, 02:50:53 PM »

As has been said, in FM the amplitude of the carrier changes with amount of modulation.

In AM the amplitude of the carrier is unchanged by modulation.

Confusing, isn't it?
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G3RZP
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Posts: 4957




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« Reply #14 on: January 25, 2013, 01:05:13 AM »

Even worse is the fact that some of the old AM rigs also had the carrier frequency shift with modulation. But that was not intended!
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