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Author Topic: Yes/No question--should be easy.  (Read 2846 times)
KB4QAA
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Posts: 2265




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« Reply #15 on: January 25, 2013, 02:50:39 AM »

a carrier has no sidebands.

What does that mean?  Jim
A carrier is a signal on a single frequency which has no intelligence impressed upon it.  It is of nominal width. 

That is a carrier considered on it's own.  It doesn't have sidebands, (they imply additional width).   Sidebands are not part of the carrier.  They are related but separate parts of the total transmitter signal.
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K9YLI
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Posts: 848




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« Reply #16 on: January 25, 2013, 08:20:15 AM »

Now  I'm  confused.     In  broadcast school  I learned that  an   AM  carrier, modulated by a sine wave  at  100%, would  have the  carrier  amplitude  doubled on the  plus   half cycle and  driven to  zero  on the  minus  half cycle.
Thats why  chicken banders  cant   over modulate without distortion.
You can go as high as your  modulator will drive the  carrtier  amplitude, 200% if you want,
but  on the negative half cycle  you can't  go below  ZERO..

FM should in practice  have no  amplitude change,,  just  frequency excursions.
The two components are   how  "loud" is the modulation signal, and   how "fast' (audio frequency) does it change.    If I remember right   'loud' determines  how far the freq changes, and  'freg' (audio) how quickly it changes.

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G3RZP
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« Reply #17 on: January 25, 2013, 09:52:07 AM »

If you look with a narrow band receiver or a spectrum analyser, in the AM case the carrier stays constant in power and in frequency. The extra power is the provision of the AM sidebands, and that power comes from the modulator - in the case of straight screen grid modulation, you use the gain of the PA tube, which is why you don't need  as much modulation power as you do in plate and  screen modulation or, with a triode, straight plate modulation. So as you modulate, the total output power increases. You also have  the situation that where the receiver detector is a diode, modulation in the downwards direction leads, as the mod depth exceeds about 90 or 95%, to increased distortion.

In the FM case, the total output power across the "bandwidth" (we'll come to what that means later) of the signal is constant: you don't see any change on your dummy load with a thermoammeter. Look with the spectrum analyser or narrow band receiver, and as you increase the modulation level i.e. deviation and modulation index, the various sidebands vary in level, as does the carrier, because they have to all add up to the unmodulated carrier power. An if you use a bandwidth in the spectrum analyser much less than the modulating frequency, you will see the carrier in FM doesn't move in frequency, only in amplitude. The carrier is the signal at the frequency radiated when there is no modulation.

So what you see depends on the bandwidth of the measuring device: in 20KHz measurement bandwidth with comms type speech, you won't see a change with FM, and with AM, the thermo ammeter will increase by its reading by 22% at 100% modulation. This is because the thermoammeter method is reading rms current. Not totally sure what the Bird Thruline would read on AM.

The 'bandwidth' of the signal is usually taken as the 'occupied bandwidth i.e. the difference in frequencies between which 99% of the energy lies.

And for FM don't start me with Bessel functions. I could never get them right over 40 years ago, and I surely am not going to even start to try again at my age!
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WS3N
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Posts: 649




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« Reply #18 on: January 25, 2013, 10:49:36 AM »

And for FM don't start me with Bessel functions. I could never get them right over 40 years ago, and I surely am not going to even start to try again at my age!

But that's the prettiest part!
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G3RZP
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« Reply #19 on: January 25, 2013, 01:14:45 PM »

OK, you explain them, then!
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WS3N
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Posts: 649




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« Reply #20 on: January 25, 2013, 08:15:07 PM »

OK, you explain them, then!

I should have said it's the prettiest part, but it's beauty is not widely appreciated.  Cheesy
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W5LZ
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Posts: 477




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« Reply #21 on: February 04, 2013, 09:17:38 AM »

An FM, Frequency Modulated, signal has no side bands as such.  The 'sideband' that's normally thought of as the applied modulation of an AM signal is the amount of deviation in frequency from the center frequency of the signal.  The amplitude or strength of an FM signal remains constant, doesn't vary  The amplitude isn't changed by the application of intelligence to that signal,  the frequency changes 'slightly'.  That change in frequency is called the variation and contains the intelligence applied to that base frequency.
An FM signal displayed on a 'scope is entirely different that that of an AM signal.  It's all in the name, as in 'amplitude' modulation and 'frequency' modulation.  The difference is in HOW that modulation is applied to an RF signal.
 - 'Doc
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K8AXW
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Posts: 3651




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« Reply #22 on: February 04, 2013, 09:26:29 AM »

GUB:  Of all the years I've haunted the eHam forums I have NEVER seen a question that could be answered by a YES or NO!   Roll Eyes
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AA4PB
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Posts: 12672




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« Reply #23 on: February 04, 2013, 09:30:08 AM »

"An FM, Frequency Modulated, signal has no side bands as such"

Oh yes it does. The sidebands are dependent upon the modulation index (the ratio of maximum deviation to modulating frequency). The FM signal occupies more bandwidth than twice the maximum deviation.
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G3RZP
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Posts: 4366




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« Reply #24 on: February 04, 2013, 11:40:02 PM »

"An FM, Frequency Modulated, signal has no side bands as such"

It definitely does. You only need to use a spectrum analyser or a narrow band receiver to see them.

The math says it does, too.
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AA4HA
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Posts: 1378




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« Reply #25 on: February 05, 2013, 04:19:45 AM »

It may be clearer to think of it as "deviation" from the center frequency carrier and the frequency of the deviation is the frequency of the intelligence. Like FSK tones on FM, that makes things intuitively obvious.

That is oversimplified of course as with an FM signal like from a commercial broadcast in the 88-108 MHz band you have sub-carriers (FM pilot, R+L, R-L, sub-carriers like SCA broadcast) that may run 100 KHz away from the center frequency. It all was so theoretical in school and only really made sense when I had plenty of time to look at what was going on with a spectrum analyzer.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
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