Amplitude modulation - sidebands
Hi, I'm trying to understand AM/sidebands intuitively:
https://en.wikipedia.org/wiki/File:Amfm3-en-de.gif
(I don't want a mathematical definition - no Fourier and Sin
representation)
Is the AM wave-output the sum of three vectors - where the two sidebands cancel out and leave only the carrier wave suitably modulated?
Basically if you look at that diagram (just that red graph) - there are three waves:
1'st is the red line
2'nd and 3'rd are the upper envelope and lower envelope
Would it be reasonable to think of the two envelopes as being created by two vectors that cancel out? With the modulated carrier being traced by the third vector? In which case how are the vectors being generated from the signal and carrier?
The signal to be modulated has Amplitude, Phase and Frequency info that needs to be stored in the carrier. So what data is going where because the carrier also has a certain amplitude, frequency and phase.
If the amplitude of signal is stored in the amplitude of the carrier thus creating the sidebands (by stretching the carrier over/under a mean) then why does the spectrum analyzer show the sidebands at two different frequencies - where is that information coming from?
https://en.wikipedia.org/wiki/File:Amfm3-en-de.gif
(I don't want a mathematical definition - no Fourier and Sin
representation)
Is the AM wave-output the sum of three vectors - where the two sidebands cancel out and leave only the carrier wave suitably modulated?
Basically if you look at that diagram (just that red graph) - there are three waves:
1'st is the red line
2'nd and 3'rd are the upper envelope and lower envelope
Would it be reasonable to think of the two envelopes as being created by two vectors that cancel out? With the modulated carrier being traced by the third vector? In which case how are the vectors being generated from the signal and carrier?
The signal to be modulated has Amplitude, Phase and Frequency info that needs to be stored in the carrier. So what data is going where because the carrier also has a certain amplitude, frequency and phase.
If the amplitude of signal is stored in the amplitude of the carrier thus creating the sidebands (by stretching the carrier over/under a mean) then why does the spectrum analyzer show the sidebands at two different frequencies - where is that information coming from?
It's really nothing more than constructive and destructive cancellation when the carrier and signal become in and out of phase.
In it's simplest text book example with sine waves, the two frequencies are the carrier waveform and the modulation (signal) waveform.
If these were produced as sound and propagated independently, three frequencies are observed:
a) fundamental (hf carrier) = fc
b) information (audible frequency amplitude-modulation signal) = fm
c) the beat frequency of the two combined (harmonic frequency produced at carrier frequency +/- audible frequency) = |fc - fm|
The sidebands in AM are nothing more than the harmonics produced by the addition and subtraction of the two as they become in and out of phase.
The complex maths (Fourier) describes this beat phenomena as a trigonometric series expansion and allows modelling of complex waveforms by decomposing to an infinite series of summed cosine waveforms.
In it's simplest text book example with sine waves, the two frequencies are the carrier waveform and the modulation (signal) waveform.
If these were produced as sound and propagated independently, three frequencies are observed:
a) fundamental (hf carrier) = fc
b) information (audible frequency amplitude-modulation signal) = fm
c) the beat frequency of the two combined (harmonic frequency produced at carrier frequency +/- audible frequency) = |fc - fm|
The sidebands in AM are nothing more than the harmonics produced by the addition and subtraction of the two as they become in and out of phase.
The complex maths (Fourier) describes this beat phenomena as a trigonometric series expansion and allows modelling of complex waveforms by decomposing to an infinite series of summed cosine waveforms.
(edited 8 years ago)
https://www.youtube.com/watch?v=dNx70orCPnA
This video has two sources interfering (diffraction) - it's a mechanical wave - but we can treat it as EM for the discussion.
The surface of that mech-wave is our modulated carrier right? (One sinusoidal src-carrier busy interfering with another-signal src)
Where are the harmonics? Where exactly in that mech wave would the information be stored?
There are some dark-shadow-lines radiating out from the center - that's cancellation (diffraction - a trough and a crest).
The ripply stuff are the data stored in a changed amplitude? Why then we do we mark out the envelopes from peak to peak?
This video has two sources interfering (diffraction) - it's a mechanical wave - but we can treat it as EM for the discussion.
The surface of that mech-wave is our modulated carrier right? (One sinusoidal src-carrier busy interfering with another-signal src)
Where are the harmonics? Where exactly in that mech wave would the information be stored?
There are some dark-shadow-lines radiating out from the center - that's cancellation (diffraction - a trough and a crest).
The ripply stuff are the data stored in a changed amplitude? Why then we do we mark out the envelopes from peak to peak?
for radio the carrier is typically much higher frequency than the signal.
e.g. carrier 1152 kHz signal 4kHz
most diagrams of amplitude modulation in books and on the internet are not to scale
the time between peaks of the 1152 kHz carrier would be 0.868 μs
the time between peaks of the 4 kHz signal would be 0.25 ms
it's difficult so show what's going on in a scale diagram
if you amplitude modulated a 1152 kHz carrier with a 4kHz sine wave you'd have RF energy at the 1152 carrier frequency as well as at the sum and difference frequencies uber mentioned (1148 kHZ and 1156 kHz)
This isn't similar to having 2 sources of the same frequency set up some distance apart and creating an interference pattern and there isn't any modulation with a signal occurring in that video.
e.g. carrier 1152 kHz signal 4kHz
most diagrams of amplitude modulation in books and on the internet are not to scale
the time between peaks of the 1152 kHz carrier would be 0.868 μs
the time between peaks of the 4 kHz signal would be 0.25 ms
it's difficult so show what's going on in a scale diagram
if you amplitude modulated a 1152 kHz carrier with a 4kHz sine wave you'd have RF energy at the 1152 carrier frequency as well as at the sum and difference frequencies uber mentioned (1148 kHZ and 1156 kHz)
This isn't similar to having 2 sources of the same frequency set up some distance apart and creating an interference pattern and there isn't any modulation with a signal occurring in that video.
Original post by veekm
https://www.youtube.com/watch?v=dNx70orCPnA
This video has two sources interfering (diffraction) - it's a mechanical wave - but we can treat it as EM for the discussion.
The surface of that mech-wave is our modulated carrier right? (One sinusoidal src-carrier busy interfering with another-signal src)
Where are the harmonics? Where exactly in that mech wave would the information be stored?
There are some dark-shadow-lines radiating out from the center - that's cancellation (diffraction - a trough and a crest).
The ripply stuff are the data stored in a changed amplitude? Why then we do we mark out the envelopes from peak to peak?
This video has two sources interfering (diffraction) - it's a mechanical wave - but we can treat it as EM for the discussion.
The surface of that mech-wave is our modulated carrier right? (One sinusoidal src-carrier busy interfering with another-signal src)
Where are the harmonics? Where exactly in that mech wave would the information be stored?
There are some dark-shadow-lines radiating out from the center - that's cancellation (diffraction - a trough and a crest).
The ripply stuff are the data stored in a changed amplitude? Why then we do we mark out the envelopes from peak to peak?
As Joinedup said, you are confusing yourself unnecessarily.
Look at this video which gives a much simpler explanation:
The presenter uses two similar sound frequencies but the principle is exactly the same as for amplitude modulation. i.e. with a voltage representation of the carrier frequency >> than an audio frequency which is used to modulate the amplitude of the carrier.
[video="youtube;1ZC4Rc3sYEw"]https://www.youtube.com/watch?v=1ZC4Rc3sYEw[/video]
(edited 8 years ago)
@uberteknik i think one of my posts got nuked by the system - anyway, modulation is multiplication/convolution - that video is addition and is used in generation of square waves from sinusoids - at least that's what i think 
for now.. 
Maybe i should re-type that.. here goes.. (hope no one thinks I'm opinionated)
When you create a square wave you add sinusoids - one main sin wave at a fundamental f, and a whole bunch of odd frequency sin waves at 1/3, 1/5, 1/7 the main freq and amplitude. So when you sum them all up, they construct/destruct and presto you get a square wave.
Modulation is different. Here you multiply (probably by varying the gain of the amplifier) the carrier amplitude with the signal amplitude. It's called a convolution. Anyway what I found on the wiki was this:
https://en.wikipedia.org/wiki/Envelope_detector
Basically the HF wave is chopped with the stuff below 0v getting blocked by the diode. So the capacitor sees only the upper HF wave. There is NO ENVELOPE per-se. What happens is that the HF is varying so fast that the cap can't follow so it basically traces the outer envelope (that red line in the wiki image) and that's basically the signal - so in many ways, the HF wave/carrier is basically used for sampling our data signal with the peaks of the carrier being each point..
At least that's what i think is going on..
for now.. Maybe i should re-type that.. here goes.. (hope no one thinks I'm opinionated)
When you create a square wave you add sinusoids - one main sin wave at a fundamental f, and a whole bunch of odd frequency sin waves at 1/3, 1/5, 1/7 the main freq and amplitude. So when you sum them all up, they construct/destruct and presto you get a square wave.
Modulation is different. Here you multiply (probably by varying the gain of the amplifier) the carrier amplitude with the signal amplitude. It's called a convolution. Anyway what I found on the wiki was this:
https://en.wikipedia.org/wiki/Envelope_detector
Basically the HF wave is chopped with the stuff below 0v getting blocked by the diode. So the capacitor sees only the upper HF wave. There is NO ENVELOPE per-se. What happens is that the HF is varying so fast that the cap can't follow so it basically traces the outer envelope (that red line in the wiki image) and that's basically the signal - so in many ways, the HF wave/carrier is basically used for sampling our data signal with the peaks of the carrier being each point..
At least that's what i think is going on..
(edited 8 years ago)
Original post by veekm
@uberteknik i think one of my posts got nuked by the system - anyway, modulation is multiplication/convolution - that video is addition and is used in generation of square waves from sinusoids - at least that's what i think for now..
for now.. It makes no difference. Multiplication is simply the gain of the system.
Quick Reply
Related discussions
- Please help me understand?
- stuck on this question
- Car travelling towards a person- physics
- HNC MATHS A2 Task 3 (Radio Transmitters)
- How can 2 waves be in phase if they have different amplitudes?
- A Level physics- Waves question (1)
- M3 Further Dynamics - Simple Harmonic Motion HELP!
- hnc mechanical engineering, struggling to expand question
- eeg scan drowsy patient
- Edexcel GCSE Computer Science Paper 1 (1CP2 01) - 19th May 2023 [Exam Chat]
- physics
- Physics question phase difference
- Physics Homework question - Simple Harmonic Motion
- Physics A Level unit 4 questions
- AQA A Level Physics Paper 1 7408/1 - 26 May 2022 [Exam Chat]
- maths question help
- Physics A G482 EWP Unofficial mark Scheme
- The Weighing Machine - Isaac physics
- What happens if...?
- University Modules
