AM Modulation

[[email protected]]

when v see the o/p of a AM Modulated signal in frequency domain ,why
there r three frequencies ? in spite of modulating signal is made of
single frequency?

 

[Rene Tschaggelar]

when v see the o/p of a AM Modulated signal in frequency domain ,why
there r three frequencies ? in spite of modulating signal is made of
single frequency?

The upper, the lower sideband plus the carrier.
Do a fourier transform of an AM modulation.

Rene

 

[[email protected]]

when v see the o/p of a AM Modulated signal in frequency domain ,why
there r three frequencies ? in spite of modulating signal is made of
single frequency?

You see the carrier frequency, the carrierfreqiuency plus the
modulation frequency, and the carrier frequency minus the modulation
frequency.

When you modulate a carrier, you are essentially multiplying two sine
waves toegther

sine A. sin B = 0.5 .(cos (A-B) - cos(A+B))

This implies that you shouldn't see the carrier frequency at all, but
few modulators are perfect multipliers.

Single side band modulation schemes use 90-degree pahse shifters to
generate cosine A and cosine B so that one can form the second product

cosine A. cosine B = 0.5.(cos (A-B) + cos (A+B)

which cn then be summed with the first to suppress the sum frequency
(or - with very little more ingenuity) the difference frequency,

Electronics is really just a branch of trigometry .

 

[Joe G \(Home\)]

http://en.wikipedia.org/wiki/Amplitude_modulation

 

[John Larkin]

When you modulate a carrier, you are essentially multiplying two sine
waves toegther

sine A. sin B = 0.5 .(cos (A-B) - cos(A+B))

This implies that you shouldn't see the carrier frequency at all, but
few modulators are perfect multipliers.

The other reason the carrier doesn't disappear is that most AM
stations don't run at 200% modulation.

John

 

[Mochuelo]

You see the carrier frequency, the carrierfreqiuency plus the
modulation frequency, and the carrier frequency minus the modulation
frequency.

When you modulate a carrier, you are essentially multiplying two sine
waves toegther

sine A. sin B = 0.5 .(cos (A-B) - cos(A+B))

This implies that you shouldn't see the carrier frequency at all, but
few modulators are perfect multipliers.

The reason why most AM signals do have carrier is not because
multipliers aren't as good as the designers would want. It is because
they _want_ to transmit the carrier. And they want to transmit it
because that simplifies the receiver. You can use a simple envelope
detector (diode + capacitor + resistor) in your receiver, whereas
without a carrier you would need coherent detection, which is more
complex and expensive.

Best,

 

[John Larkin]

The other reason the carrier doesn't disappear is that most AM
stations don't run at 200% modulation.

John

Or, to put things more precisely, the equation for AM is more like

v = sin(c*w*t) * (1 + m)

where m is the modulation signal, in the range +-1 for max (100%)
modulation. The "1" is what preserves the carrier component, not the
imperfection of the modulator.


John

 

[Don Bowey]

http://en.wikipedia.org/wiki/Amplitude_modulation

(snip)

I couldn't get past the first sentence, "Amplitude modulation (AM) is a form
of modulation in which the amplitude of a carrier wave is varied in direct
proportion to that of a modulating"


Where does wikipedia get it's B.S?

Thanks for the useless reference.

 

[John Larkin]

Where does wikipedia get it's B.S?

Wiki is totally open; anybody can post, just like a newsgroup.

Not all engineers are articulate; for example, many have trouble
distinguishing between "its" (possessive) and "it's" (contraction for
"it is".)


John

 

[Tim Wescott]

(snip)

I couldn't get past the first sentence, "Amplitude modulation (AM) is a form
of modulation in which the amplitude of a carrier wave is varied in direct
proportion to that of a modulating"


Where does wikipedia get it's B.S?

Thanks for the useless reference.

Out of curiosity -- what don't you like about it? It's about as
technically correct as you can fit into one sentence, and I doubt that I
could make it any clearer while retaining the formal language that seems
to prevail on wikipedia.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

 

[Mark]

The reason why most AM signals do have carrier is not because
multipliers aren't as good as the designers would want. It is because
they _want_ to transmit the carrier. And they want to transmit it
because that simplifies the receiver. You can use a simple envelope
detector (diode + capacitor + resistor) in your receiver, whereas
without a carrier you would need coherent detection, which is more
complex and expensive.

Best,

The stumbling block for understanding AM is the difference between the
"carrier" and the envelope". They are not the same.

Imagine a spectrum analyzer with a very narrow filter. Imagine it can
seperate or resolve all the components and that you will be able to see
the "carrier" seperatley from the sidebands. In this case you will see
that the carrier sits there unchanged all the time despite the
modulation. The sidebands jump up and down but the carrier is
unchanged.

If however, the spectrum analyzer is not narrow enough to resolve the
carrer apart from the sidebands, then they will merge and you will be
looking at the "envelope". The envelope is the picture we are all
familiar with of AM on an oscilloscope. The ampliture of the ENVELOPE
varies up and down in step with the modulation.

The sidebands combine with each other and with the carrier to vary the
amplitude of the ENVELOPE. The carrier itself is unchanging.

Hope this helps.

Mark

 

[Rudolf Drabek]

the best understanding for the 3 freq. you can get if you make a small
drawing like that:
One vector representing the carrier wave. on the top of the carrier
wave you draw 2 smaller vectors rotating clock and anticlockwise at the
top of the carrier vector. Taking care that the resulting vector of the
2 smaller vectors is always in phase with the carrier vector. The
rotating freq. represents the modulating freq.
So the sum of the 3 vectors is a change in length of only one vector =
modulated carrier.
In the thread you can find the math. to that, namely the solving of the
multiplication of the carrier freq. with the modulating freq (additions
theorem for trig. functions).

rotating small vectors = the carrier freq.+ and - the modulating freq.
\ /
\/
| fixed vector = the carrier
|
|
|
if the small vectors are 50% of length of the carrier vector, as drawn,
you get 100% modulation.
If you delete the carrier vector and one small vector = the sideband
you have SSB lower or upper sideband, depending which small vector you
delete.

Rudi

 

[Pig Bladder]

Out of curiosity -- what don't you like about it? It's about as
technically correct as you can fit into one sentence, and I doubt that I
could make it any clearer while retaining the formal language that seems
to prevail on wikipedia.

He's too young and pig-headed to go look up "amplitude", "carrier wave",
"varied", "direct proportion", things like that.

 

[Boris Mohar]

when v see the o/p of a AM Modulated signal in frequency domain ,why
there r three frequencies ? in spite of modulating signal is made of
single frequency?

It was hammered into my thick skull forty years ago and it still
reverberates.

"... once you start modulating it no part of it is a sine wave.."

 

[Phil Allison]

When you modulate a carrier, you are essentially multiplying two sine
waves toegther

sine A. sin B = 0.5 .(cos (A-B) - cos(A+B))

** Shame that is NOT the formula for a normal AM wave.

This implies that you shouldn't see the carrier frequency at all,

** Because it is the wrong formula.

but few modulators are perfect multipliers.

** Normal AM ones are certainly not.

Unless the signal is "double sideband, suppressed carrier".



** Horse manure.

"Electronics" was initially a branch of physics, then expanded to become a
branch of engineering in its own right separate from "electrical
engineering".

It is all about exploiting nature for human benefit.

Maths is just a tool to help achieve that end.



......... Phil

 

[Phil Allison]

"Mark"

The stumbling block for understanding AM is the difference between the
"carrier" and the envelope". They are not the same.

** When the modulation level falls to zero - they certainly are.

Imagine a spectrum analyzer with a very narrow filter. Imagine it can
seperate or resolve all the components and that you will be able to see
the "carrier" seperatley from the sidebands. In this case you will see
that the carrier sits there unchanged all the time despite the
modulation. The sidebands jump up and down but the carrier is
unchanged.

If however, the spectrum analyzer is not narrow enough to resolve the
carrer apart from the sidebands, then they will merge and you will be
looking at the "envelope". The envelope is the picture we are all
familiar with of AM on an oscilloscope. The ampliture of the ENVELOPE
varies up and down in step with the modulation.

The sidebands combine with each other and with the carrier to vary the
amplitude of the ENVELOPE. The carrier itself is unchanging.

Hope this helps.

** I think it only deepens the mystery of AM for novices.

Carrier and envelope amplitude seem to be the same in practice.

Thinking in terms of the "frequency domain" is the problem.




........ Phil

 

[[email protected]]

** Shame that is NOT the formula for a normal AM wave.




** Because it is the wrong formula.

As if Phil Allison knew the "right" formula - the cantankerous poseur
would have posted it if he did.

If fact, given the range of applications for amplitude modulation,
there is no one "right" formula. The one I posted is probably the right
one to answer the OP's question, but that's a level of comprehension
that Phil can't manage.

 

[Phil Allison]

[email protected] a criminal, a lying pile of shit & an autistic pig.

As if Phil Allison knew the "right" formula - the cantankerous poseur
would have posted it if he did.

** No - it would be superfluous to bother.

It is in all the texts, the Wiki that was posted AND had already posted in
this thread.

PLUS .......

It would go WAY over the demented autistics brain of the Slow Man fuckwit.

If fact, given the range of applications for amplitude modulation,
there is no one "right" formula.

** Yawn - yet another pathetic " straw man " fallacy.

The Slow Man fuckwit's favourite line of BULL SHIT !!

The one I posted is probably the right
one to answer the OP's question,

** ROTFL !!!

Can't be - since that question was about THREE frequencies.

Bill Sloman, Nijmegen

** I hear the rest of Nijmegan' inhabitants are planning to LYNCH the
slimy **Son of a Bitch** ASSHOLE for slandering them !


Good luck to them - I say !!!




........... Phil

 

[Le Chaud Lapin]

when v see the o/p of a AM Modulated signal in frequency domain ,why
there r three frequencies ? in spite of modulating signal is made of
single frequency?

I noticed that everyone seems to be using trig to explain the addition
of frequencies.

Having been deprived of trig, I prefer Euler's formula, which, IMO, is
simply unforgettable:

e^jw=cosw +jsinw ---> cosw=1/2(e^jw+e^-jw)

You can express all your signals in this form so that not only do the
resulting signals from modulation becomes easy compute, but you can
*see* the additions happening in the exponents.

-Le Chaud Lapin-

 

[Jim Thompson]

[email protected] a criminal, a lying pile of shit & an autistic pig.


** No - it would be superfluous to bother.

It is in all the texts, the Wiki that was posted AND had already posted in
this thread.

PLUS .......

It would go WAY over the demented autistics brain of the Slow Man fuckwit.





** Yawn - yet another pathetic " straw man " fallacy.

The Slow Man fuckwit's favourite line of BULL SHIT !!





** ROTFL !!!

Can't be - since that question was about THREE frequencies.





** I hear the rest of Nijmegan' inhabitants are planning to LYNCH the
slimy **Son of a Bitch** ASSHOLE for slandering them !


Good luck to them - I say !!!




.......... Phil

Phil, During my recent system crash I lost all my kill filters.

So I've been slowly rebuilding the file on a "need-be" basis.

Bill Sloman has been quite a reasonable fellow during this
reconstruction, as has Frank Bemelman and even John Fields; so I
continue to read their posts.

You, on the other hand, continue to be one of the most ignorant
assholes on the face of the earth... worse even than Roy ;-)

AND I resent your use of "autistic" as a derogatory term... clearly
you've not really been exposed to a child with autism, smart as a
tack, but unable to speak... I have an autistic grandson.

So PLONK!

...Jim Thompson