amplitude modulation

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E=MC2

Joined: 21 Jan 2005
Posts: 12

26 Jan 2005 16:30 amplitude modulation

i have a question about amplitude modulation. in an AM transmitter the signal and the carrier frequency are multiplied with a mixer whose output is fed to the final stage amplifier which is then fed to an antenna. right????

the frequency of the carrier is about ten times the frequency of the signal (i read this somewhere). but when both the signals are multiplied the resulting amplitude modulated signal has the frequency of the carrier and the shape resembles the signal to be transmitted.

i understand the part that the output of the mixer resembles the signal. what i dont understand is why is the frequency of the modulated signal equal to the signal of the carrier. doesnt multiplication do anything to the frequency???

and my second question is that how are two analog signals multiplied?

actually i have a communications course this semester and i want to know everything about communications from A to Z. so any book recommendations would be greatly appreciated

usernam

Joined: 09 Mar 2004
Posts: 698
Helped: 129

26 Jan 2005 16:57 Re: amplitude modulation

E=MC2 wrote:

The AM equation is something like A[1+k.m(t)]cos(ωt)
where m(t) is the signal. So you could now look at this as a cosine wave with a time varying amplitde of A[1+k.m(t)]. Suppose m(t) was also a sinusoid if you were to multiply out and expand the modulated signal expression there would be 3 different frequency components. So saying that the frequency of the modulated signal is equal to the signal of the carrier not actually true.
As to a good book Communication Systems by Simon Haykin would be a good option.

van_gift

Joined: 11 Jan 2005
Posts: 26
Helped: 1

26 Jan 2005 17:29 amplitude modulation

Normally the carrier frequency is much higher than the bandwidth of the modulated signal.

Borber

Joined: 01 Jan 1970
Posts: 1561
Helped: 117
Location: Slovenia

26 Jan 2005 17:46 Re: amplitude modulation

Frequency of the modulated carrier is equal to the carrier frequency! Product of the modulation proccess are two sideband frequencies if modulation signal is pure sin(ωm), ω-ωm and ω+ωm besides ω.

Andre3000

Joined: 25 Aug 2004
Posts: 18
Helped: 2

27 Jan 2005 0:01 amplitude modulation

In AM the modulation is for the carrier's amplitude and not for the frequency.

You can read for more info IN HAYKIN'S bOOK

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IanP

Joined: 05 Oct 2004
Posts: 6492
Helped: 1542
Location: West Coast

27 Jan 2005 3:50 Re: amplitude modulation

Some additional explenations with examples you can find here:
http://www.seas.upenn.edu/~ee111/modulation/Modulation.html

nand_gates

Joined: 19 Jul 2004
Posts: 907
Helped: 120

27 Jan 2005 7:52 Re: amplitude modulation

how are two analog signals multiplied?
Answer: Using a gain control amplifier.

An amplifier multiplies input signal with a constant "A" known as gain of that
amplifier. If you make this constant vary according to another input signal then
what you get is a gain controlled amplifier or amplitude modulator. In this case
the output is given by K*vin1*Vin2;

In a transistor amplifier you can vary the gain by changing transistor biasing.
This is because the transconductance of transistor depends on biasing.

E=MC2

Joined: 21 Jan 2005
Posts: 12

27 Jan 2005 18:25 Re: amplitude modulation

actually Andre3000 i decided not to have a look at Haykin's book by looking at the reviews for that book at amazon.com

i know that i shouldnt believe on reviews at book store sites and that this book is quite like "THE" industry standard. but i dont know why i havent read that book. im looking for Lathi's book. but i havent found it anywhere yet.

and nand_gates i read that in a book but acutally someone told me that in the analog world multiplication is really hard to do. he told me that first you have to take the log of the two signals by log amps and then add them by an adding amp and then take the antilog of the result by an antilog amp. so i just wanted to confirm. thanx alot everyone

Mina Ayman

Joined: 15 Oct 2004
Posts: 45
Helped: 1

27 Jan 2005 18:29 amplitude modulation

This is an Excellent book that will be very helpful to you in your communications study
http://www.edaboard.com/viewtopic.php?t=84979&highlight=communications+handbook

Bukitoo

Joined: 12 Oct 2003
Posts: 140
Helped: 2
Location: Argentina

28 Apr 2005 4:44 Re: amplitude modulation

The variable gain amplifier (to generate the multiplication) is one way. Another way (talking about a random signal x a cosine carrier) is to input you signal to a switch (switching at the carrier frequency) and then filtering the result with a band pass filter of the bandwith of the random signal.

Another way is to buy a little IC which does the a(t) x b(t) directly.

cedance

Joined: 24 Oct 2003
Posts: 704
Helped: 28
Location: Germany

02 May 2005 7:59 Re: amplitude modulation

well, reg. the references or materials u asked for, apart from the books and papers u get from here, i would like u to go thro the A*ilent website. they issue a free student version CD rom and a 6 cd pack containing various tutorials. I bet none can match them for a beginner course.

browse this site completely to explore the free CD offers which are undoubtedly the best.

/http://www.educatorscorner.com/index.cgi

/cedance

shafee001

Joined: 06 May 2004
Posts: 116
Helped: 12
Location: Egypt

02 May 2005 8:20 Re: amplitude modulation

hi all;
1. frequency of the carrier not have to be 10 time of base band signal, any frequency more that 2*base band signal frequency is good engough
2. frequency of the modulated signal is centered at carrier frequency, and have side bands (it basnd width) equals to base band frequency to the right side and to the left side, to undersnad consider that
W is carrier frequency
w is base band frequency
cos(wt)*cos(Wt) = cos(W+w)t + cos (W-w)t,
so the resulted signal is cos centered at W and have two side loops to the right +w, and to the left -w, so band width is w but centered at W.

3. produsing AM is simply using and swich transistor, carrier is connected to base, and base band signal is connected to it's emmiter

baseband signel
|
|
/
\
/
|_________||______ output modulated signal
carrier ------/\/\/\/-----| < ||
|
|
_
\/ gnd

kokabanga

Joined: 02 May 2005
Posts: 45
Helped: 4

02 May 2005 9:42 Re: amplitude modulation

E=MC2 wrote:

there are commercial multiplier circuits such as Ne602 which maybe used to accomplish multiplication in the time domain!

Very Happy

shmoib

Joined: 09 Apr 2005
Posts: 29

02 May 2005 16:07 Re: amplitude modulation

k, first the amplitue modulation is done using a mixer, which is a non-linear device so it produces ( ......,fc+fm , fc-fm, fc-2fm,2fc+2fm,......... ) so we use after it a filter to bypass the required signals only (always : fc+fm , fc-fm). so the final action is not multiplication it is just frequency shift for the signal frequency.

mixer may be done by any non linear device like transistor

hermin

Joined: 19 Apr 2005
Posts: 135
Helped: 9

13 May 2005 3:02 amplitude modulation

first, try to look for topics using a search engine about that, look for ebooks on this forum,

when the carrier frequency if multiplied using the modulating signal(basedband signal) the carrier will retain its own frequency, but will generate two sidebands. try multiplying two sine functions, e.g sin(w*fc*t) x sin(w*fm*t), and you'll arrive at an answer like sin(w*fc*t) + cos(w*t( fc - fm)) - cos(w*t(fc+fm)...im not sure with this equation but you can check out books and ebooks and you'll have a greater understanding about AM concepts
goodluck with your studies

emadi

Joined: 03 May 2005
Posts: 127
Helped: 4

28 May 2005 13:04 Re: amplitude modulation

These two sites will help you about thinking these matter.

http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/RadCom/part9/page1.html

http://www.media.mit.edu/physics/pedagogy/fab/fab_2002/help_pages/networking_resources/more%20networking/robotics.eecs.berkeley.edu/_sastry/ee20/modulation/node3.html

comsians

Joined: 22 May 2005
Posts: 46
Helped: 1

10 Jun 2005 9:30 Re: amplitude modulation

Answer1
the basic definitions of AM is to "vary the amplitude of the carrier according to the instantanious value of the modulating signal's amplitude.
the modification is made only in the carrier, therefore, the frequency of the AM modulated wave is that of the carrier and the amplitude (variation) is that of the modulating signal.

Answer2
two analog signals are multiplied by multiplying each of their respective instantanious values.
each time domain unit (time) gets a multiplied answer of the value of the two waves.

suvendu

Joined: 10 Oct 2004
Posts: 442
Helped: 25

11 Jun 2005 8:53 Re: amplitude modulation

In any modulation technique we vary one of the parameter(amplitude ,phase,frequency) of carrier signal.In A.M we vary amplitude of carrier not frequency.