Register to reply 
Sidebands in AM Transmission 
Share this thread: 
#1
Jan3113, 04:51 PM

Mentor
P: 12,013

Say you are transmitting AM radio signals. You input an audio signal of 1khz which is used to modulate a signal of 1 mhz. The heterodyning process outputs 4 different frequencies, the audio, carrier, sum, and difference. But where are the sum and difference?
If I look at a graph of the modulated waveform, I can see the carrier and the audio (as amplitude variation in the carrier), but where are the other two? The sidebands as they are also known. I'm not seeing how you add and subtract the frequencies if you are using the audio signal to modify the amplitude of the carrier. When you transmit the signal from your antenna, are you sending power through the two sidebands as well, or only on the main carrier frequency? 


#2
Jan3113, 05:42 PM

P: 663

When you measure a modulated waveform there are several ways to picture the changes. If you only look at the time vs level domain it's hard to see the changes. What you need is a way to see the other domains of the signal.
http://www.naic.edu/~phil/hardware/M...r_basis_of.pdf https://www.youtube.com/watch?v=Lp6sTn55N4 With today's electronics some pretty cool stuff can be made for cheap. http://www.youtube.com/watch?v=ZuRcaxpbYCw 


#3
Jan3113, 05:55 PM

Mentor
P: 12,013




#4
Jan3113, 06:11 PM

P: 663

Sidebands in AM Transmission
You can't normally see sideband signals with a timedomain display. 


#5
Jan3113, 06:34 PM

Mentor
P: 12,013

Edit: Whoops, just noticed the link. Reading it now. Ok, that's interesting. According to the link: 


#6
Jan3113, 07:24 PM

Mentor
P: 41,351

See if post #4 in this thread by the_emi_guy helps:
http://www.physicsforums.com/showthread.php?t=606315 . 


#7
Jan3113, 09:01 PM

Sci Advisor
PF Gold
P: 2,085

An ideal mixer multiplies two signals. Use trig identities to write
cos(fm)*cos(fc) = cos(fm+fc) + cos(fmfc)] / 2 A real mixer is nonideal and allows some of fc to leak through to the output. (Leakage of fm is far away and is filtered out). Real mixers also produce harmonics and highorder mixing products, which again are filtered away. 


#8
Jan3113, 09:31 PM

Mentor
P: 12,013

Maybe this is a bad question, but are the sidebands actually propagating outwards from the antenna along with the modulated carrier? Or is this just something that happens when you "do the math"?



#9
Jan3113, 10:17 PM

Emeritus
Sci Advisor
PF Gold
P: 5,196




#10
Jan3113, 10:27 PM

Emeritus
Sci Advisor
PF Gold
P: 5,196

The modulating signal (the one that contains the actual information content) already contains a whole continuous spectrum of frequencies. It's a theorem from Fourier analysis that you can represent an arbitrary timevariable signal as a sum* of sinusoids of different frequencies and amplitudes. So that information signal contains a continuum of frequencies, from 0 up to some maximum frequency (that defines the bandwidth). You would see this if you were to look at the frequency spectrum of the signal (which you do by taking its Fourier transform). Anyway, what multiplying this signal by the carrier does is simply to shift the frequency spectrum so that instead of one spectrum being centred on zero, there are now two identical such spectra centred on +carrier frequency and carrier frequency. To see why this is, you'd need to understand more about Fourier transforms and convolution. *I use the term "sum" loosely, it's actually an integral called an inverse Fourier transform 


#11
Jan3113, 10:47 PM

Mentor
P: 12,013

You say it doesn't exist in real space? Could you elaborate? I'm unfamiliar with frequency domain as well. If this is too complicated without understanding both frequency domain and fourier analysis just say so. 


#12
Jan3113, 11:02 PM

Emeritus
Sci Advisor
PF Gold
P: 5,196

A sideband is a feature on this plot. The upper sideband is the portion of the spectrum that lies above the carrier frequency, and the lower sideband is the portion of the spectrum that lies below the carrier frequency. 


#13
Jan3113, 11:09 PM

Emeritus
Sci Advisor
PF Gold
P: 5,196

I just read the OP and saw that you were considering a simpler case where the modulating (informationcontaining) signal is also just a sinusoid at a single frequency. Sorry, I hope I haven't confused things by talking about a more general case with a broadband spectrum.



#14
Jan3113, 11:18 PM

Mentor
P: 12,013




#15
Feb113, 12:45 PM

P: 663

One way to look at the creation of sideband frequencies is to work the problem in reverse. A function (signal) can be decomposed into purely sinusoidal components. You want to create a AM single frequency modulation timedomain display on your oscilloscope display. To create this display you have RF frequency generators and a summing network. What set of frequencies would you have to set the RF generators to recreate the AM modulation display.



#16
Feb113, 01:27 PM

Mentor
P: 12,013




#17
Feb113, 01:39 PM

HW Helper
P: 7,181

It might help to note that components of a sine wave undergoing a change in amplitude appear similar to a higher or lower frequency sine wave of fixed gain (steeper or milder ramp rates near the crossover point). The rate of change in the gain determines the bandwidth consumed by the modulated signal. Morse code AM transmistters, which turn signals on and off, are designed to take 5 ms to switch the signal on and off, in order to reduce the bandwidth.



#18
Feb113, 02:38 PM

P: 663

http://users.ece.gatech.edu/~vkm/nii/node35.html Yes, the sideband frequencies are actually transmitted along with the carrier (the actual carrier average power does not change during AM modulation). http://www.technology.heartland.edu/...modulation.ppt 


Register to reply 
Related Discussions  
Transmission Lines  Electrical Engineering  4  
Ehv dc transmission  Electrical Engineering  5  
HIV transmission  Biology  5  
Power Transmission  Engineering, Comp Sci, & Technology Homework  2  
Why can some waves pass through materials but others can't  Advanced Physics Homework  5 