Ssb Am

Thread Starter

MWalden

Joined Apr 26, 2007
24
I've been reading about SSB amplitude modulation, and I am kind of confused about something. Book says you can filter/phase out the carrier wave which would eliminate the LSB or USB thus increasing efficiency and cuting used bandwidth in half. What I don't under stand how this work? First off, I thought it was aplitude modulation. It shows a picture of a spectrum analyzer (Frequency [horizontal] vs. Amplitude in dBm [vertical]) graph and shows that a voice is deviated away from the carrier signal evenly in both ways equal to the bandwidth of themodulated signal! I thought it was amplitude modulation how is it getting away from the carrier freaquency??

Sorry for my ignorance I am new to electronics. I understand the voice has frequences too but I thought the frequencies of the voice only affected the amplitude of the carrier wave??

And why is it a mirror image? If I had to guess I would figure it would go Half the bandwidth of the modulated signal in both ways????

Thanks for any help!
 

Papabravo

Joined Feb 24, 2006
14,382
Amplitude Modulation is actually the nonlinear combination of two signals. One of the signals is the carrier and its frequency is fixed and stable, and its amplitude is fixed and stable. The other signal might be a voice, which is the modulating signal. The amplitutude is time varying and there is a rich spectral content.

The basic math is the multiplication of two sine waves. When you work this out you see the following four components:

  1. The modulating frequency
  2. The carrier frequency
  3. The sum of the modulating frequency and the carrier frequency
  4. The difference of the modulating frequency and the carrier frequency
 

John Luciani

Joined Apr 3, 2007
477
AM is created by multiplication of a message signal with a carrier
signal. The equation for a modulated carrier signal (single carrier)
is ---

x(t) = m(t)cos(wct)

m(t) is the message signal and wc is the radian carrier frequency.

For standard AM radio (LSB, Carrier, USB) the message signal is
defined as

m(t) = 1 + m * s(t)

where m is a modulation constant that is between 0 and 1, s(t) is the
standardized message signal. For this message signal the values of
m(t) >= 0. Since the signal is unipolar the envelope is easily
detectable which makes for a low cost receiver.

The amplitude spectrum diagrams that show the the LSB, Carrier and USB
are depicting a signal that is a sine wave. If you substitute a cosine
wave for the signal ---


x(t) = (1 + m * cos(wmt)) * cos(wct)

= (cos(wct) + m/2 * cos((wc + wm)t) + m/2 * cos((wc - wm)t)

term 1 is the carrier frequency, term 2 is the USB and term 3 is the
LSB. For traditional AM you require a bandwidth of 2 * wm where wm is
the maximum frequency that you wish to transmit.

The trade-off is a simple receiver at the expense of transmitter power
and bandwidth. The USB and LSB contain the same information. If you
generate a carrier at the receiver you can save the power to transmit
it.

(* jcl *)

PS These are from my notes and "Signal Analysis and AM Theory" by
Walter H. Lob

PPS sorry about the lack of subscripts
 
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