Can someone explain side bands 4 me?
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Single Sideband Modulation (SSB) :: Electronics and Radio Today
In summary, SSB has only one of the sidebands transmitted and no carrier. Basically you take the AM signal, remove one of the sidebands and ditch the carrier and then transmit just the chosen sideband.
cheers,
george.
In summary, SSB has only one of the sidebands transmitted and no carrier. Basically you take the AM signal, remove one of the sidebands and ditch the carrier and then transmit just the chosen sideband.
cheers,
george.
- Thread starter
- #3
For those interested, here is the sentence that I needed to here that cam as a result of Georges link:
"The process of modulating a carrier is exactly the same as mixing two signals together, and as a result both sum and difference frequencies are produced. Therefore when a tone of 1 kHz is mixed with a carrier of 1 MHz, a "sum" frequency is produced at 1 MHz + 1 kHz, and a difference frequency is produced at 1 MHz - 1 kHz, i.e. 1 kHz above and below the carrier."
Amplitude Modulation, AM :: Electronics and Radio Today
Thanks George.
Karl
"The process of modulating a carrier is exactly the same as mixing two signals together, and as a result both sum and difference frequencies are produced. Therefore when a tone of 1 kHz is mixed with a carrier of 1 MHz, a "sum" frequency is produced at 1 MHz + 1 kHz, and a difference frequency is produced at 1 MHz - 1 kHz, i.e. 1 kHz above and below the carrier."
Amplitude Modulation, AM :: Electronics and Radio Today
Thanks George.
Karl
AM is a form of modulation. Modulation is a shifting of the signal. The reason why we shift the signal to the carrier frequency is that in reception, we need an antenna that is some multiple or whole fraction of the wavelength of the signal. Since voice is in the 5000 Hz range, we'd need an antenna that is some whole fraction of about 50,000 meters. Any whole fraction of 50,000 meters is ridiculously long. So we modulate the signal to a much higher frequency (carrier frequency) such as 144 MHz (2 m wavelength) which will allow us to use a much shorter antenna. Here the modulated signal at a carrier frequency of 144 MHz needs an antenna that is a fraction of two meters.
The signal is sound, and all sounds are a mixture of sinusoids. We modulate the signal (a sinusoid) centered around 5KHz or so and we modulate (mulitply) it with another sinusoid centered much higher (let's say 144 MHz.)
Ordinary AM :
a(t) : modulated signal
sin(a) : original signal with frequency a
sin(b) : modulating signal withfrequency b
a(t) = [1 + sin(a)]sin(b)
= sin(b) + (1/2)sin(b+a) + (1/2)sin(b-a)
Note that the multiplication of two sinusoids results in :
sin(a)sin(b) = (1/2)sin(b+a) + (1/2)sin(b-a)
So for ordinary AM, we see two side components at half amplitude, at frequency b+a and b-a, and we see a center component centered at frequency b.
DSB-SC AM (double side band suppressed carrier) :
a(t) = sin(a)sin(b)
= (1/2)sin(b+a) + (1/2)sin(b-a)
Results in only the sidebands, centered at b+a and b-a. You'll notice that the carrier is eliminated (or supressed).
SSB-SC AM (single sideband suppressed carrier) :
a(t) = sin(a)sin(b)
= (1/2)sin(b+a) + (1/2)sin(b-a)
Results are the same as with DSB-SC, but SSB modulation takes on a second step. It passes through a filter that eliminates either the upper side band or the lower side band. Thus you have
SSB-SC-LSB :
a(t) = (1/2)sin(b-a)
or
SSB-SC-USB :
a(t) = (1/2)sin(b+a)
You'll notice that with these different modulation types that the transmitted power is greatly reduced with SSB modulation. In fact, signal power is 1/8 th that of ordinary AM.
As an example, with 100 watts and ordinary AM, 50 watts would be used for the carrier, and 25 watts for each side band. Our signal is contained entirely within half of a sideband, thus only 12.5 watts is used out of the 100.
Compare that to SSB with 100 watts. The full power would be used for the chosen sideband.