Detailed Explanation of different types of Sidebands, Including Single Sideband Suppressed carrier, Dual-Sideband Suppressed Carrier, and Vestigial Sideband.
What is Sideband in AM Modulation:
Whenever a carrier signal is modulated by the information signal, new signals at different frequencies are generated as the part of AM modulation. These new frequencies are called Sidebands or Side frequencies. These new frequencies can be seen in the frequency spectrum as shown in the figure below. The Signal component above the carrier frequency is known as Upper Sideband and the signal below the carrier frequency is known as Lower Sideband.
The upper sideband f(USB) and lower sideband f(LSB) are calculated as:
fUSB = fc +fm and fLSB = fc – fm
- fc is the carrier frequency
- fm is the modulating frequency
For example, if 800 kHz carrier is amplitude modulated by a 2 kHz audio signal, there will be components at 799 kHz and 801 kHz as well as 800 kHz in the generated AM Modulation frequency spectrum.
The existence of sidebands can be proved mathematically:
Using the equation for an AM signal described previously:
υAM =Vc sin 2πfct + (Vm sin 2πfmt) (sin 2πfct)
Now applying trigonometric Identity of a product of two sine waves are:
And substituting this identity into the expression of AM modulation equation we get:
Here you can see the first term is the carrier, the second term is, contains the different of lower sideband and the third term contains the upper sideband.
Now it is obvious that during AM Modulation process sidebands are generated.
Single Sideband Modulation:
In AM Modulation (Amplitude Modulation) process two third of the power is consumed by the carrier wave, which itself does not convey any information. The real information is contained within the sidebands. In order to make AM modulation efficient, we can suppress the carrier and eliminate one sideband because each sideband contains the same information the best trick will be eliminating one of the sidebands. This is known as Single Sideband Modulation.
Signal Sideband or (SSB) is the form of Modulation which has lots of benefits in Electronic Communication and Radio transmission.
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Two Achieve Single Sideband Modulation we have to first understand:
- Dual-Sideband Suppressed Carrier (DSSC or DSB):
- Single-Sideband Suppressed Carrier (SSB):
Dual-Sideband Suppressed Carrier (DSSC or DSB)
In Dual-Sideband Suppressed carrier we suppress carrier wave, The benefit, of course, is that no power is wasted on the carrier. In simple words, Dual-Sideband Suppressed carrier is the special case of AM modulation with no carrier. A frequency-domain display of a DSB signal is shown below. Note that the spectrum occupied by AM Modulation and DSB Signal is same.
Double-sideband suppressed carrier signals are generated by a circuit called a balanced modulator. The purpose of balance modulator to produce fUPS and fLPS and suppress carrier. The elimination of carrier wave does not make DSB to be used widely because it saves one-third power but the regeneration of information signal is not easy in DSB modulation.
Here in Time domain, you can see that by eliminating carrier wave the modulated signal undergoes a phase reversal whenever the message signal m(t) crosses zero. The envelope of a Dual-Sideband Suppressed Carrier modulated signal is different from the information signal.
The transmission bandwidth required by Dual-Sideband Suppressed Carrier modulation is the same as that for amplitude modulation which is twice the bandwidth of the information.
Single-Sideband Suppressed Carrier (SSB):
In DSB-SC there were no carrier but only two sidebands, and in Single- Sideband Suppressed Carrier there is suppressed carrier and one sideband elimination. Making it more useful in Electronic Communication and RADIO transmission.
Few advantages of SSB Modulation are:
- The primary benefit of an SSB signal is that the spectrum space it occupies is only one-half that of AM and DSB signals. This allows more signals to be transmitted in the same frequency range.
- The power which was being utilized in a carrier and one sideband is reduced hence the more power will be consumed by only one sideband and the greater distance can be achieved by SSB Modulation.
- Because SSB signals occupy a less bandwidth, the amount of noise in the information signal is also reduced.
Disadvantages of DSB-SC and SSB-SC
The main disadvantage of Dual-Sideband Suppressed carrier and Single-Sideband Suppressed Carrier signals is that they are harder to recover or demodulate at the receiver end. Because demodulation depends on the carrier being present. If the carrier is not present at the receiver end then it must be regenerated at the receiver and reinserted into the signal, in order to achieve faithful recovery of the information signal. The reinserted carrier must be in same phase and frequency as the original carrier had.
This is the very difficult requirement in DSB and SSB Modulation in order to make a proper modulation we use vestigial side modulation.
VSB (Vestigial) Modulation:
A vestigial-sideband system is a combination of DSB and SSB It has the advantages of DSB and SSB but avoids their disadvantages. The Idea is to transmit one of the sidebands similar to SSB but also transmit a vestige (small trace) of the other sideband. So, VSB has a larger bandwidth compared to the SSB but smaller than DSB-FC
Why Use VSB?
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- In DSB, we wasted power and bandwidth but the receiver was simple
- In SSB, we saved power and bandwidth but the receiver was complex, Complex means more expensive
- In VSB, we can reduce the complexity and save bandwidth to an extent
To generate a VSB modulated wave, we pass a Dual-Sideband Suppressed Carrier modulated wave through a sideband shaping filter as shown in Fig.
Vestigial sideband modulation has the advantage of having bandwidth almost as efficiently as single sideband modulation while retaining the low-frequency characteristics of double sideband modulation. The VSB modulation has become very helpful for analog transmission of television and RADIO signals.