Evolution and Description of SSB techniques
Single side band suppressed carrier: (SSB-SC)
- In SSB-SC any one of the unwanted side band is suppressed along with the carrier signal.
- Since DSB-FC consumes more power as both the side bands are transmitted and the same information is contained n both side bands one of it can be eleminated.
- Also the bandwidth consmption is more in DSC-FC.
- To overcome all these things one of the side band can be eleminated as it is symmetrical and the information contained in other band will be redundant.
- There are two techniques are used to generate the SSB-SC signal.
- Filter method.
- Phase shift method.
Working of filter method:
- A crystal oscillator produces a stable carrier frequency(fc) signal.
- This carrier frequency is then fed to the balanced modulator through a buffer amplifier which isolates these two satges.
- The audio signal from the modulating amplifier modulates the carrier in the balanced modulator. Audio frequency range is 300 to 2800 Hz.
- The carrier is also suppressed in this stage but allows only to pass the both side bands. (USB & LSB).
- A band pass filter (BPF) allows only a single band either USB or LSB to pass through it. It depends on our requirements.
Let we want to pass the USB then LSB will be suppressed. Inthis case.
fc = 100 KHz
Audio range = 300 – 2800 Hz
USB frequency range = fc + 300 to fc + 2800
= 100000 + 300 to 100000 + 2800
= 100300 to 102800 Hz
So this band of frequency will be passed on through the USB filter section
- The frequency of the SSB signal generated at output of filter is very low as compared to transmitter frequency.
- The frequency is boosted up to the transmitter frequency by balance mixer and crystal oscillator.
- This process of frequency boosting is also called as up conversion.
- The SSB signal havng frequency equal to the transmitter frequency is then amplified by the linear amplifiers.
2.Phase shift method:
Working of phase shift method:
- It uses two balanced modulators instead of one.
- The balanced modulators effectively eliminate the carrier.
- The carrier oscillator is applied directly to the upper balanced modulator along with the audio modulating signal.
- Then both the carrier and modulating signal are shifted in phase by 90oand applied to the second, lower, balanced modulator.
- The two balanced modulator output are then added together algebraically.
- The phase shifting action causes one side band to be canceled out when the two balanced modulator outputs are combined
Input of BM1 are, sin wmt and sin (wct +90 ̊ )
Therefore output of BM1= sin (wct +90 ̊ ) sin wmt
= cos (wct +90 ̊ – wmt) – cos (wct +90 ̊ + wmt)
LSB 90 ̊ phase shift USB 90 ̊ phase shift
Input of BM2 are, sin wct and sin (wmt +90 ̊ )
Therefore output of BM2= sin (wmt +90 ̊ ) sin wct
= cos (wct- wmt -90 ̊ ) – cos (wct+ wmt +90 ̊ )
LSB -90 ̊ phase shift USB 90 ̊ phase shift
Output of summing amplifier,
= cos (wct +90 ̊ – wmt) – cos (wct +90 ̊ + wmt) + cos (wct- wmt -90 ̊ ) – cos (wct+ wmt +90 ̊ )
- One of the side band is suppressed.
The advantages of single side band SSB transmission:
- It allows better management of the frequency spectrum. More transmission can fit into a given frequency range than would be possible with double side band DSB signals.
- All of the transmitted power is message power none is dissipate as carrier power.
- The noise content of a signal is an exponential function of the bandwidth: the noise will decrease by 3dB when the bandwidth is reduced by half. There fore, single side band SSB signals have less noise contamination than DSB double side band.
- The cost of a single side band SSB reciver is higher than the double side band DSB counterpart be a ratio of about 3:1.
- The average radio user wants only to flip a power switch and dial a station. Single side band SSB recievers require several precise frequency control settings to minimize distortion and may require continual readjustment during the use of the system.
Advantages of amplitude modulation:
- Noise reduction.
- Increase the system fidelity.
- Efficient use of power.
- Low transmission power efficiency.
- Poor quality reception.
- Noisy signal reception.
- Limited operating radio range.
- Complex circuits.
- Medium and high frequency bands for long-distancet radio broadcast applications.
- Two way mobile radio communication.
- Point to point long distant telephony(SSB).
- VSB(television app).