Mumbai University > Electronics > Sem 4 > Priniciples of Communication Engineering

The methods employed for SSB generation are as follows:

1. Filter Method
2. Phase-Shift Method
3. Third Method

Third Method of SSB generation:

Operation:

• The third method of generating SSB was developed by Weaver.
• From the block diagram, we see that the latter part of this circuit is identical to that of the phase-shift method, but the way in which appropriate voltages are fed to the last two balanced modulators at points C and F has been changed.
• Instead of trying to phase-shift the whole range of audio frequencies, this method combines them with an AF carrier fo, which is a fixed frequency in the middle of the audio band, 1650 Hz.
• A phase shift is then applied to this frequency only, and after the resulting voltages have been applied to the first pair of balanced modulators, the low-pass filters whose cut-off frequency is fo ensure that the input to the last pair of balanced modulators results in the proper eventual sideband suppression.
• It may be shown that all lower sideband signals will be cancelled for the configuration of the given Figure, regardless of whether audio frequencies are above or below fo.
• If a lower sideband signal is required, the phase of the carrier voltage applied to M1 may be changed by 180°.

Advantages:

• It has the advantages of the phase-shift method, such as its ability to generate SSB at any frequency.
• It uses low audio frequencies, without the associated disadvantage of an AF phase-shift network required to operate over a large range of audio frequencies.

Disadvantages:

• The third method is in direct competition with the filter method, but is very complex.
• It is expensive and so cannot be used commercially.