Draw and Explain the Block Diagram of Phase Shift Method for Generating SSB Signal .

Phase Shift Method for the SSB Generation

Fig. 1 shows the block diagram for the phase shift method of SSB generation .

This system is used for the suppression of lower sideband .

This system uses two balanced modulators M₁ and M₂ and two 90^o phase shifting networks as shown in fig. 1 .

Fig 1: Phase shift method for generating SSB signal

Working Operation

The message signal x(t) is applied to the product modulator M₁ and through a 90^o phase shifter to the product modulator M₂ .

Hence, we get the Hilbert transform

at the output of the wideband 90^o phase shifter  .

The output of carrier oscillator is applied as it is to modulator  M₁ whereas it s passed through a 90^o phase shifter and applied to the modulator M_2 .

The outputs of   M₁ and M₂ are applied to an adder .

Therefore,

This expression represents the SSB signal with only LSB i.e. it rejects the USB .

SSB with Upper Sideband (USB)

The block diagram of the phase shift method to generate SSB with LSB is shown in fig. 2 .

Fig 2 : Phase shift method to suppress the LSB

Note that the adder polarities for the in-phase is positive and  for the quadrature paths is negative .

Suppression of the upper sideband

We can suppress the LSB and generate the SSB signal consisting of the USB by arranging the blocks as shown in fig. 2 .

Here, the modulating and the carrier signals are applied to the upper balanced modulator directly (without any phase shift ) .Whereas, both these signals are 90^o phase shifted and then applied to the lower balanced modulator .

Advantages of Phase Shift Method

The advantages of the phase shift method are as under :

1. It can generate the SSB signal at any frequency, so the frequency up converter stage is not required .
2. It can use the low audio frequencies as modulating signal .(In filter method, this is not possible ) .
3. It is easy to switch from one sideband to other .

Drawbacks of Phase Shift Method

1. The main drawback is that the design of the 90^o phase shfting network for the modulating signal is extremely critical .
2. This network has to provide a correct phase shift of 90^o at all the modulating frequencies which is practically difficult to achieve .