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 90o phase shifting networks as shown in fig. 1 .
Fig 1: Phase shift method for generating SSB signal
The message signal x(t) is applied to the product modulator M1 and through a 90o phase shifter to the product modulator M2 .
Hence, we get the Hilbert transform
at the output of the wideband 90o phase shifter .
The output of carrier oscillator is applied as it is to modulator M1 whereas it s passed through a 90o phase shifter and applied to the modulator M2 .
The outputs of M1 and M2 are applied to an adder .
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 90o 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 :
- It can generate the SSB signal at any frequency, so the frequency up converter stage is not required .
- It can use the low audio frequencies as modulating signal .(In filter method, this is not possible ) .
- It is easy to switch from one sideband to other .
Drawbacks of Phase Shift Method
- The main drawback is that the design of the 90o phase shfting network for the modulating signal is extremely critical .
- This network has to provide a correct phase shift of 90o at all the modulating frequencies which is practically difficult to achieve .