Filter Method for Generating SSB Signal
The filter method can be used for generating the SSB modulated wave if the message signal satisfies the following conditions :
- The message signal should not have any low frequency content . The audio signal posses this property, e.g. the telephone signal will have a frequency range extending from 300 Hz to 3.4 kHz . The frequencies in the range 0-300 Hz are absent .
- The highest frequency in the spectrum of the message signal i.e. W Hz should be much smaller than carrier frequency fc .
System Block Diagram
Fig. 1 shows the block diagram of an SSB modulator which operates on the principle of frequency discrimination .
Fig.1 : Block Diagram of SSB modulator using frequency discriminator method
This modulator consists of a product modulator, carrier oscillator and bandpass filter designed to pass the desired sideband.
At the output of the product modulator, we get the DSB-SC modulated wave which contains the two sidebands only .
The bandpass filter will pass only one of these sidebands and produce the SSB modulated wave at its output .
Let us consider fig. 1 which shows the DSB-SC signal at the output of the product modulator which contains both the sidebands .
The frequency difference between the highest frequency in LSB and the lowest frequency in USB is too small as shown in fig.2 .
Fig.2 : spectrum of message signal and product modulator output
This makes the design of the bandpass filter extremely difficult because its frequency response need to have very sharp change over from attenuation to pass band and vice versa.
Design of bandpass Filter
The design of bandpass filter must be based on satisfying the following conditions :
- Passband of the BPF should occupy the same frequency range as that occupied by the spectrum of the desired SSB modulated wave .
- The width of the guard band which separates the passband from stopband be twice the lowest frequency component of the message signal . i.e. Guard band = 2f1 Hz