State advantage of SSB over DSBFC. Explain filter method to generate SSB AM
1 Answer

The advantages of SSB over DSB-FC signal are:

  1. Less bandwidth requirement as SSB requires a BW of $f_m$ .This will allow more number of signals to be transmitted in the same frequency range.

  2. Lots of power saving. This is due to the transmission of only one sideband component .At 100% modulation, the percent power saving is 83.33 %.

  3. Reduced interference of noise .This is due to the reduced bandwidth .As the bandwidth increases the amount of noise added to the signal will increase.

Filter method

  • The filter method of SSB generation produces suppressed carrier double sideband signals (using a balanced modulator), one of which is then filtered to leave USB or LSB.

  • This can be achieved using one of two methods. The first uses two filters that have different passband centre frequencies for USB and LSB respectively.

  • The second uses only one filter but switches different carrier frequencies tuned to pass the USB or LSB to pass through the filter.

  • Since steep roll off filters are required to suppress the opposite sidebands, the carrier frequency is usually an IF frequency so that practical filters can be implemented.

  • The resultant SSB signal is then mixed (heterodyned) to shift its frequency higher as shown in Figure

  • The performance of this type of SSB generation is dependent on carrier rejection using a high quality balanced modulator as well as sideband suppression through the use of a suitable carrier frequency and filters with good rejection of out of band signals.

  • A disadvantage is the need for mixing to produce a suitable SSB output.

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