Coherent SSB Demodulation
The product modulator is a type of coherent SSB demodulator . To recover the modulating signal from the SSB-SC signal, we require a phase coherent or synchronous demodulator .
The block diagram of the coherent SSB-SC demodulator is shown in fig.1 .
Fig 1: Block Diagram of coherent SSB demodulator
The received SSB signal is first multiplied with a locally generated carrier signal . The locally generated carrier should have exactly the same frequency as that of the suppressed carrier .
The product modulator multiplies the two signals at its input and the product signal is passed through a low pass filter with a bandwidth equal to fm . At the output of the filter, we get the modulating signal back .
Analysis of the Coherent Demodulator
Let the SSB wave at the input be given by :
The locally generated carrier is cos (2π fct)
Therefore, output of the product modulator is given by :
When v(t) is passed through the filter, it will allow only the first term to pass through and will reject all other unwanted terms . Thus, at the output of the filter we get the scaled message signal and the coherent SSB demodulation is achieved .
Phase Error and Frequency Error in Coherent Detection
In the coherent detection explained above, it is assumed that the ideal operating conditions in which the locally generated carrier is in the perfect synchronization .But, in practice, a phase error Φ may arise in the locally generated carrier wave.
The detector output will get modified due to this phase error as follows :
In the above expression, the plus sign corresponds to the SSB input signal with only USB whereas the negative sign corresponds to SSB input with only LSB.
Due to the presence of the Hilbert transform in the output, the detector output will suffer from the phase distortion.
Such a phase distortion does not have serious effects with the voice communication. But in the transmission of music and video, it will have untolerable effects .