SSB Demodulation using Coherent Detection

SSB(Supressed Sideband) demodulation is the process of recovering message signal from the SSB modulated wave. SSB is also called SSB-SC where SC means suppressed carrier as the carrier signal in the SSB signal is completely removed. Here we illustrate how we can demodulate SSB modulated signal to recover message signal. The SSB demodulation method demonstrated here is called coherent detection. The coherent detection means that the frequency of the local oscillator signal generated at the SSB AM receiver is synchronized with the carrier signal frequency.

The following circuit diagram shows SSB modulator that uses phase discrimination method and SSB receiver demodulator circuit that uses coherent detector.

The above circuit shows SSB modulator as well as SSB demodulator. The SSB modulator is explained in SSB modulation Transmitter Circuit.

The SSB demodulator circuit consist of the last DSB-SC modulator, the passive low pass filter and the operational amplifier. The following shows simplified SSB demodulation block diagram that uses the coherent method for demodulation.

The DSB modulator, commonly called product modulator, is made using the AD633 analog multiplier IC. It multiplies the incoming SSB modulated signal $$s(t)$$ with locally generated carrier signal $$c(t)$$ to generate signal $$r(t)$$ as follows.

The SSB signal is,

$$s(t) = cos(w_ct) m(t) \pm \hat{m(t)} sin(w_ct)$$  -------->(1)

and $$c(t) = cos(w_c t)$$  -------->(2)

So, $$r(t) = s(t) c(t)$$  -------->(3)

or,  $$r(t) = cos(w_c t)[cos(w_ct) m(t) \pm \hat{m(t)} sin(w_ct) ]$$

or, $$r(t) = [cos^2(w_ct) m(t) \pm \hat{m(t)} sin(w_ct) cos(w_c t)]$$

or,  $$r(t) = [\frac{1}{2}\{1+cos(2w_ct)\} m(t) \pm \frac{1}{2} \hat{m(t)} sin(2w_ct)]$$

that is,  $$r(t) = \frac{1}{2}m(t) + \frac{1}{2}m(t) cos(2w_ct)\} \pm \frac{1}{2} \hat{m(t)} sin(2w_ct)]$$ --->(4)

The second and the third terms can be removed using the low pass filter. After low pass filtering we get,

$$v(t) = \frac{1}{2}m(t)$$  --->(5)

We can then use operational amplifier since the message signal is in audio range to amplify the message signal and obtain recovered message signal.

$$m_{demod}(t) = m(t)$$  --->(6)

So in this way we showed SSB modulation and demodulation circuit works. The SSB signal was generated using SSB modulator that uses phase discrimination method but the SSB demodulator shown here can also be used for SSB signal generated by frequency discrimination SSB modulator. We explained how we can demodulate SSB  signal using coherent detection. Coherent detector uses standard DSB modulator and then low pass filter to remove high frequency components. The low pass filtered signal is then fed into LM358 audio amplifier since the demodulated message signal is weak for speaker. Coherent detection relies on the assumption that the frequency of the carrier signal at the SSB transmitter and the frequency of the local oscillator signal at the SSB receiver is in synchronization. Also ideally, the transmitted carrier signal phase and the locally generated signal phase should be in synchronization otherwise we will get phase error and therefore phase distortion. This is the main disadvantage of the coherent detection method for SSB-SC demodulation.

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