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Matlab Examples – Amplitude Demodulation Using Envelope Detection

Another detection scheme is called envelope detection consisting of a simple diode and a low pass filter ( a parallel connection of a resistor and capacitor).The low pass filter gets rid of the higher frequencies coming out of the diode and to allow the low frequency message signal to pass through the filter. The detection scheme is associated with Double Sideband Large Carrier AM (DSB-LC AM).A dc or average value is needed in the message so a large carrier can be generated at the output of the first multiplier. The dc signal in the message must be large enough so that it can conduct a diode and to recover the message from the envelope of the DSB-LC AM signal. The dc value must be large enough such that the message plus dc signal is always positive.The disadvantage of this scheme occurs when there is no message . In this case, wasted carrier energy is used up in the unmodulated carrier waveform. Another way to look at the disadvantage is when dc value is added to the message, it provides no additional information to the message.The advantage of this scheme is that adding a dc value simplifies the receiver and does not require a more complex circuitry like the phased-locked loop (PLL) used in synchronous detection discussed earlier.Since envelope detection does not require any phase information about the transmitted carrier, this approach is often called non-coherent detection.Note that the rectified signal coming out of the diode can be modeled as a sampled signal. In this case, we multiply the message plus the dc value with a periodic pulse train with an peak-to-peak amplitude of one and an average value of 0ne-half.This above model and envelope detection are shown in the video below. Again, we are using Simulink found in Matlab.

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