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I've created a simulation on Matlab of amplitude modulation. After demodulating the signal I need to add a low-pass filter to remove the high frequency components.

I'm confused as to why this is? I though it was to get smooth out some of the additive white Gaussian nose I added, but surely that noise is added across all frequencies as its 'white' noise.

What is the need in filtering the demodulated signal and why is this needed before we can recover the signal?

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  • \$\begingroup\$ Please look at the answer by @DanBoschen here: dsp.stackexchange.com/a/51670/41790. In the simplest case, a sine wave of unknown amplitude, in noise, is demodulated by multiplying by a reference sine wave of known amplitude, with the same frequency and phase as the noisy sine wave. Then the difference frequency is at DC, so the DC magnitude is proportional to the noisy sine wave’s amplitude. The low pass filter estimates that DC magnitude and kills the sum frequency. This is a simple lock-in amplifier or phase synchronous demodulator. \$\endgroup\$ – Ed V Apr 28 '20 at 20:21
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If your required bandwidth is (say) 1 MHz then based on what you know about the noise, how much noise power is contained in the range DC to 1 MHz compared to how much is contained between 1 MHz and 99999999999 MHz (or more)?

What is the need in filtering the demodulated signal and why is this needed before we can recover the signal?

Because it helps improve the signal to noise ratio and, in turn, makes data recovery more plausible.

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  • \$\begingroup\$ I was thinking of posting an answer, but your excellent answer here appears to fully answer the OP’s question. Please correct me if I am wrong, but I am going back to that answer to upvote and bookmark it. \$\endgroup\$ – Ed V Apr 29 '20 at 13:51
  • \$\begingroup\$ @EdV thank you. \$\endgroup\$ – Andy aka Apr 29 '20 at 15:15

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