# Block diagram of a demodulator with a low-pass filter

I am having some difficulty designing a block diagram of a demodulator with a low pass filter - I don't know how to add it to the schematic(apply the LP filter to the diagram). I have an input signal (sinusoidal) given to me, it then goes through a mixer which adds a signal that is also harmonic, to then mix both of these signals together to form the output signal.

This looks more or less like in the picture. Now I need a low pass filter to filter out the upper frequencies to create a multi-level rectangular wave.

I've been told that mixers in fact multiply signals but it only happens when the signal is in time domain. In this case it is based on Angular frequency/pulsatance domain and I've been taught that mixers on pulsatance add or subtract the signal.

I would also appreciate it if it was based on any literature

• Can you clarify what you mean by "adding it to the schematic"? Are you using a particular drawing tool?
– MBaz
May 25, 2022 at 21:00
• When you say "input signal (harmonic)", do you mean that the input is sinusoidal? And are you sure that your mixer is performing $y(t) = r(t) + s(t)$? If you're doing RF signal processing, then a mixer performs $y(t) = r(t)s(t)$, i.e. there's some sort of multiplication going on. Audio signal processing says "mixer" when they mean two signals are being added. (Yes, it's confusing. But there it is, we all have to live with it). May 25, 2022 at 21:41
• Please edit your question with any clarifications. May 25, 2022 at 21:41
• @TimWescott uh yes, (i used translator maybe its wasnt the proper name for that) its sinusoidal. I've been told that mixers in fact multiply signals but it only happens when the signal is 'based on' time. In this case it is based on Angular frequency/pulsatance and I've been taught that mixers on pulsatance add or subtract the signal. May 25, 2022 at 22:02
• @MBaz I just basically need to apply a LP filter to this diagram, the problem is I don't know whether just drawing an additional line like it happend for s(t) would work May 25, 2022 at 23:05

In the middle of the block you either put a transfer function (i.e. $$\frac{\omega_0}{s + \omega_0}$$, or you call out a transfer function (In the Laplace domain, i.e. $$H(s)$$ if continuous time, in the $$z$$ domain if discrete), or you put in a little graphic -- folks will call out filters with three horizontal squiggly lines, with diagonal "block" marks through some of them. So a lowpass filter would have the top two lines "blocked", and the bottom one "passing".