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This circuit is a bit of a contradiction: it uses analog components to simulate a digital implementation of a low-pass filter, usually done at the code level of a microcontroller or DSP.
The input frequency and waveform are adjustable from the leftmost signal generator, which is fed into the network.
The frequency of the input sine wave can go up to F = 1/2*Ts = 1KHz (Nyquist frequency).
You can also substitute it with a unitary step generator to see the step response typical of a low-pass filter.
The output voltage is defined by this equation:
y(k) = a*y(k-1) + b*x(k)
where the "a" part is called "autoregressive" and makes the filter behave like a IIR (analog) one. The "b" part is called "moving average" and makes the filter have a FIR component as well.
The cutoff frequency, with the chosen coefficients, is at about 600Hz.
To change the coefficients of the filter, change the ratios of the two pots located on the top-left amplifier (controls the "b" coefficient) and at the down-center location (controls the "a" coefficient) of the schematic.
Careful with the values: the value of the "a" part (IIR component) CANNOT be greater or equal than 1 (so pot must not cross 50%), otherwise the filter results UNSTABLE.
The b part can be any value, and only affects the gain.
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