LOW-PASS FILTERS:

A low-pass filter is a circuit that offers easy passage to low-frequency signals and difficult passage to high frequency signals. There are two basic kinds of circuits in low pass filters,

The inductive low-pass filter and the capacitive low-pass filter,

The inductance impedance increases with increasing frequency. This high impedance in series tends to block high-frequency signals from getting to the load.

The capacitor’s impedance decreases with increasing frequency. This low impedance in parallel with the load resistance tends to short out high-frequency signals, dropping most of the voltage across series resistor R.

All low-pass filters are rated at a certain cutoff frequency. That is, the frequency above which the output voltage falls below 70.7% of the input voltage. In a simple capacitive/resistive low-pass filter, it is the frequency at which capacitive reactance in ohms equals resistance in ohms. In a simple capacitive low-pass filter (one resistor, one capacitor), the cutoff frequency is given as:

fcutoff =1/2*3.14*RC