Low-Pass Filter

Figure 3.2 shows one possible low-pass filter. The circuit is essentially a frequency-sensitive voltage divider. At high frequencies the output behaves as if it is shorted while at low frequencies the output appears as an open circuit.

 
Figure 3.2:  RC low-pass filter

Mathematically we have

The approximations are

At the corner

Therefore

is the corner frequency of the filter. At the corner frequency

We say that the output is down by at the corner frequency.