A low pass filter eliminates high frequency signals from an input signal.
There are two main types of passive low pass filters, a **RC LowPass Filter**
Is made from a capacitor and a resistor while a **RL LowPass Filter**
Is made from an inductor and a resistor.
![[rc-rl-lp-filter.png]]
We can also see that for the RL LowPass Filter the positions of the resistor and the reactive component are switched. This is because the inductor works in the opposite way of the capacitor, allowing low frequencies to pass and attenuating high ones.
**Example:**
Lets design a RC LowPass Filter with a [[glossary#Cutoff Frequency|Cutoff Frequency]] of $15.9kHz$. The Formular for calculating the cutoff frequency is the following:
Now we still have two unknown variables to fill in, for my personal taste that is one variable to much. Usually when designing a low pass filter we first choose the value of $R$.
We do not want $R$ to be too high, as that will limit the current flow. We also do not want it to be *too* low because then we would need a big capacitor. For now we will choose:
$\displaystyle R =1k\ohm$
With this our formula now looks something like this:
$\displaystyle 15900 = \frac{1}{2*\pi*1000*C}$
Now we only have one unknown variable, which is $C$ so we can solve the equation:
