What Does Low Pass Filter Do

Depression Pass Filter- Explained

Low Pass Filter

A low laissez passer filter is a filter which passes low-frequency signals and blocks, or impedes, high-frequency signals.

In other words, low-frequency signals go through much easier and with less resistance and loftier-frequency signals have a much harder getting through, which is why it's a low pass filter.

Depression pass filters can be constructed using resistors with either capacitors or inductors. A low laissez passer filter equanimous of a resistor and a capacitor is called a low pass RC filter. And a low pass filter with a resistor and an inductor is called a low pass RL filter.

We volition become through both of these type of circuits on this folio and show how both RC and LC low laissez passer filters are synthetic. Both circuits take the effect of passing through low frequency signals while impeding high-frequency ones.

Low Pass RC Filter

A Low pass RC filter, again, is a filter circuit composed of a resistor and capacitor which passes through low-frequency signals, while blocking high frequency signals.

To create a low pass RC filter, the resistor is placed in serial to the input signal and the capacitor is placed in parallel to the input indicate, such as shown in the circuit below:

RC low pass filter

So, with this setup, the in a higher place circuit is a depression pass filter. As a capacitor is a reactive device, information technology offers differing resistance to signals of different frequencies inbound through it. A capacitor is a reactive device which offers very high resistance to low-frequency or DC signals. And it offers low resistance to high-frequency signals. Equally it offers very high resistance to DC signals, in this circuit, it will block DC from inbound and pass them off to an culling part in the circuit, which is shown to the right past the arrow. High-frequency signals will go through the capacitor, since the capacitor offers them a very low-resistance path. Call up that current always takes the path of least resistance. Existence that a capacitor represents a depression resistance in a circuit for high-frequency signals, they volition take the path through the capacitor, while low-frequency signals will take an culling, lower-resistance path.

How to Build a Low Laissez passer RC Filter

At present that we've gone through what a low laissez passer RC filter is, let's go over a practical example of building one.

To build a low pass filter, the components we will utilize are a role generator, a 10nF ceramic capacitor, and a 1KΩ resistor.

This is the schematic of the excursion we will build, shown below:

RC low pass filter example

The formula to find the frequency cutoff signal of an RC circuit is, frequency= 1/2πRC. Doing the math, with the values shown above, we get a frequency of, frequency= ane/2πRC = i/2(iii.14)(1KΩ)(10nF)= 15,923 Hz, which is approximately xv.9KHz.

This ways that all frequencies above 15.9KHz are attenuated. And as yous go further (higher) from the 15.9KHz region, the attenuation becomes greater and greater.

Frequencies beneath xv.9KHz are passed through without attenuation.

So if nosotros input an Air conditioning point into the excursion from the function generator and make the signal a depression frequency such as 10Hz, the excursion will pass this signal to output virtually completely unattenuated. This is because low frequency signals do not take the path of the capacitor. You tin can check this if you have an oscilloscope. If you at present increase the frequency of the signal to 30KHz, the betoken will pass through to output with neat attenuation. This is because high-frequency signals go through the capacitor and not to output, because capacitor is low resistance to them.

Low Laissez passer RL Filter

A low pass RL filter, again, is a filter circuit composed of a resistor and inductor which passes through depression-frequency signals, while blocking high-frequency signals.

To create a depression pass RL filter, the inductor is placed in serial with the input signal and the resistor is placed in parallel to the input bespeak, such as shown in the circuit beneath:

Low Pass RL Filter

This circuit above is a depression pass RL filter. How it works is based on the principle of anterior reactance. Anterior reactance is how the impedance, or resistance, of the inductor changes based on the frequency of the point passing through the inductor. Unlike a resistor, which is a nonreactive device, an inductor offers differing impedance values to signals of differing frequencies, just as capacitors do. All the same, unlike capacitors, inductors offer very high resistance to high-frequency signals and offers low resistance to low-frequency signals. So it's the reverse of a capacitor. Therefore, the placement of the resistors are switched in RC and RL filter circuits. So, based on this, the to a higher place RL excursion works finer as a depression pass filter. Information technology blocks loftier-frequency signals from entering and allows low-frequency signals to pass through unimpeded.

How to Build a Low Pass RL Filter

So, now that RL filters accept been summarized, permit'due south get over a practical example of edifice 1.

To build a depression laissez passer filter, the components we will use are a function generator, a 470mH inductor, and a 10KΩ resistor.

This is the schematic of the circuit we will build, shown beneath:

Low pass RL filter example

The formula to find the frequency cutoff point of an RL circuit is, frequency= R/2πL. Doing the math, with the values shown above, nosotros get a frequency of, frequency= R/2πL = (10KΩ)/(ii(3.14)(470mH))= 3,388 Hz, which is approximately iii.39KHz.

This means that all frequencies in a higher place 3.39KHz are attenuated. And as yous get further (higher) from the 3.39KHz region, the attenuation becomes greater and greater.

Frequencies beneath 3.39KHz are passed through without attenuation.

And so, again, you can check this on an oscilloscope to see that very low-frequency signals are passed through to output unattenuated, while high-frequency signals undergo attenuation.

To see how a low pass filter works in a existent circuit, see the following video. Below y'all can see a video of an RC low pass filter. This video demonstrates how the RC low laissez passer filter lets low-frequency signals laissez passer through to output with full gain, no attenuation, while profoundly attenuating high-frequency signals.