I understand that capacitors will charge until it’s “full” and voltage and current = 0 because no more electrons can go through. In AC circuits, capacitors will charge and discharge according to the ups and downs of the sin graph. But how does this filter noise?
Noise being different frequencies than the sin wave that we want?
In: Physics
Lets say you send sine waves through a capacitor. **At a low frequency the capacitor fills long before the wave changes sign**. A fast frequency does not last long enough for that, it changes sign before it gets close to full and then even depletes it again.
But even for high frequencies it isn’t just letting them through unhindered. **The fuller a capacitor already is, the less current will flow** in (at any fixed voltage). So the slower a sine wave changes, the more it fills the capacitor, and thus the less current will flow on average.
But **the ratio of voltage per current is _resistance_**. So what we have observed is that a capacitor acts like a resistor, but not one of fixed value. Instead **the capacitor acts like a resistor that depends on frequency**; slow waves face high resistance, higher frequencies lead to low resistance. If you do the math you will find that the resistance is proportional to 1/frequency.
In practical terms this means that **putting a capacitor in series will strongly hinder low frequencies**, while letting high ones through. A _high pass filter_, it lets high frequencies pass through. **If instead you put a capacitor in parallel to a device, then all high frequencies will be diverted through the capacitor**, the device will mostly get the low ones. A _low pass filter_.
For all intents and purposes you can see DC as AC of zero frequency whenever it matters.
Okay, but to understand filtering you need one more crucial ingredient: **the superposition principle**.
Certain basic components such as resistors, capacitors, inductances(coils), transformers, and a bunch more are (almost) _linear_. This means that **combining two inputs leads to the combined result of whatever they would do individually**.
Lets as an example look at the combination of a low frequency and a high frequency wave such as a radio signal (sound waves, 20-20000 Hz) on a carrier wave (megahertz range or above). Say we again put a capacitor in parallel to our sensor or a loudspeaker:
– Most of the high frequency wave is diverted through the capacitor and thus not seen by our sensor.
– The low frequency wave however flows mostly through he sensor, as the capacitor acts like a high resistance.
Thus by the superposition principle our sensor will primarily see the low frequency part. It filters the carrier wave of the radio signal and leaves us with the sound part. This is far from the only or best way to make a radio receiver, but it definitely is a starting point.
Lastly it should be noted that _inductances_ such as coils act exactly the opposite way: they strongly block high frequencies but let low ones through.
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