Electrical - What type of filter is this (Low Pass?)

[pat666]

The title says it all, what type of filter is this?
I thought it was a "low pass" filter but the image on Wikipedia is different.


Thanks

 

[uart]

Yeah it's a second order LPF with a Q factor of approx unity.

What was it described as on Wikipedia?

 

[pat666]

Looks more like a series band pass in Wikipedia. Not sure what that is?

 

[uart]

Looks more like a series band pass in Wikipedia. Not sure what that is?

Post a link. If it had a very high Q factor it could be considered "like a bandpass filter". But as it stands, no it's not BP.

 

[pat666]

http://en.wikipedia.org/wiki/RLC_circuit

 

[uart]

http://en.wikipedia.org/wiki/RLC_circuit

Ok that site shows a lot of bandpass like responses, but not for the circuit in that particular configuration (output across the capacitor).

The simple story is that a series RLC circuit can operate as either lowpass, bandpass or highpass, depending on from where you tap the output voltage. If you take the output from across the capacitor then it's always lowpass, if you take the output from across the inductor then it's always highpass, and if you take the output from across the resistor then you get a bandpass response.

Like I said earlier however, in the LPF (or HPF) configuration you can get a very "peaky" response near the cutoff frequency if the damping factor is too low. When this happens it looks a little bit like a BPF response but technically it's not (as DC still passes in the case of the "peaky" LPF and high frequencies still pass in the case of the "peaky" HPF).

 

[pat666]

Thanks for the explanations - clears things up