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

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Discussion Overview

The discussion revolves around identifying the type of filter depicted in a circuit diagram, specifically whether it is a low pass filter (LPF) or another type, such as a band pass filter (BPF). Participants explore the characteristics of RLC circuits and their configurations, focusing on theoretical aspects and practical implications.

Discussion Character

  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant initially suggests the filter is a "low pass" filter but questions this based on a comparison with an image on Wikipedia.
  • Another participant identifies it as a second order LPF with a Q factor of approximately unity.
  • Some participants argue that the filter resembles a series band pass filter according to Wikipedia, expressing uncertainty about its classification.
  • A later reply clarifies that a series RLC circuit can function as low pass, band pass, or high pass depending on the output voltage tap point, stating that output across the capacitor yields a low pass response.
  • It is noted that a low pass or high pass configuration can exhibit a "peaky" response near the cutoff frequency, which may visually resemble a band pass filter response, but technically remains classified as low pass or high pass.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the classification of the filter, with multiple competing views presented regarding its nature and behavior.

Contextual Notes

The discussion highlights the dependence on the configuration of the RLC circuit and the output voltage tap point, which influences the filter's classification. There are unresolved aspects regarding the specific characteristics of the filter in question.

pat666
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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
 

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

What was it described as on Wikipedia?
 
Looks more like a series band pass in Wikipedia. Not sure what that is?
 
pat666 said:
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.
 
Last edited:
pat666 said:
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).
 
Last edited:
Thanks for the explanations - clears things up
 

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