At what frequency will the frequency be attenuated by -20dB?

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Discussion Overview

The discussion revolves around determining the frequency at which a high pass filter, consisting of a 1kΩ resistor and a 330nF capacitor, will experience an attenuation of -20dB. The scope includes theoretical calculations and practical application of filter design principles.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant suggests using the equation gain=20log(Xc/Z) to find the frequency but expresses uncertainty about its correctness.
  • Another participant calculates the frequency to be approximately 48.47 Hz.
  • A request for clarification on the calculation method is made by a third participant.
  • A fourth participant provides a formula involving -20 dB and the magnitude of the impedance, indicating they used trial and error to find the frequency.
  • One participant notes that first order RC filters have a power attenuation rate of 20dB per decade and suggests finding the corner frequency and adjusting accordingly.
  • A similar point is reiterated by another participant, who provides a different frequency of 53.6 Hz but expresses confusion over their own calculation yielding 48.23 Hz.
  • There is a discussion about the interpretation of the original question regarding whether the attenuation refers to power or voltage, with some participants questioning the clarity of the phrasing.

Areas of Agreement / Disagreement

Participants express differing calculations for the frequency at which -20dB attenuation occurs, with no consensus reached on the correct value. Additionally, there is uncertainty regarding the interpretation of the original question about power versus voltage attenuation.

Contextual Notes

Participants rely on different interpretations of the question and various methods for calculating the frequency, leading to discrepancies in results. There is also a lack of clarity regarding the definitions of power and voltage ratios in the context of the discussion.

liam1992
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For a high pass filter with a 1kΩ resistor and a 330nf capacitor at what frequency will the frequency be attenuated by -20dB?

I thought about using the equation gain=20log(Xc/Z) but I am not sure if this is correct, I saw it on a website and even if it is I can't seem to calculate the frequency.
 
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I get about 48.47 Hz.
 
Can you show me how you got that answer please.
 
-20 dB = 10*Log(Mag(R/(R+jXc)))
Xc = -1/(2*pi*F*3.3*10^-7)
Mag is short for magnitude = sqrt(R^2 + Xc^2)

I did it by trial and error substituting values for F.
 
First order RC filters have a power attenuation rate of 20dB per decade. So find the corner frequency from the part values and add a decade (in the appropriate direction).
 
gneill said:
First order RC filters have a power attenuation rate of 20dB per decade. So find the corner frequency from the part values and add a decade (in the appropriate direction).

The answer is 53.6Hz but by doing it this way I get an answer of 48.23Hz so I don't know where I have gone wrong
 
In your original post you phrased the question as, "at what frequency will the frequency be attenuated by -20dB?". I think everyone has interpreted that to mean at what frequency will the power be attenuated by 20 dB. Is it indeed the power ratio that we're looking for or is it the voltage ratio? Or something else?
 
gneill said:
In your original post you phrased the question as, "at what frequency will the frequency be attenuated by -20dB?". I think everyone has interpreted that to mean at what frequency will the power be attenuated by 20 dB. Is it indeed the power ratio that we're looking for or is it the voltage ratio? Or something else?

I don't know, what I wrote was the exact wording of the question.
 

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