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

1. Apr 9, 2012

### liam1992

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.

2. Apr 9, 2012

3. Apr 9, 2012

4. Apr 9, 2012

### skeptic2

-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.

5. Apr 9, 2012

### Staff: Mentor

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).

6. Apr 10, 2012

### liam1992

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

7. Apr 10, 2012

### Staff: Mentor

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?

8. Apr 10, 2012

### liam1992

I dont know, what I wrote was the exact wording of the question.