How to Determine dB Attenuation for Power Filters

Low-Pass Filter

• 1

  Multiply the filter capacitor's capacitance (in farads) by the frequency of the input signal (in hertz).

• 2

  Multiply this result by 6.28.

• 3

  Divide 1 by the answer from Step 2. This value represents the "reactance" exhibited by the capacitor at that particular input frequency.

• 4

  Square the resistance (in ohms) of the filter's resistor.

• 5

  Square the filter's reactance (from Step 3).

• 6

  Add together the results from Steps 4 and 5.

• 7

  Calculate the square root of the result from Step 6.

• 8

  Divide the reactance (from Step 3) by the result from Step 7. This value is the ratio of the output voltage to the input voltage for the filter.

• 9

  Calculate the standard logarithm (base 10) for the output/input ratio from Step 8.

• 10

  Multiply by 20. This is the attenuation, in decibels, of the signal at that particular frequency.

High-Pass Filter

• 11

  Multiply the filter capacitor's capacitance (in farads) by the frequency of the input signal (in hertz).

• 12

  Multiply this result by 6.28.

• 13

  Divide 1 by the answer from Step 2. This value represents the "reactance" exhibited by the capacitor at that particular input frequency.

• 14

  Square the resistance (in ohms) of the filter's resistor.

• 15

  Square the filter's reactance (from Section 2, Step 3) and add it to the result from Step 4.

• 16

  Calculate the square root of this product.

• 17

  Divide the resistor's resistance by the result from Section 2, Step 6. The resulting value represents the filter's ratio of output voltage to input voltage.

• 18

  Calculate the standard logarithm (base 10) of this ratio.

• 19

  Multiply by 20. This is the calculated decibel attenuation for the high-pass filter at that particular signal frequency.