[mlundy57]

If a first order in-line filter is designed to be -3dB at 100Hz, it would be -9dB at 50Hz. How do you determine where it will be -6dB since the drop is not linear?

[jmimac351]

.

[HAL]

For a single pole high pass filter you can use the equation:

C=1/(2*Pi*R*F-3dB), so for a R=47000 Ohm input impedance, and a F-3dB=100Hz, C= 0.0338uF. 

Linear Technologies has a free version of SPICE for circuit simulation.  I used it to plot the frequency response of that filter.  I put the cursor at the -6.02dB point as close as possible with the system.  The -6dB point is ~58Hz.
 

[jmimac351]

Hal, thanks for sharing that formula...

High Pass Calculator:
http://www.learningaboutelectronics.com/Articles/High-pass-filter-calculator.php

Capacitance Conversion:
https://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-capacitance

[HAL]

The 0.0338uF would be 33.8nF or 33800pF.

If you start with the original uF value and multiply x1000, then it is nF, do it again and it is pF.

[jmimac351]

The 0.0338uF would be 33.8nF or 33800pF.

If you start with the original uF value and multiply x1000, then it is nF, do it again and it is pF.

Got it! I see the mistake I made... Should have entered "47" into the calculator instead of 47,000, given that I had the value set to "Kohms".  Thank you. 

[jmimac351]

Here's what I got for a spreadsheet for capacitor value / formula for 100hz high pass filter... or given whatever values are entered into the yellow cells.

[mlundy57]

For a single pole high pass filter you can use the equation:

C=1/(2*Pi*R*F-3dB), so for a R=47000 Ohm input impedance, and a F-3dB=100Hz, C= 0.0338uF. 

Linear Technologies has a free version of SPICE for circuit simulation.  I used it to plot the frequency response of that filter.  I put the cursor at the -6.02dB point as close as possible with the system.  The -6dB point is ~58Hz.
 

Thanks Rich,

I've downloaded the software. Now I have to learn how to use it to get what you are showing. Then I can manipulate the parameters. Based on your example, the -6 point is 84% of the way through the octave, not 50% like it would be if it was linear instead of logarithmic. It will be interesting to see if the -6 point is 84%  of the way through the octave with other -3 points between 40Hz and 200Hz.

[HAL]

Mike,
Glad to help.

Rich