Hey all,

I'm working on a little hobby project which has led me to designing a low-pass filter with a cut-off frequency of 150Hz (300pi rad/s). The input signal will have an amplitude of 1V and frequencies varying from 40Hz - 20kHz. Since this will be dealing with sound I've chosen a second-order Butterworth filter in a Sallen-Key configuration. I've derived the following two equations using the normalized Butterworth polynomials:

\(C_{2}(R_1 + R_2) = \frac{\sqrt{2}}{\omega_c}\)

\(C_1C_2R_1R_2 = \frac{1}{\omega_c^2}\)

(Using the circuit configuration shown here: http://upload.wikimedia.org/wikipedia/commons/thumb/3/31/Sallen-Key.svg/450px-Sallen-Key.svg.png)

Now, I have a couple of questions:

1. For this filter what kind of Capacitor should I use? This range of frequencies seems pretty much a grey-zone Tantalum Capacitors. Do I need to choose capacitors which are ideal for the range I want to pass, or the range I want to filter out?

2. What assumptions should I make when determining the components? Is it better to choose capacitors, then figure out resistor values? Is it safe to say C1 = C2 (so I don't have to buy two different capacitor values?)

I'm working on a little hobby project which has led me to designing a low-pass filter with a cut-off frequency of 150Hz (300pi rad/s). The input signal will have an amplitude of 1V and frequencies varying from 40Hz - 20kHz. Since this will be dealing with sound I've chosen a second-order Butterworth filter in a Sallen-Key configuration. I've derived the following two equations using the normalized Butterworth polynomials:

\(C_{2}(R_1 + R_2) = \frac{\sqrt{2}}{\omega_c}\)

\(C_1C_2R_1R_2 = \frac{1}{\omega_c^2}\)

(Using the circuit configuration shown here: http://upload.wikimedia.org/wikipedia/commons/thumb/3/31/Sallen-Key.svg/450px-Sallen-Key.svg.png)

Now, I have a couple of questions:

1. For this filter what kind of Capacitor should I use? This range of frequencies seems pretty much a grey-zone Tantalum Capacitors. Do I need to choose capacitors which are ideal for the range I want to pass, or the range I want to filter out?

2. What assumptions should I make when determining the components? Is it better to choose capacitors, then figure out resistor values? Is it safe to say C1 = C2 (so I don't have to buy two different capacitor values?)

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