RC Filters

In this instructable I will show you how to make simple RC high-pass and low-pass filters.

Why? They filter out certain frequencies, which may be useful when building upon the circuit to make it more complex. You may want a specific range of frequencies from your original signal. For example, high-pass filters are often used in speakers.

What you need:

Capacitor (I used 100nF)

Resistor (I used 1K-ohm)

Input signal (Function generator)

Oscilloscope

Breadboard

Step 1: High-pass Filters

These circuits let high frequencies “pass”, thus removing low frequency signals.

Set up your circuit as shown above. The input signal goes through the capacitor first, then the resistor. Your output signal should be read between the capacitor and the resistor.

In my circuit, I hooked Vout up to an oscilloscope. Reading the oscilloscope, you can see that as the frequency increases, the output signal gets larger until the output and input frequencies are the same. Note that although the frequency amplitudes can become equal, there still may be a phase shift between the input and the output.

Taking data measurements from the circuit of Vin and Vout, you can plot the gain versus frequency (called a Bode Plot). I plotted the data in a log-log plot and marked where the 3dB point is. The 3dB point if where the amplitude has dropped by √2, or falls off at “6 dB per octave”, or most simply the “cut-off” point. The 3dB point should be equal to 1/RC. I also measured the phase shift for the same range of frequencies and plotted them as well.

Step 2: Low-pass Filters

These circuits do just the opposite. They remove high frequency signals and allow low frequency signals to pass.

Set up your circuit as shown above. You can easily do this by just switching the resistor and capacitor. This section is very similar to the section on high-pass filters, but your results will be different. Repeat everything from the previous section. This time reading the oscilloscope, you can see that at low frequencies our output is pretty much the same as our input signal. The signal then gets filtered out as the input signal increases.

Again, taking data measurements of Vin and Vout, you can plot the Bode plot. My plot is shown below, with the 3dB point marked.