AC2 Fundamentals Unit 5 – Low- and High-Pass Filters
Exercise 1 – Low-Pass Filters
EXERCISE OBJECTIVE
When you have completed this exercise, you will be able to calculate the cutoff frequencies and
attenuations of RC and RL low-pass filters. You will verify your results with an oscilloscope.
DISCUSSION
- Several ways exist for the implementation of low-pass filters, each of which consist of a
voltage-divider network containing a resistor and a frequency-varying component (inductor
or capacitor). - Output voltage from the filters is “tapped off” the voltage divider.
- Changes in the frequency of the supply voltage cause changes in the circuit reactance,
resulting in output voltage variations. - In RC filters, the capacitive reactance is high at low frequencies compared to the resistance,
causing most of the input voltage to appear across the output capacitor. - Capacitive reactance decreases as the generator frequency increases, causing larger voltage
drops across the R and decreasing the voltage across the output capacitor. - Low-pass filters are designed so that frequencies below the cut-off frequency are passed
while higher frequencies are attenuated. - In low-pass RL filters, the inductive reactance is small at low frequencies compared to the
resistance, and most of the input voltage falls across the output resistor. - Inductive reactance increases as the generator frequency increases; therefore, more and more
voltage is dropped across the inductor and less across the output resistor. - Cutoff frequency is defined as the frequency where the output signal is 3 dB down, or 0.707
x Vo. - For RC circuits: fc = 1/2πRC
- For RL circuits: fc = R/2πL