AC2 Fundamentals Unit 5 – Low- and High-Pass Filters
Exercise 2 – High-Pass Filters
EXERCISE OBJECTIVE
When you have completed this exercise, you will be able to calculate and measure the cutoff
frequencies and observe the attenuation rates of RC and RL high-pass filters. You will verify
your results with an oscilloscope.
DISCUSSION
- Several ways exist for the implementation of high-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 formed by the series resistor
and reactive component. - Changes in the frequency of the supply voltage cause changes in the circuit reactance,
resulting in output voltage variations. - In high-pass RC filters, the capacitive reactance is low at frequencies above cutoff compared
to the resistance, causing most of the input voltage to appear across the output resistor. - Capacitive reactance increases as the generator frequency decreases causing larger voltage
drops across the C and decreasing the voltage across the output resistor. - In high-pass RL filters, the inductive reactance is large at high frequencies compared to the
resistance, and most of the input voltage falls across the output inductor. - Inductive reactance decreases as the generator frequency decreases; therefore, more and more
voltage is dropped across the resistor and less across the output inductor. - 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