GopalKrishna Advanced Rural Research Foundation
GARRF Virtual Engineering & Electronics Laboratory
KALAM ZERO RESEARCH FUNDING LAB
EXPERIMENT 15 RC FILTER LOW-PASS FILTER SIGNAL PROCESSING
KALAM ZERO RESEARCH FUNDING LAB
In this experiment, students investigate one of the most fundamental signal-processing circuits: the RC low-pass filter. The virtual laboratory allows the student to change the resistance, capacitance and input frequency and observe the resulting output amplitude and phase behavior.
Simple circuits such as an RC filter are the foundation of much more advanced engineering systems. Filters are used in sensors, instrumentation, communications, robotics, biomedical electronics, structural health monitoring and signal conditioning.
A research engineer first understands the fundamental behaviour of a simple circuit and then asks: “Can this principle solve a real-world problem?”
This is the same engineering pathway that can lead from a basic laboratory experiment to a practical prototype and eventually to new intellectual property.
A low-pass filter allows low-frequency components of a signal to pass while reducing the amplitude of higher-frequency components.
The basic circuit consists of a resistor in series with the input and a capacitor connected from the output node to ground. The output is measured across the capacitor.
where fc is the cutoff frequency.
At the cutoff frequency, the output magnitude is approximately 70.7% of the input magnitude, corresponding to approximately −3 dB.
At frequencies much lower than the cutoff frequency, the capacitor has a relatively high impedance. Therefore, most of the input signal appears at the output.
At high frequencies, the capacitor impedance becomes small and the signal is increasingly diverted toward ground. Therefore, the output amplitude decreases.
The input signal is shown together with the attenuated output signal after passing through the RC low-pass filter.
The frequency-response curve demonstrates why an RC low-pass filter is useful for removing unwanted high-frequency components.
| Trial | R | C | fc | Input f | Output | Gain | Phase |
|---|
Design an RC low-pass filter with a cutoff frequency close to 1 kHz.
Try several combinations of R and C and determine which combination gives the desired cutoff frequency.
For:
The cutoff frequency is:
Therefore, signals substantially below approximately 159 Hz pass with relatively little attenuation, while signals substantially above this frequency are increasingly attenuated.
Piezoelectric sensors can produce signals containing useful information together with unwanted high-frequency noise. RC filtering is one of the simplest ways of understanding how frequency-selective signal conditioning works.
In advanced Structural Health Monitoring systems, filtering is important because the engineer must distinguish meaningful sensor responses from noise and unwanted frequency components.
The simple RC filter studied here therefore provides a foundation for understanding much more advanced digital filters and signal-processing algorithms.
Students can use CHITTI to explore filters, signal processing, frequency response, sensors and robotics electronics.
“What is an RC low-pass filter?”
“How is cutoff frequency calculated?”
“Why does a capacitor block low frequencies?”
“How can I design a 1 kHz filter?”
“How are filters used with PZT sensors?”
This is a virtual experiment. If reproduced physically, use a current-limited laboratory supply and verify component ratings before connecting the circuit.