GopalKrishna Advanced Rural Research Foundation
GARRF Virtual Engineering & Electronics Laboratory
KALAM ZERO RESEARCH FUNDING LAB
EXPERIMENT 22 ACTIVE FILTER LOW-PASS FILTER SIGNAL CONDITIONING
KALAM ZERO RESEARCH FUNDING LAB
In Experiment 21, we learned how an operational amplifier can provide controlled voltage amplification. In this experiment, we use an op-amp together with a resistor-capacitor network to create an active low-pass filter.
A low-pass filter allows lower-frequency components of a signal to pass while progressively reducing higher-frequency components.
A low-pass filter is a frequency-selective circuit that passes low-frequency signals while attenuating high-frequency signals.
A simple first-order RC low-pass filter consists of a resistor and capacitor.
where:
At the cutoff frequency, the magnitude of the simple first-order low-pass response is approximately β3 dB relative to its low-frequency gain.
The input is shown together with the filtered output. As frequency increases relative to the cutoff frequency, the output amplitude decreases and phase shift increases.
The vertical axis represents relative magnitude in decibels. The horizontal axis is logarithmic frequency.
Below the cutoff frequency, the filter response is comparatively flat. Above the cutoff frequency, the response progressively decreases. For a first-order RC response, the attenuation approaches approximately β20 dB/decade at high frequency.
If you double the resistance while keeping the capacitance constant, what should happen to the cutoff frequency?
The cutoff frequency is controlled by the product RC. Try the following combinations and compare their cutoff frequencies.
| Trial | R | C | Expected Relationship |
|---|---|---|---|
| 1 | 10 kΞ© | 100 nF | Reference |
| 2 | 20 kΞ© | 100 nF | Cutoff decreases |
| 3 | 10 kΞ© | 200 nF | Cutoff decreases |
| 4 | 5 kΞ© | 100 nF | Cutoff increases |
Design a filter with a cutoff frequency as close as possible to 160 Hz.
Try to achieve this using approximately:
Then set the input frequency approximately equal to the cutoff frequency and investigate the response.
| Trial | Frequency | R | C | Cutoff | Gain | Output | Attenuation |
|---|
A PZT sensor can respond to a wide range of mechanical and electromagnetic disturbances. A measurement system may need to retain the frequency range containing useful structural information while reducing unwanted high-frequency noise.
A low-pass filter can therefore form part of an analog signal-conditioning chain before digitization.
PZT β Signal Conditioning β Filter β ADC β Computer β AI/ML β SHM Decision
The filter does not "understand" the structure. It simply performs frequency-selective processing. The subsequent digital processing or AI system can then work with a cleaner signal.
Use CHITTI to explore filters, electronics, sensors, robotics, signal conditioning and engineering applications.
"Why does a capacitor block high-frequency signals in this circuit?"
"How do I calculate the cutoff frequency?"
"What happens at β3 dB?"
"How can I design a filter for a PZT sensor?"
"What is the difference between analog and digital filtering?"
This is an educational virtual simulation. When constructing a physical filter, verify component ratings, op-amp supply voltage, wiring and measurement equipment before applying power.
Never connect experimental electronics directly to mains voltage.
In this experiment, we used resistance and capacitance to create a frequency-selective low-pass filter and investigated its behaviour with an active amplifier stage.
The experiment demonstrates one of the central ideas in signal processing: different frequencies can be treated differently by an engineered system.
This principle is fundamental to instrumentation, robotics, communications, vibration analysis and structural health monitoring.
From analog filtering to intelligent signal analysis β this is the beginning of the signal-processing pathway.