GopalKrishna Advanced Rural Research Foundation
GARRF Virtual Engineering & Electronics Laboratory
KALAM ZERO RESEARCH FUNDING LAB
EXPERIMENT 24
ACTIVE BAND-PASS FILTER
SIGNAL CONDITIONING
PZT / SHM
KALAM ZERO RESEARCH FUNDING LAB
A band-pass filter combines the fundamental ideas of the high-pass filter and low-pass filter. Instead of passing everything above or below one frequency, it allows a selected range of frequencies to pass.
This is an important step toward real engineering signal-processing systems because many sensors do not require the complete frequency spectrum. Engineers often want to isolate the frequency range that contains useful information.
A band-pass filter allows a selected range of frequencies to pass while attenuating frequencies below and above that range.
It therefore has two important cutoff frequencies:
A higher Q means a narrower and more selective passband. A lower Q means a wider passband.
A practical band-pass filter can be constructed by combining high-pass and low-pass sections:
Input → High-Pass → Low-Pass → Amplifier → Output
The high-pass section establishes the lower cutoff. The low-pass section establishes the upper cutoff.
The output amplitude depends on the frequency position relative to the lower and upper cutoff frequencies.
The response rises after the lower cutoff, reaches the passband, and falls after the upper cutoff.
Can you design a filter that passes approximately 400–600 Hz while strongly rejecting frequencies below 100 Hz and above 2 kHz?
| Frequency Region | Relationship | Expected Behaviour |
|---|---|---|
| Below fL | f < fL | Strong attenuation |
| Near fL | f ≈ fL | Transition |
| Inside passband | fL < f < fH | Strong transmission |
| Near fH | f ≈ fH | Transition |
| Above fH | f > fH | Strong attenuation |
| fL | fH | Bandwidth | Centre Frequency | Q |
|---|---|---|---|---|
| 100 Hz | 1000 Hz | 900 Hz | 316 Hz | 0.35 |
| 400 Hz | 600 Hz | 200 Hz | 490 Hz | 2.45 |
| 450 Hz | 550 Hz | 100 Hz | 497 Hz | 4.97 |
| 900 Hz | 1100 Hz | 200 Hz | 995 Hz | 4.97 |
Narrowing the passband increases the Q factor. A high-Q filter is more selective.
Structural systems can generate vibration over a wide range of frequencies. However, a particular structural mode, resonance or damage-sensitive feature may occupy only a limited frequency region.
A band-pass filter can isolate that region and reduce irrelevant components outside the target band.
PZT → Amplifier → High-Pass → Low-Pass → ADC → AI/ML → SHM
For example, if an engineering investigation is interested primarily in a vibration band around a particular structural resonance, filtering can reduce unwanted low-frequency drift and high-frequency noise before feature extraction.
Filtering must be designed from the physics of the measurement. An incorrectly selected passband can remove information that is actually important for detecting damage.
Explore electronics, filters, sensors, robotics, signal processing and engineering applications with CHITTI.
"What is the difference between low-pass, high-pass and band-pass?"
"What is Q factor?"
"Why does a narrow band-pass filter have a high Q?"
"How can band-pass filtering help PZT SHM?"
"How would I select a passband for a structural vibration?"
This is a virtual educational simulation. When constructing the physical circuit, verify op-amp supply voltage, component ratings, breadboard connections and measurement equipment.
Never connect experimental electronics directly to mains voltage.
In this experiment we studied the operation of an active band-pass filter.
Unlike a low-pass or high-pass filter, the band-pass filter has both a lower and an upper cutoff frequency.
We also saw how a carefully selected frequency band can be used to extract useful information from a much more complicated signal.
This concept forms an important bridge between basic electronics, sensor instrumentation, vibration analysis, PZT systems and AI-assisted Structural Health Monitoring.
From filtering a signal to understanding a structure — this is where electronics becomes intelligent engineering.