GopalKrishna Advanced Rural Research Foundation
GARRF Virtual Engineering & Electronics Laboratory
KALAM ZERO RESEARCH FUNDING LAB
EXPERIMENT 23
ACTIVE HIGH-PASS FILTER
SIGNAL CONDITIONING
PZT / SHM
KALAM ZERO RESEARCH FUNDING LAB
In Experiment 22, we studied an active low-pass filter. Now we reverse the filtering concept. An active high-pass filter allows high-frequency components to pass while attenuating low-frequency components.
This experiment is particularly important when a sensor system must remove slow changes, DC offsets, baseline drift or very low-frequency disturbances while preserving useful dynamic information.
A high-pass filter is a frequency-selective circuit that attenuates low frequencies and passes higher frequencies.
For a simple first-order RC high-pass filter:
The magnitude response is:
At frequencies much higher than the cutoff frequency, the magnitude approaches unity. At frequencies much lower than the cutoff, the output is strongly attenuated.
At the cutoff frequency the first-order high-pass filter has a magnitude of approximately โ3 dB relative to its high-frequency passband magnitude.
The input and high-pass filtered output are shown. At low frequencies the output amplitude is strongly reduced. As frequency rises above the cutoff frequency, the output approaches the amplifier's passband gain.
Notice the opposite behaviour compared with Experiment 22: the high-pass response rises with frequency and eventually reaches the passband.
At very low frequency:
f โช fc โ output is strongly attenuated.
At the cutoff frequency:
f = fc โ approximately โ3 dB.
At very high frequency:
f โซ fc โ response approaches the passband gain.
If capacitance is increased while resistance remains constant, does the high-pass cutoff frequency increase or decrease? Explain why.
The combination of low-pass and high-pass filtering can create a band-pass filter, which passes only a selected frequency range. That concept will become increasingly important in sensor instrumentation and SHM.
| Trial | R | C | Expected Cutoff | Observation |
|---|---|---|---|---|
| 1 | 10 kฮฉ | 100 nF | โ159 Hz | Reference |
| 2 | 20 kฮฉ | 100 nF | โ80 Hz | Cutoff decreases |
| 3 | 10 kฮฉ | 200 nF | โ80 Hz | Cutoff decreases |
| 4 | 5 kฮฉ | 100 nF | โ318 Hz | Cutoff increases |
Design a high-pass filter with a cutoff frequency close to 160 Hz.
Start with:
| Trial | Frequency | R | C | Cutoff | Gain | Output | Attenuation |
|---|
Sensor systems may contain unwanted very-low-frequency components, DC offsets, baseline drift, environmental variations or slow changes that are not relevant to the dynamic phenomenon being investigated.
A high-pass filter can reduce those components while retaining higher-frequency dynamic information.
A simplified signal-conditioning chain can be represented as:
PZT โ Pre-amplifier โ High-Pass Filter โ Low-Pass Filter โ ADC โ Computer โ AI/ML โ SHM Decision
The filter itself does not diagnose structural damage. It prepares the measured signal so that later digital processing, feature extraction and AI algorithms can operate on an appropriate frequency range.
Explore filters, electronics, sensors, robotics, signal processing and engineering applications with CHITTI.
"Why does a high-pass filter remove DC?"
"What happens at the cutoff frequency?"
"Why does increasing capacitance lower the cutoff frequency?"
"How can high-pass filtering help PZT measurements?"
"How can low-pass and high-pass filters create a band-pass filter?"
This is a virtual educational simulation. When building the physical circuit, verify the op-amp supply voltage, component ratings, breadboard wiring and measurement equipment.
Never connect experimental electronics directly to mains voltage.
In this experiment, we investigated the operation of an active high-pass filter.
We learned that the cutoff frequency is determined by the RC product:
Below the cutoff frequency, the signal is increasingly attenuated. Above the cutoff frequency, the output approaches the passband gain.
Together with Experiment 22, this experiment introduces the student to the fundamental concept of frequency-selective signal conditioning.
From simple RC networks to PZT signal conditioning, instrumentation and intelligent SHM โ engineering begins with understanding the signal.