In today's electronic devices, many practical applications use hardware filtering implementation. These applications include electrocardiograms (ECG), image processing cameras, etc. Electrocardiograms are medical devices that record the activity of the human heart using a non-invasive approach, encoding heartbeats into signals. But before reading the information from these electrical signals, they are filtered from unwanted noisy signals introduced from the environment using many hardware filters such as low-pass, high-pass and notch filters.
In this article, we will try to practice using hardware electronic components by designing some hardware filters to capture their input and output waveforms on the oscilloscope.
Low Pass Filter: As we know, low pass filter passes the low frequency components of any signal without attenuation (decrease in power). Low frequency components can be defined using the cutoff frequency of the designed hardware filter.
Fig. 1: The image shows the specification of the designed first-order low-pass filter
The above design has a cutoff frequency of approximately 1KHz. Therefore, this low pass filter will attenuate frequencies above 1KHz and pass frequency components below 1KHz.
The input signal has a peak-to-peak amplitude of 5 Volts.
On the oscilloscope screen below, the input is in red and the output is in yellow. And it is clearly shown that a low frequency of 500 Hz passes through the above designed filter, which has a cutoff frequency of 1 KHz.
There is no attenuation in the output signal.
Red Problem – Input Signal Frequency
Yellow problem – Output signal frequency
Fig. 2: The image shows the output variation for a low frequency of 500 Hz that passes through the filter designed above
But when the input signal frequency is increased from 500Hz to 5KHz. High frequency components are attenuated. This can be seen in the figure below. The signal amplitude is attenuated from 5 V to 1 V.
Fig.3: The image shows the attenuated output when the input frequency is increased from 500 Hz to 5KHz
We study the first-order low-pass filter, which is designed using a resistor and a capacitor in series. But sometimes, it is not necessary that all noisy frequency components can be removed using just a single-order low-pass filter.
So a designer needs a second order filter.
Second order low pass filter
Second order low pass filter can be designed by adding one more low pass filter stage to the first order low pass filter. Higher-order low-pass filters are obtained to completely remove or attenuate the high-frequency components of the input signal.
A certain formula calculates the second-order cutoff frequency.
Fig.4: The image shows the specification of the second order filter
Fig.5: Image shows input and output on the oscilloscope screen for second order filter
In the figure above, a signal with a frequency of 300 Hz passes through a second-order filter with a cutoff frequency of 1 kHz. Thus, the above circuit passes the signal frequency below 1 kHz and stops the frequency components above 1 kHz.
When a 3 kHz frequency signal passes through the filter designed above. The output signal is almost lost or attenuated because this low-pass filter attenuates the frequency above its 1 kHz cutoff frequency.
Fig. 6: The image shows the final attenuated signal for the designed filter
High-pass filter: The high-pass filter is the functional complement of the low-pass filter. As we studied, the high-pass filter has high-frequency components in the pass band and low-frequency components in the stop band.
A high-pass filter was designed, which has a cutoff frequency of approximately 1KHz. The electronic components and their values used in the project are shown in the figure below.
Fig. 6: The image shows the specification of the designed high-pass filter.
When a 500 Hz signal passes through the high-pass filter, which has a cutoff frequency of around 1 kHz, this filter attenuates the input signal, as shown in the figure below. The signal amplitude drops from 4.64 V to 2.80 V peak to peak.
Red Problem – Input Signal Frequency
Yellow problem – Output signal frequency
Fig. 7: The image shows the attenuated output signal for 500 Hz input signal at the cutoff frequency of 1 KHz
And when a signal with high frequency components passes from the above circuit. It is observed that the high frequency components pass without any filter attenuation.
This can be seen in the figure below.
Fig. 8: The image shows the output for high frequency input for high pass filter
In the figure above, an input signal is 2 kHz (shown in RED color) and the filtered signal (output) is also 2 kHz (shown in yellow color). This is how the high pass filter works.
