Band pass and band reject filters have the combined benefit of low pass and high pass. They are made by connecting the low pass and high pass of different cutoff frequencies. It is beneficial when you only want or want to reject a certain band of frequencies. They are often used in investigating audio released from an airplane's black box to isolate the human voice from the rest of the noise.
This article will attempt to create simple filters using hardware electronic components. We will also analyze the oscilloscope's inputs and outputs.
Bandpass filter
As we know, the bandpass filter passes the band of frequency components of any signal without attenuation (decrease in power). The bandpass frequency components can be set using the cutoff frequency of the designed hardware filter.
The specification of the designed bandpass filter is given in the project below.
In figure 1 , the given circuit has a passband frequency of approximately 4.8 KHz-16 KHz. Therefore, this filter will attenuate frequencies below 4.8 KHz and above 16 KHz and pass frequency components above 4.8 KHz and below 16 KHz. The input signal has a peak-to-peak amplitude of 5 Volts.
High Pass Filter Stage: The required capacitor value with a resistor value of 10 KOhm to form a high pass filter is calculated as
C = 1/2PiF i R
Low Pass Filter Stage: The required capacitor value with a resistor value of 10 KOhm to form a high pass filter is calculated as
C = 1/2PiF H R
The figure below shows the complete design of the bandpass filter.
Figure 1: 4.8 KHz to 16 KHz Bandpass Filter
This circuit is a basic low-pass circuit with a cutoff frequency of 16 KHz and a high-pass circuit with a cutoff frequency of 4.8 KHz connected in series.
On the screen below the oscilloscope, the input is in red and the output is in yellow. And it is clearly shown that a low frequency of 1000 Hz ( Figure 2 ) and 50 KHz (Figure 4) are attenuated while the frequency of 4.8 (5KHz) (Figure 3) KHz is passed as is.
Red probe – Input signal frequency
Yellow probe – Output signal frequency
Figure 2. Attenuated 1KHz frequency
1KHz Frequency – Frequency outside the passband, therefore, attenuated.
Figure 3. The 5KHz frequency is passed as is
Frequency 5 KHz – Part of the passband frequency, therefore passed as is
Figure 4. The 50KHz frequency is attenuated
Frequency 50 KHz – Frequency outside the passband, therefore, attenuated.
Band Reject Filter
Bandpass filter attenuates a band of frequency components of any signal. The pass rejection frequency components can be set using the cutoff frequency of the designed hardware filter.
The specification of the designed bandpass filter is given in the project below.
High Pass Filter Stage: The required capacitor value with a resistor value of 10 KOhm to form a high pass filter is calculated as
C = 1/2PiF H R
Low Pass Filter Stage: The required capacitor value with a resistor value of 10 KOhm to form a high pass filter is calculated as
C = 1/2PiF i R
The specification of the designed bandpass filter is given in the project below.
Figure. 5: Schematic of the 4.8 Hz -16 Khz band rejection filter
The above circuit consists of a low-pass filter and a high-pass filter with outputs connected through a low forward voltage Schottky diode. This diode acts as an OR gate and prevents reverse current.
The above design has a frequency band rejection of approximately 4.8 KHz-16 KHz. Therefore, this filter will attenuate frequencies below 16 KHz and above 4.8 KHz and pass frequency components above 16 KHz and below 4.8 KHz. The input signal has a peak-to-peak amplitude of 5 Volts.
On the screen below the oscilloscope, the input is in red and the output is in yellow. And it is clearly shown that a low frequency of 1000 Hz (Figure 5) and 30 KHz (Figure 7) is passed as is while the frequency of 6 KHz (Figure 6) is attenuated.
Red probe – Input signal frequency
Yellow probe – Output signal frequency
Figure 6. 1 KHz frequency is passed
Frequency 1 KHz – Not part of the band rejection frequency, therefore, it is passed as is.
Figure 7. The 6KHz frequency is attenuated
Frequency 6 KHz – Part of the band rejects the frequency and is therefore attenuated.
Figure 8. 30KHz frequency is passed
Frequency 30 KHz – It is not part of the band rejection frequency, therefore, it is passed as is. Above all, the figure explains the theory with practical implementations.