Practical implementation of bandpass and band reject filters

Bandpass and band reject filters have the combined benefit of both low- and high-passes. They are made by connecting the low-pass and high-pass of different cut-off frequencies. It is beneficial when you require or want to reject only a particular band of frequencies. They are used heavily in investigating audio released from a black box of the airplane to isolate the human voice from the rest of the noises.

This article will try to create simple filters using hardware electronics components. We will also analyze the inputs and outputs on the oscilloscope.

Bandpass filter
As we know, the bandpass filter passes the band of frequency components from any signal without attenuation (decrease in power). Bandpass frequency components can be defined using the cut-off frequency of the designed hardware filter.

Specification of the designed bandpass filter is given in the design below.

In Figure 1, the given circuit has a pass band frequency of approx 4.8 KHz-16 KHz. So this filter will attenuate the frequencies below 4.8 KHz and above 16 KHz and pass the frequency components above 4.8 KHz and below 16Khz. The input signal has a peak-to-peak amplitude of 5 Volt.

High pass filter stage: The capacitor value required with a resistor value of 10 KOhm to form a high pass filter is calculated as

C = 1/2PiF_LR

Low-pass filter stage: The capacitor value required with a resistor value of 10 KOhm to form a high-pass filter is calculated as

C = 1/2PiF_HR

The figure below shows the complete bandpass filter design.

Figure 1: Bandpass filter of 4.8KHz to 16KHz

This circuit is a basic low-pass with a cut-off frequency of 16 KHz and a High pass circuit with a cut-off frequency of 4.8 KHz connected in series.

In the below oscilloscope screen, the input is in red, and the output is in yellow. And it is clearly shown that a low frequency of 1000Hz (Figure 2) and 50 KHz (Figure 4) is attenuated while the 4.8 (5KHz) (Figure 3) KHz frequency is passed as it is.

Red probe – Input signal frequency
Yellow probe – Output signal frequency

Figure 2. Frequency of 1KHz is attenuated

Frequency 1KHz – Out of Pass band frequency hence attenuated.

Figure 3. Frequency of 5KHz is passed as it is

Frequency 5 KHz – Part of pass band frequency hence passed as it is

Figure 4. Frequency of 50KHz is attenuated

Frequency 50 KHz – Out of Pass band frequency hence attenuated.

Band reject filter
The Bandpass filter attenuates a band of frequency components from any signal. Pass reject frequency components can be defined using the cut-off frequency of the designed hardware filter.

Specification of the designed Band Pass filter is given in the design below.

High-pass filter stage: The capacitor value required with a resistor value of 10 KOhm to form a high-pass filter is calculated as

C = 1/2PiF_HR

Low-pass filter stage: The capacitor value required with a resistor value of 10 KOhm to form a high-pass filter is calculated as

C = 1/2PiF_LR

Specification of the designed bandpass filter is given in the design below.

Figure. 5: Band reject filter schematic 4.8HZ -16Khz

The above circuit consists of a low-pass filter and a high pass filter with outputs connected via a low forward voltage Schottky diode. This diode acts like an OR gate and prevents reverse current.

The above design has a band reject frequency of approx 4.8 KHz-16 KHz. So this filter will attenuate the frequencies below 16 KHz and above 4.8 KHz and pass the frequency components above 16 KHz and below 4.8 KHz. Input signal has a peak-to-peak amplitude of 5 Volt.

In the below oscilloscope screen, the input is in red, and the output is in yellow. And it is clearly shown that a low frequency of 1000Hz (Figure 5) and 30 KHz (Figure 7) is passed as it is while the 6 KHz (Figure 6) frequency is attenuated.

Red probe – Input signal frequency

Yellow probe – Output signal frequency