Band Pass Filter is a filter used in signal processing to allow wanted frequency components and to remove the unwanted components from the signals. This post will discuss about what is Band Pass Filter, Wide Band Pass & Narrow Band Pass Filter, how it works, applications, advantages and disadvantages.
What is Band Pass Filter
Filters are generally used in electronics and communications systems especially in signal and image processing systems. Band Pass Filter is a filter that allows specific band frequencies to pass and eliminates the other frequencies.
Fig. 1 – Introduction to Band Pass Filter
It is a circuit that permits signals between two particular frequencies to pass. The main objective of Band Pass Filter is to primarily work with wireless transmitters and receivers. It’s function is to limit the bandwidth of the signal to a minimum required and then transmit data in a particular speed and form.
In a receiver, this Filter lets the signals to pass through a particular range emitting repetitive frequencies. It also enhances the signal to noise ratio in a receiver. It has two cut off frequencies i.e. lower frequency and higher frequency and eliminates any signals outside these frequencies.
Fig. 2 – Basic Structure of Band Pass Filter
When there is maximum gain in the center frequency, it’s referred to as total Band Pass Gain. At first, the signal passes through the High Band Pass section and the output of the signal would be infinity. The low pass filter has high frequency signal. Hence, when a High Pass Filter cascades with Low Pass Filter, a simple Band Pass Filter is formed. The Transfer Function ‘H’ is defined as:
Types of Band Pass Filter
Any Filter is determined by its Q Factor. The Q Factor is defined as the ratio of Center Frequency to Bandwidth. Filter in which the Q factor is less than 10 is called as Narrow Band Pass Filter and if the Q factor is above 10, it is called as Wide Band Pass filter. Q factor works as a measure of selectivity. For example, higher the Q factor value, narrow will be the bandwidth.
Let us discuss two types of Band Pass Filters. They are:
- Wide Band Pass Filter
- Narrow Band Pass Filter
Wide Band Pass Filter
This Filter is formed when High Pass and Low Pass Filters are cascaded. Multiplying the low-frequency response by the high-frequency response gives the output as shown in the Fig. 3 below. The Center Frequency is defined as:
Where: fL = Low Cut- off Frequency.
fH = High Cut-off Frequency.
Fig. 3 – (a) Wide Band-Pass Characteristic Curve (b) Product of Low-Pass and High-Pass Characteristics
Narrow Band Pass Filter
It provides multiple feedback. It only utilizes one operational amplifier. It has many unique features in comparison to all other filters. Due to the two feedback paths, it is called as multiple feedback. Commonly, this filter is made for three components i.e., Center Frequency, Q factor and bandwidth. The key advantage of multiple feedback filter is that the center frequency can be changed to new frequency without altering the bandwidth.
Also, the Operational Amplifier works in an inverting mode as shown in the Fig. 4. This type of filter is used across various fields. It is used in research and designing of optical systems in Biochemical Analyzer. The Biochemical Analyzer is required in clinical diagnosis. It is also utilized in reducing the timing jitters and noise intensity in fiber lasers.
Fig. 4 – (a) Frequency Response Curve (b) Filter Circuit
How does Band Pass Filter Work
They work with a particular band of frequencies without disturbing the i/p signal. The range of frequency between two cut off frequencies is called bandwidth. The difference between the low and high cut-off frequency is known as widespread frequency bandwidth.
For the accurate functionality of this filter, the low cut-off frequency should be higher than high cut-off frequency. It is also called as second order filters as they have two reactive components. The circuit can be built with two capacitors as shown in the Fig. 5.
Fig. 5 – Filter Circuit
The components are selected in such a way that, the cut-off frequency for the High-Pass Filter is lower than the critical frequency of the Low-Pass Filter. Frequency f1 may pass through the Low-Pass Filter but this will have a minimal effect on output voltage V0, due to reject characteristics of the High-Pass Filter.
Similarly, frequency f2 may pass through High-Pass Filter but is restricted from reaching High-Pass filter by the Low-Pass reject characteristics. Frequency fo near the center of the pass band will pass through both filters with very less attenuation. The circuit will produce Frequency Response characteristics curve as shown in the Fig. 6. Vout is never equal to Vi, but the output voltage level can reach close to input voltage with proper designing. For, Vo max ≠ Vi max, the bandwidth is defined at 0.707 of the resulting Vo max.
Fig. 6 – Frequency Response Characteristics
The applications include:
- They are widely used in wireless transmitters and receivers in signal processing.
- The main function of this filter is to restrict the bandwidth of the o/p signal that is allocated for transmission.
- It evades the transmitter from meddling with other stations.
- It decodes signals from a particular range of frequencies while stopping signals from unwanted frequencies to pass through. It also enhances the signal to noise ratio and sensitivity in a receiver environment.
- They are deployed in all kinds of electronic and communication devices, biomedical devices like EEG’s, electrocardiograms, seismology etc.
- It is popularly used in optical field too, like Lasers, LIDARs, etc.
- It is used in finding meteorological data to understand the weather forecasts during a particular range of time. For example, finding out weather report of specific area from a time range of 3-10 days time period.
- It takes the form of a transparent colored film in the theatre lighting work and photography.
The advantages are:
- When the filters are used in conjunction with switches, it offers low insertion loss and performance increases.
- Passive filters are easier to design.
- Available in portable size.
- As active components in the circuit provide amplification, Active Filters have power Gain.
The disadvantages include:
- Active Filters requires power supply.
- Highly sensitive to variations in circuit components.
- They have inherent insertion loss.