Filtering is a technique used to retain desired components and remove unwanted components from the system input signal. In signal processing, this can be done by filtering out all other frequencies while maintaining a specific frequency range. The signal can be filtered using two types of filters: software and hardware.
Software and hardware filters work the same way regarding frequency response; the only difference is in its implementation. Software filters are designed in code during creation, therefore called digital filters. Hardware filters are physically present in the circuit design.
Both filters have their pros and cons:
- Hardware filters are designed using active and passive components such as resistors, capacitors, inductors, and operational amplifiers. But software filters are designed through coding.
- The whole design needs to be changed to change something in the hardware filter, but in the software filter, only the encoding needs to be changed.
- Software filters or digital filters are more reliable and easier to implement compared to hardware filters or analog filters.
- Higher-order analog filters get bulky; Digital filters don't work as they are designed into the code.
- Digital filters are not affected by environmental noise; Hardware or analog filters are affected by environmental noise.
- Analog filters are affected by temperature rating and component aging over time; no digital filters.
- Digital filters require high-performance DSP, DAC and ADC. But analog filters don't.
- Digital filters are more accurate than hardware filters.
Hardware filters are the realization of signal filtering techniques using the hardware electronics mentioned above. In signal processing, noise is a signal that is unwanted or lacking valuable information. It can also be the modification of the main signal during transmission, storage, conversion or processing.
In this article we will study hardware filters, including their types, how they can be designed using active or passive components, and more.
Terminologies
Below are some of the terminologies used in this article.
Attenuate : Decreasing the amplitude of the electronic signal without distortion.
Signal : Signal is an electromagnetic field or electrical energy (voltage*current) that transmits significant information between different electronic systems.
Noise: Unwanted signal that adds useless values to any signal, making it uninformative.
Passive Components: Passive components do not need electricity to function like capacitors, resistors, diodes, transformers, etc.
Active Components: Active components need electricity to function like transistors.
Passband: A region from which a group of frequencies can pass a filter.
Stop band: In this region, frequencies are attenuated when a signal passes through a filter.
3dB frequency (cutoff frequency): frequency at which the signal changes its region from the passband to the endband or from the endband to the passband. It is also called the signal cutoff frequency.
Definition
A filter is an electronic circuit designed to modify, reshape, or reject all unwanted frequency components of a signal and transmit only the necessary informational signals. The figure below shows a signal with noise and a filtered signal.
Fig.1 Signal with noise and filter
In the field of electronic signal processing, signals are processed in a frequency-dependent manner by filters. Passive components such as inductors and capacitors have impedances in their nature. The basic concept of filters can be explained by examining the frequency-dependent nature of passive components.
There are many practical applications of filters. At high frequencies, a unipolar low-pass filter provides stability to a system by eliminating gain.
DC offset can be blocked in single-supply circuits or high-gain amplifiers using a unipolar high-pass filter. There are many applications of filters, such as signal separation and attenuation of unwanted frequency components. For example, in radio communication, an attenuating signal is normally transmitted with gain. The effect of aliases can also be eliminated by using filters in the A/D system when converting data. Filters can be used to reconstruct the D/A output signal and can smooth a signal by removing higher frequency components such as sampling frequency and generated harmonics.
Fig.2 A/D and D/A with filtered signal.
For the ideal filter, the amplitude response will be unity or fixed gain for the frequencies of interest (passband frequency) and zero everywhere else (endband frequency). The frequency at which the response changes from the passband to the endband is called the cutoff frequency.
Features of hardware filters include:
- It can be implemented using only passive components such as resistors and capacitors or inductors and capacitors.
- It can be designed in various orders according to signal filtering requirements.
- Simple to analyze. No software is needed to see the response.
- It can be used in many fields such as telecommunications, analog signals, data acquisition, etc.
- Power supply noise reduction or filtering.
The electronic filter is the circuit that passes some frequency components of the circuit and rejects or attenuates all other frequency components. Based on the frequency band passing through the filter, filters can be classified into four different types.
Low pass filter
An ideal low-pass filter has low frequencies in the pass band and higher frequencies in the stop band. The active and passive components available in the low-pass filter attenuate the high-frequency components of the signal passing through it. For example, an electronic guitar has a tone knob equipped with a low-pass filter to reduce the highs of the sound.
Fig.3 Electronic guitar tone knob.
The figure below shows the frequency response of an ideal low-pass filter.
Fig. 4 Ideal low-pass filter frequency
High pass filter
An ideal high-pass filter has high frequencies in the pass band and low frequencies in the stop band. The active and passive components available in the high-pass filter attenuate the low-frequency components of the signal passing through it. For example, in audio signal recording, a high filter is used to eliminate the DC offset of the audio.
Fig.5 Microphone
The figure below shows the frequency response of an ideal high-pass filter.
Fig.6 Frequency for high-pass filter
Bandpass filter
A bandpass filter is a filter that has two transition points – one from stopband to passband and the second from passband to stopband. This means that this filter has two cutoff frequencies in its frequency response. The filter passes through a set of frequencies that are in the passband region. For example, in wireless communication (RF communication), a bandpass filter is used to support a frequency band.
Fig.7 RF communication system
The figure below shows the frequency response of an ideal bandpass filter.
Fig. 8 Bandpass filter frequency
Band rejection or notch filter
A band-reject filter is a filter that attenuates a frequency band that is below this stopband area. Just like the bandpass filter, it also has two transition points or cutoff frequencies. This filter is also called a notch filter and is used to remove a specific frequency from the signal that passes through the band-reject filter. For example, in audio system, band reject filter is used to remove interfering power line hum of a specific frequency such as 50 Hz in India.
Fig. 9 Frequency response of the ideal band-rejection filter
A practical filter has five parameters and typically displays one or more variables. The parameters:
- The cutoff frequency is the frequency at which the filter response leaves the passband.
- The stop band frequency is the frequency at which minimum attenuation is achieved in the stop band.
- Passband ripple is the variation in filter response across the passband.
- The acute angle of the filter is defined as the order (M) of the filter.
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The number of poles in the transfer function is also called the order of the filter. A pole is the root of the denominator of the transfer function and a zero is the root of the numerator of the transfer function.
It is not necessary for all filters to have these features. For example, if there are no zeros in the transfer function (all pole configurations), there will be no ripple in the stop band. Butterworth and Bessel filters are all polar filters that have no ripple in the passband.
If you are designing an anti-aliasing filter for an ADC, you will need to know the cutoff frequency (maximum frequency you want to pass), the stopband frequency, and the maximum attenuation required. From there you can go to other parameters such as filter order, f 0 and Q. These parameters will be discussed in the next article.