Signal filtering is one of the most fundamental practices in signal processing. The filters are widely incorporated in the signal processing circuits due to their indomitable role in data acquisition and also to get rid of all the unnecessary signals and frequencies which interfere with the quality of the main signal. These unwanted signals are commonly referred to as noises. Once the signal has been filtered, they are then processed as either analog or digital depending on the signals application. Some signals are however weak and thus require an amplification circuit so that the filtering circuits can be consistent.
In signal filtering, the cutoff frequency is usually considered to be the corner frequency that marks the range of frequencies that a specific filter can operate within. This means that every filter circuits has the minimum and the maximum range of frequencies that are desired. All the other frequencies are filtered out, and these are said to be the stop band frequencies while those that are desired are the pass band frequencies.
In general, the filters are widely classified into two main distinctive classes, and these are the low pass filters and the high pass filters. In the low pas filters, all the frequencies that are below the cut-off frequencies are allowed to pass while signals whose frequencies exceed this frequency are stopped and filtered out. The low pass filter has widely been used for its versatility to provide smooth signals free from environmental noises. On the other hand, the high pass band filters only allow signals that exceed the cut-off frequencies to pass. These are majorly used in filtering off DC related signals and maintaining a stable signal without unnecessary fluctuations.
The RC and RL circuits are used in designing filters either low pass or high pass filters depending on the circuit requirements. In a simple RC circuit, the major components of the circuits are resistors and capacitors arranged in parallel towards each other. The RL circuits have their resistors arranged in series with an inductor material.
For a low pass filter we use these two circuits:
We use a simple RC circuit
Take R= 1000 Ohms and C= 47nF and
Take R= 470 Ohms and L= 0.47mH
We then compute the approximately equally spaced frequencies, phase shift and the gain.
The formula for the gain is given by
While that of the phase shift is given by
The voltage in for the circuits is 3.0V.
RC filters are commonly used in electronic circuits and needless to mention in the signal processing circuits. This is because of their ability to attenuate the ripple voltages. When the filters are excluded from the main circuit, the output voltage depicts some variance from as low as zero voltage to peak voltages. The filters are not prone to changes in the circuit components and interestingly, the more the changes happens rapidly, the rigid the circuit becomes in adopting to these changes.
When the filter resistors are placed in series to the power supply, they behave like physical resistive elements. The capacitors, on the other hand, act as reservoirs. They charge rapidly, especially when the rectified voltage in the circuit exceeds that of the capacitor voltage. The RC and RL circuits are preferred due to their ease of assembly without compromising on their functionality.
A combination of RLC filter is flexible as it can be used to filter the band pass region of the signal.
While RC and RL circuits are used for low and high pass filters, the RLC can be used to achieve both of these characteristics. RC circuits are less cheap and compact in nature although they are affected by attenuation. LC filters, on the other hand, do not have resistors, and therefore, they do not dissipate energy. For industrial purposes, the RLC circuits are the most appropriate.
References
1. Bakshi, U.A, & A.V Bakshi (2008).Circuit Analysis. Technical Publication Pue. 2. Bakshi, U.A, & A.V Bakshi (2008).Electrical Circuit Analysis. Technical Publication Pue. 3. Godse, A.P, & U.A Bakshi (2008).Basic Electronics. Technical Publication Pue. 4. Kal, Santaram (Eastern Economy Edition).Basic Electronics Devices, Circuits and Fundamentals. PHI: 5. Santiago, John (April 2013).Circuit Analysis For Dummies. For Dummies.
