A frequency filter is a circuit that passes a specific frequency and reduces the level of signals at other frequencies. Learn about the various types of inductive filters and their power and communication system applications.
Filters block unwanted signal or noise voltages and pass the desired signal with little or no attenuation. Filters are typically classified based on frequency characteristics; low-pass filters pass low frequency signals and attenuate all frequencies above the selected cutoff frequency. Similarly, high-pass filters pass only inputs with frequencies above the selected cutoff frequency. Band-pass filters pass signals in the selected frequency band, and trap (or bandstop) filters block frequencies in their band.
Frequency filters are used to eliminate harmonics. Power lines also pick up interference (noise) caused by lightning strikes and machine radiation from devices such as high-power transmitters or radiating equipment. This type of interference can cause problems with other types of equipment on the line. Frequency filters are used to remove these unwanted signals.
Basic Types of Inductive Filters
The waveforms and typical frequency response graphs given below illustrate the functions of the various types of filters.
Low-pass filters
A low-pass frequency filter is a filter that allows all frequencies below a selected frequency to be applied to the load and blocks all frequencies above that point, as shown in Figure 1. The simplest filter consists of an inductor connected in series with the load. Low-pass filters are typically used in DC power and audio applications.
At 0 Hz, the reactance of the inductor is zero and the total source voltage is located at both ends of the load. In this case, there is a maximum current flowing through the load. As the frequency increases, the reactance increases proportionally. Some of the source voltages across the inductor drop, thus reducing the current flowing through the load.
Figure 1. Waveform and frequency response of a low-pass filter. Image courtesy of Amna Ahmed
The input waveform of the low-pass filter in Figure 1 consists of a low-frequency signal superimposed on an unwanted high-frequency input. The low-frequency input is reproduced at the output with very little attenuation, while the high-frequency noise voltage is severely attenuated.
The gain/frequency response plot of the filter is v the plot o/v I plotted versus frequency (f). As shown in Figure 1, the filter has a low-frequency passband where the output voltage is approximately equal to the input (vo/v I ≈ 1). The passband applies to all signal frequencies from zero to the cutoff frequency. For input frequencies above the cutoff frequency, vo/v I is less than 1 and drops rapidly to a lower value. Therefore, as shown in the figure, the signal c at frequencies above f is attenuated.
High-pass filter
The high-pass filter in Figure 2 passes the high-frequency input voltage and attenuates the low-frequency input. In this case, the passband is above the cutoff frequency (fc) and the attenuation band is below fc. Therefore, the low-frequency input is attenuated while the high-frequency signal is passed to the output terminals. As with the low-pass filter, the inductor-resistor-capacitor circuit combination is designed for this purpose.
Figure 2. Waveform and frequency response of a high-pass filter. Image courtesy of Amna Ahmed
The simplest of these filters is an inductor connected in parallel with the load. High-pass filters are commonly used in communication applications, such as radio and television broadcasting, and in household appliances to eliminate the effects of 60 Hz noise.
The rejection frequency is the frequency that is rejected due to amplitude or size. Inductors have low reactance at the rejection frequency, which shunts the current around the load resistor. As the frequency increases, the reactance increases until it equals the load value. The sharp cutoff curve of the high-pass filter can be obtained by resonant circuit. At this point, the high pass band flows through the load, providing half or more of the power.
Band-pass filter
A bandpass frequency filter is a filter that passes a signal between the selected frequencies. The band-pass filter application is shown in Figure 3. In this case, the frequency band is at two cutoff frequencies (f1 and f2). As shown in the figure, the bandpass filter passes the signal frequencies in the middle range within its passband and blocks the signal frequencies above and below the passband.
Figure 3. Waveform and frequency response of the bandpass filter. Image courtesy of Amna Ahmed
Resonance can be used to allow a band of frequencies to be transmitted to a load. The simplest circuits are series resonant circuits and parallel resonant circuits. Bandpass filters are typically used in communication applications to tune the frequency band.
In a series resonant circuit, the capacitor has a high reactance below the resonant frequency and the inductor has a low reactance below the resonant frequency. Above the resonant frequency, the capacitor has low reactance and the inductor has high reactance. At the resonant frequency, the inductive reactance is equal to the capacitive reactance and the maximum current passes through the load resistance. Using a band-pass filter, the frequency band passed to the load includes the frequency band between the half-power points of the curve.
The same bandpass circuit can be obtained using a parallel resonant circuit connected in parallel with the load. Below the resonant frequency, the inductive resistance is low and the current bypasses the load. Above the resonant frequency, the capacitive reactance is lower and shunts the current around the load. At resonant frequencies, the impedance of the parallel circuit is at its maximum, causing the maximum current to flow to the load. When the capacitive or inductive reactance is equal to zero, the resistor limits the current in the circuit. Bandpass filters are used to reproduce audio in stereo speakers. A typical stereo speaker system has three speakers: small (tweeter), medium and large (woofer). A crossover network is an arrangement of bandpass filters used to route the appropriate signal to the appropriate speaker.
Bandstop (trap) filter
A bandstop frequency filter is a filter that reduces the signal between the selected frequencies. With a bandstop filter, the selected frequency band is prevented from reaching the load, as shown in Figure 4.
Figure 4. Waveform and frequency response of the trap filter. Image courtesy of Amna Ahmed
Filtering the selected frequency band is done using a series/parallel resonant circuit designed with a resistive band. Band-stop filters are typically used for applications where specific frequencies (e.g. 60 Hz) are removed.
When a series resonant circuit is connected in parallel with a load resistor, the series resonant circuit will short out most of the current around the load resistor at its resonant frequency. Since this arrangement acts as a short circuit, a resistor is used to limit the current from the source current to a safe level.
The current from the source can also be limited by connecting the resonant circuit in series parallel with the load. At resonance, connecting a parallel resonant circuit in series with the load creates a very high impedance to the source, thus limiting the current flowing through the load. The maximum current in this circuit is limited by the value of the load and the value of the source voltage.
Key Points of Frequency Filters
Frequency filters are combinations of inductors, capacitors and resistors that select or suppress frequency bands. The characteristics of different combinations of these components make it possible to select broadcasts from one radio or television station while excluding others. Frequency filters are commonly used in power applications (to eliminate harmonics) and in communications applications (to attenuate unwanted frequency signals).