Equalizers are used to change the performance of signal circuits. In some cases, this means linearizing the performance of the signal circuit. In other cases, equalizers are used to introduce nonlinearities. As a result, equalizer designs range from simple passive circuits to complex multi-stage designs and active circuits.

This frequently asked question looks at the basics of equalizer, looks at equalizers for radio frequency (RF) and audio applications, and concludes with a brief look at how equalizers are applied in visible light communications and renewable energy systems.

The term ‘equalizer’ is commonly used for networks designed to compensate for different amplitude characteristics of a system; in some cases this type of equalizer may provide some level of phase correction as a secondary result. The terms “phase equalizer” or “phase corrector” are used for networks where the main purpose is phase correction.

Grilles and bridge equalizers are examples of filters for all passages. They are often used to correct amplitude and / or phase errors in a signal circuit, to provide a system with a flat amplitude characteristic and a constant delay in a certain frequency range. (Figure 1). A lattice phase equalizer has a constant attenuation of all frequencies, but the relative phase between input and output varies with frequency. The grid filter topology is a constant resistance network and can be easily combined with other constant resistance filters, such as bridge-T equalizers.

Figure 1: Lattice (left) and bridge-T (right) equalizers are filters for all passages that can correct phase and / or amplitude errors in signal circuits. (Images: Lattice / Mostovo-T)

The bridge T can be used to insert a constant delay of all frequencies in the signal circuit. It is often the choice when two or more signals need to be matched based on a time criterion. When balancing a single circuit, the bridge-T is often used, and when balancing a balanced transmission line, a grid configuration is usually preferred.

RF equalizers for amplification

The gain of most RF amplifiers decreases at higher frequencies. In addition, other components of the signal circuit, such as cables and passive components, may add insertion losses that increase at higher frequencies. The RF amplifier equalizer can only compensate for the performance of the RF amplifier or the combination of the RF amplifier and other elements in the signal circuit (Figure 2).

Figure 2: An equalizer can only be used to compensate for the performance of the RF amplifier (left) or to provide more compensation for the combination of the RF amplifier and other elements in the signal circuit (right). (Image: Mini chains)

Due to the widespread use of equalizers for RF amplification, there is an equally wide variety of design considerations. Three of the most common include:

Operating frequency range – For example, L-Band boost equalizers or microwave ovens. L-band designs are used between 950 MHz and 2.15 GHz. Microwave equalizers with frequencies from 2GHz to 20GHz are usually used.

Inclination slope – RF equalizers can have positive, negative, positive parabolic or negative parabolic amplification functions. Positive gain inclinations have an increasing loss of insertion with frequency. Negative gain inclinations have decreasing insertion losses with frequency. Parabolic RF equalizers have non-linear parabolic gain inclinations.

Active versus passive – Active gain equalizers need external power and use an amplifier to generate positive gain. Passive boost equalizers do not use an amplifier and produce a loss of insertion.

Audio equalizers

While RF equalizers compensate for various sources of nonlinearity in the RF signal circuit, audio equalizers are often used to introduce nonlinearities and “shape” sound. So-called graphic equalizers (EQs) are a set of fixed-frequency peak and clipping filters designed to cut or amplify several predefined frequencies simultaneously (Figure 3). For example, the 31-band, one-third octave equalizers have 31 center frequencies spaced at one-third octave intervals, so that three adjacent filters cover one musical octave and each filter provides a gain adjustment range of up to 15 dB.

Figure 3: Stereo equalizer tuned to cut the midrange frequencies on both the left and right. (Image: Wikipedia)

Parametric EQs have controls for frequency, gain, and bandwidth (Q) and offer more flexibility than simpler graphical EQs. Available in analog design with up to five tapes or digital design with more tapes. In addition, they are available in semi-parametric and fully parametric design. Semi-parametric designs have frequency and gain controls, but no Q controls for each band. Fully parametric designs have frequency, gain, and Q controls for each bandwidth to support complex tone shaping.

T-bridge for visible light communications

Real-time Gb / s Visible Light Communication Systems (VLC) based on non-zero manipulation (NRZ-OOK) using LED is a new technology. A cascading bridge-T equalizer network is required (Figure 4) to implement these systems. With the bridge-T circuit, the 3-dB bandwidth of the VLC system can be extended from 1 to 520 MHz. The bit error rate of the system is 7.36 × 10−4and the data transfer rate is 1 Gb / s at a distance of 1.5 m.

Figure 4: Cascade bridge-T equalization circuit for VLC system. (Image: IEEE Photonics Journal)

Grid for DER network connected

Accurate information on the phase, frequency and amplitude of voltages at the common connection point (PCC) is required by the controller of single-phase distributed inverters for distributed energy resources (DER). Phase Blocked Circuit (PLL) is used for fast synchronization with the network, which is necessary to meet the requirements of network codes and optimize the performance of the inverter. The main task of the PLL is to find accurate real-time voltage information in the PCC. The grid filter for all passes can accurately determine the required values, even in voltage harmonics, and generate an orthogonal voltage system for the PLL (Figure 5).

Figure 5: A lattice filter for all passages can help generate the orthogonal voltage system information needed by grid-connected inverter controllers. (Image: Advances in electrical and electronic engineering)

Figure 5: A lattice filter for all passages can help generate the orthogonal voltage system information needed by grid-connected inverter controllers. (Image: Advances in electrical and electronic engineering)

Summary

Equalizers can be used to linearize the performance of RF signal circuits or to introduce nonlinearities in audio signal circuits. They can be used to manipulate the gain, phase or other characteristics of the signal circuit. In addition to communication, radio and audio systems, equalizers are used in various applications such as visible light communications and renewable energy systems.

References

Bridge equalizer to delay TWikipedia
Comparative analysis of a lattice-based filter for all passages and a generalized second-order integrator as a generator of an orthogonal PLL systemAdvances in electrical engineering and electronic engineering
Alignment (audio)Wikipedia
Gb / s Real-time visible light communication system based on white LEDs using T-Bridge cascade circuit for pre-alignmentIEEE Photonics Journal
Lattice phase equalizerWikipedia
RF / microwave equalizers: an essential ingredient for the modern system designermini chains

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