How to Design Ferrite Bead Audio Filters
How a crucial EMI solution poses an issue to audio signal integrity.
Filter topics and how to maximize a ferrite bead’s dissipation.
Countering noise introduced by ferrite beads.
Ferrite bead audio filters function much the same as external chokes found on cables
There’s a pretty good chance that somewhere within arm’s or leg’s reach, there’s a ferrite bead to be found. An excellent candidate for a social media showcase of “here’s an everyday product you’ve always seen but never understood,” ferrite beads are found externally in a wide range of electronics ranging from computer and videogame peripherals to laptop chargers. These aren’t the only place they can be found, however. In-circuit, ferrite beads are an inductive element that can act as a low-pass filter through their construction and incorporation of fundamental electromagnetic device theory.
The low-pass ability of a ferrite bead endears it to many uses, but audio usage in particular can extract a good amount of functionality out of a relatively simple device. Ferrite bead audio applications center around removing noise and higher harmonics to capture a sample in its purest form along with preventing unintentional signal transceiving along extended cables.
Ferrite Bead Audio Filters Block Noise From Shielding
To gain a proper appreciation of the ferrite bead’s role, it’s worthwhile to back up to the cable it is attached to. Ferrite beads, and many other components and design techniques, are used against a common culprit: RF interference. RF is insidious in the number of ways it can couple to cables and manifest as noise in carefully calibrated audio designs. Audio cables utilize shielding with the hopes of a sheathed outer conductor layer reflecting EMI as a Faraday cage. Although the shielding is essential, it also introduces a conductive layer for current to flow through, which can become internalized as noise due to:
Improper termination between cable and connector, which imparts common-mode voltage.
Poor balancing that creates a differential voltage on the signal lines.
Insufficient filtering, which fails to diminish noise to an acceptable level.
Removing the shield is out of the question, but the potential noise it could introduce must also be accounted for. Enter the ferrite bead – with it, the construction of a filter targeted for a certain frequency will help reject noise commonly associated with the high-frequency region. Designing the filter requires a working knowledge of impedance, which can be equated to the A/C equivalent of resistance. Unlike resistance, which is a real-valued lossy measurement, impedance is formed by adding the reactance – either inductive or capacitive, which serves to offset the current and voltage by some phase angle – vectorially to the resistance. As a surface-level treatment, a ferrite bead is analogous to an inductor in physical design and construction, but their usage greatly differs.
Circuit Analysis and Simulation as a Framework for Filters
Filter design also requires knowledge of the equivalent circuit model. When working theoretically or using ballpark estimates, idealized models of components are used that typically represent the singular or dominant value imparted by the component – be it resistance, capacitance, or inductance. Intentionally, these models ignore the additional contributions of impedance that arise from elements used in the construction of the package itself.
When performance or safety is on the line during product development, idealized models can no longer be considered accurate. To better simulate the actual performance of the signal and circuit, components are instead represented with an equivalent parallel or series impedance model. Datasheets will often offer resistance and reactance as the series equivalent, although a parallel model is equally valid.
The equivalent circuit measurements are valuable, as the impedance and phase angle alone cannot yield the resistance and reactance components. Engineers will want to maximize the series resistance and minimize the reactance for a given impedance targeting a frequency range. Why? The answer lies in the operation of the ferrite bead, which aims to dissipate RF energy in the form of heat to remove it from a signal line. At some impedance, a ferrite bead can maximize dissipation by elevating the real, lossy resistance over the imaginary, lossless reactance.
Improving Models With Non-Linear Analysis Methods
It is crucial to understand for modeling purposes that ferrite beads, though often modeled as an equivalent circuit of parallel and intrinsic resistance, capacitance, and inductance in series with a DC resistance, are not linear elements. This is broadly true for all linear models: most can be considered to express linearity within some operating range, but this is a reasonable assumption for simplifying the model, as nonlinear contributions during normal usage are negligible. However, this is not necessarily the case for ferrite beads due to magnetic hysteresis. The magnetic memory of the material, hysteresis curves, shows a nonlinear relationship between the magnetic field and the actual magnetization. Although soft ferrites like manganese zinc and nickel zinc are best suited to minimize losses due to hysteresis, this still introduces some noise that cannot be accounted for with standard circuit analysis methods.
For a more comprehensive evaluation, total harmonic distortion (THD) is a way to quantify the effects of hysteresis loss at the ferrite bead. An audio frequency analyzer can probe the nodes before and after a ferrite bead to determine the amount of distortion added (this measurement can be further compared against the THD of the open circuit voltage and at the source to establish background levels). THD can be analyzed with either a voltage or frequency sweep, which plotted logarithmically, shows the response of the circuit over its expected operating range.
The Cadence Software Suite Is Music to a Designer's Ears
Ferrite bead audio filters are a necessary element in limiting the effects of RF interference on sensitive cables. Maintaining signal integrity is crucial in audio applications to ensure high-quality output. Although there are intrinsic limitations with ferrite beads that can produce audible noise issues, these concerns are secondary to the critical role of removing coupled noise external to the circuit.
The best way to counteract the presence of noise on crucial signal lines starts with simulation. Cadence’s PCB Design and Analysis Software toolset offers product teams an all-in-one solution for board development, from schematic to manufacturing files and artwork. This integrated environment extends to OrCAD PCB Designer, where the layout can support dense placement and route with robust features.
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