Electronics Guide

Physical Mechanism of Magnetostriction

In ferromagnetic materials, atoms possess magnetic moments that tend to align within regions called domains. When an external magnetic field is applied, domains aligned with the field grow at the expense of misaligned domains, and the moments within domains rotate toward the field direction. These changes in domain structure produce strain because the crystal lattice dimensions depend on the direction of magnetization relative to the crystallographic axes.

The magnetostrictive strain is typically expressed as the fractional change in length:

The strain depends on both the magnitude and direction of the magnetic field relative to the crystallographic axes. Most materials exhibit saturation magnetostriction, where the strain reaches a maximum value when the material is fully magnetized. Typical saturation magnetostriction values range from a few parts per million for nickel to hundreds of parts per million for specialized magnetostrictive alloys like Terfenol-D.

An important characteristic of magnetostriction is that it depends on the magnitude but not the polarity of the magnetic field. A sinusoidal magnetic field at frequency f produces magnetostrictive strain at frequency 2f because both positive and negative field peaks produce the same sign of strain. This frequency doubling is significant for understanding transformer noise and for designing magnetostrictive transducers.

Magnetostriction in Transformer Cores

Transformer cores are perhaps the most common source of magnetostrictive acoustic noise in electronic systems. The core material, typically silicon steel or ferrite, experiences cyclic magnetostriction as the AC magnetic flux varies. For a 50 Hz or 60 Hz power transformer, the fundamental mechanical vibration occurs at 100 Hz or 120 Hz due to the frequency-doubling effect.

Several factors influence transformer core noise:

• Core material selection: Different magnetic materials have different magnetostriction coefficients. Grain-oriented silicon steel is designed with reduced magnetostriction when the flux is aligned with the rolling direction.
• Flux density: Magnetostrictive strain increases with flux density, so operating cores at lower flux densities reduces noise but requires larger cores.
• Core construction: Laminated cores with proper stacking and clamping produce less noise than poorly assembled cores. Cut cores may exhibit higher noise than wound cores due to air gaps at joints.
• Harmonic content: Distorted current waveforms with significant harmonic content cause the flux waveform to contain harmonics, each contributing to magnetostrictive noise at twice its frequency.

Transformer noise mitigation approaches include selecting low-magnetostriction core materials, reducing operating flux density, improving core clamping and impregnation, using enclosures with sound-absorbing materials, and mounting transformers on vibration-isolating supports to prevent sound transmission to chassis and enclosures.

Magnetostrictive Transducers

While magnetostriction is often an unwanted effect, it also enables useful transducer applications. Magnetostrictive actuators convert electrical signals to mechanical displacement through the magnetostrictive effect. Common applications include:

• Sonar transducers: Magnetostrictive materials can generate high-power acoustic waves in water for submarine detection and underwater communication.
• Ultrasonic generators: Industrial ultrasonic cleaning, welding, and machining often use magnetostrictive transducers to generate high-intensity ultrasound.
• Precision positioning: The highly linear strain response of some magnetostrictive alloys enables nanometer-resolution positioning systems.
• Vibration control: Active vibration damping systems use magnetostrictive actuators to generate counter-vibrations.

Giant magnetostrictive materials like Terfenol-D (terbium-dysprosium-iron alloy) exhibit magnetostriction coefficients hundreds of times larger than conventional materials, enabling compact, high-force actuators. These materials find application in active vibration control, sonar projectors, and precision machinery.

Piezoelectric Materials and Properties

Piezoelectricity occurs in materials lacking a center of symmetry in their crystal structure. When such materials are stressed, the displacement of ions creates a net polarization that manifests as surface charge. Important piezoelectric materials include:

• Quartz: Natural piezoelectric crystal with excellent stability, widely used for frequency references and precision transducers.
• Lead zirconate titanate (PZT): Ceramic piezoelectric with high coupling coefficient, the most common material for transducers and actuators.
• Polyvinylidene fluoride (PVDF): Piezoelectric polymer that is flexible and can be formed into thin films for distributed sensing.
• Lithium niobate and lithium tantalate: Single crystals used for surface acoustic wave devices and high-frequency applications.
• Aluminum nitride: Piezoelectric thin film compatible with semiconductor processing, used in MEMS devices.

The piezoelectric response is characterized by coupling coefficients that relate electric field to strain or charge to stress. The most important parameters include the piezoelectric charge constant (relating charge per unit area to applied stress), the piezoelectric voltage constant (relating open-circuit voltage to applied stress), and the electromechanical coupling factor (indicating efficiency of energy conversion between electrical and mechanical forms).

Piezoelectric Noise Sources

Piezoelectric materials in electronic systems can generate electrical noise when subjected to mechanical stress or vibration. This microphonic effect can degrade performance in sensitive circuits:

Ceramic capacitors: Many multilayer ceramic capacitors (MLCCs) use barium titanate or similar ferroelectric ceramics that exhibit piezoelectric behavior. When these capacitors experience mechanical stress or vibration, they generate voltage across their terminals. In sensitive analog circuits, this piezoelectric noise can exceed the signals being processed.

Quartz crystals: Crystal oscillators contain quartz elements that respond to vibration with frequency modulation and spurious output. This sensitivity can cause problems in frequency-sensitive applications like communications equipment and precision instrumentation.

Piezoelectric transducers: Microphones, accelerometers, and other intentional piezoelectric devices also respond to unintended inputs. An ultrasonic transducer may pick up vibration from nearby motors; a microphone may respond to electromagnetic interference coupling capacitively to its preamplifier.

Mitigating piezoelectric noise involves selecting low-piezoelectric-activity capacitor dielectrics (such as C0G/NPO ceramics), mechanically isolating sensitive components from vibration sources, and using circuit topologies that reject common-mode noise from distributed piezoelectric sources.

Piezoelectric Transducers for EMC

Piezoelectric devices serve important functions in EMC testing and monitoring:

Acoustic emission sensors: Piezoelectric sensors can detect the ultrasonic signals generated by partial discharge in high-voltage equipment, arcing in connections, and mechanical stress in components. This enables condition monitoring and fault detection without direct electrical connection.

Vibration measurement: Piezoelectric accelerometers measure vibration during combined environmental and EMC testing, helping correlate mechanical excitation with electrical effects.

Ultrasonic sources: Piezoelectric transducers generate controlled acoustic fields for testing susceptibility to acoustic interference and for inducing vibration-related EMC effects.

Electrostatic Force Mechanism

The electrostatic force between two parallel plates with area A, separation d, and voltage V is:

This force is always attractive (the plates are pulled together regardless of voltage polarity) and proportional to the square of the voltage. For an AC voltage at frequency f, the force contains a DC component and a component at frequency 2f, similar to the frequency doubling seen in magnetostriction.

In dielectric materials, the electric field induces polarization that creates internal stresses. This electrostriction effect is analogous to magnetostriction and occurs in all dielectric materials, though it is particularly pronounced in high-permittivity ceramics.

Capacitor Acoustic Noise

Capacitors can generate acoustic noise through several mechanisms:

Electrostriction: High-permittivity ceramic capacitors experience significant electrostriction strain when voltage is applied. The resulting mechanical deformation produces acoustic noise, particularly audible when large AC voltages are present in audio-frequency circuits.

Electrostatic attraction: Electrolytic capacitors with wound foil construction experience electrostatic forces between the anode and cathode foils. These forces can cause the wound element to vibrate, producing a characteristic buzzing or humming noise.

Mechanical resonances: The physical structure of a capacitor has mechanical resonant frequencies. When electrical excitation occurs at or near these frequencies, the acoustic output is greatly amplified.

Mitigating capacitor acoustic noise involves selecting appropriate dielectric materials (C0G/NPO ceramics have much lower electrostriction than X7R or Y5V), avoiding operation near mechanical resonances, mounting capacitors to minimize vibration transmission to the PCB and enclosure, and using encapsulation or potting to damp vibrations.

Electrostatic Transducers

Electrostatic forces are exploited in various transducers:

Electrostatic loudspeakers: A thin conductive diaphragm suspended between two perforated electrodes moves in response to the audio signal, producing sound. Electrostatic speakers offer excellent high-frequency response and low distortion but require high-voltage bias supplies that can present EMC challenges.

Condenser microphones: The inverse of electrostatic speakers, these devices detect sound by measuring the capacitance change as a thin diaphragm moves in response to sound pressure. The high-impedance output requires careful shielding to prevent electromagnetic pickup.

MEMS devices: Microelectromechanical systems often use electrostatic actuation for switches, mirrors, and resonators. The small dimensions allow high electric fields with modest voltages, but also create sensitivity to vibration and acoustic interference.

Sources of Microphonic Sensitivity

Various components and structures exhibit microphonic behavior:

Vacuum tubes: The electrode structures in vacuum tubes are sensitive to vibration, which modulates the electrode spacing and hence the electron current. This was a well-known problem in early electronic equipment and remains relevant in audio amplifiers that use tubes for their tonal characteristics.

Variable capacitors: Any component whose capacitance changes with mechanical deflection exhibits microphonic sensitivity. This includes variable tuning capacitors, MEMS capacitors, and even fixed capacitors with insufficient mechanical stability.

Wire-wound resistors: Resistors constructed by winding wire on a core can exhibit microphonic effects if the turns move relative to each other or to the core when vibrated.

PCB traces and components: Vibration causes relative motion between PCB traces and the ground plane, modulating stray capacitances. Components that shift on their solder connections can modulate contact resistance.

Cables and connectors: Triboelectric effects in cables (discussed in the section on vibration-induced EMC issues) and contact modulation in connectors create microphonic noise. Coaxial cables with poor contact between shield braid and outer jacket are particularly susceptible.

Microphonic Noise Reduction Techniques

Reducing microphonic noise involves a combination of component selection, mechanical design, and circuit techniques:

Component selection: Choose components with inherently low microphonic sensitivity. Metal film resistors are less microphonic than wire-wound types. C0G/NPO ceramic capacitors are less microphonic than X7R or Y5V types. Low-noise cables use materials chosen for low triboelectric effect.

Mechanical mounting: Secure components firmly to prevent relative motion. Use conformal coating or potting to dampen component vibration. Mount sensitive components away from vibration sources like fans and transformers.

Vibration isolation: Use resilient mounts to isolate sensitive circuits from chassis vibration. Shock mounts with appropriate damping characteristics can greatly reduce transmitted vibration at frequencies above the mount's natural frequency.

Circuit design: Use differential circuit topologies that reject common-mode microphonic signals. Place sensitive components symmetrically so that vibration-induced effects tend to cancel. Reduce impedance levels where practical, since microphonic voltage noise is often proportional to circuit impedance.

Shielding and enclosure design: Acoustic shielding can reduce the transmission of airborne sound to sensitive components. Enclosure design should avoid mechanical resonances at frequencies present in the acoustic environment.

Sources of Acoustic Emission

Several phenomena generate acoustic emissions in electronic equipment:

Partial discharge: In high-voltage insulation systems, small electrical discharges occur in voids, cracks, or at contaminated surfaces before complete breakdown. These discharges generate both electromagnetic pulses and acoustic emissions in the ultrasonic range (typically 20-300 kHz). Acoustic emission monitoring can detect deteriorating insulation and impending failures.

Arcing and corona: More severe discharges in air or gas gaps produce acoustic emissions with significant energy at audible as well as ultrasonic frequencies. Arcing at poor connections can be detected acoustically before it causes complete failure.

Thermal stress: Temperature changes cause differential expansion in composite structures, potentially generating acoustic emissions as materials slip or crack. Monitoring these emissions during thermal cycling helps identify thermomechanical reliability issues.

Mechanical fatigue: Crack initiation and propagation in structural elements and component leads generate acoustic emissions that can provide early warning of impending mechanical failure.

Acoustic Emission Monitoring Techniques

Acoustic emission monitoring uses piezoelectric sensors coupled to the structure being monitored. Key aspects of AE monitoring include:

Sensor selection and placement: AE sensors must be sensitive to the frequency range of interest and properly coupled to the structure. Multiple sensors allow source location through arrival time analysis.

Signal processing: AE signals are typically processed to extract parameters including amplitude, duration, rise time, and frequency content. Pattern recognition techniques can distinguish different source mechanisms.

Environmental filtering: Electrical noise, mechanical noise, and electromagnetic interference can mask genuine AE signals. Filtering, timing gates, and guard sensors help reject spurious signals.

Correlation with electrical measurements: Combining acoustic emission monitoring with electrical measurements such as partial discharge pulse detection improves fault identification and location.

Sources of Ultrasonic Interference

Common sources of ultrasonic interference in electronic environments include:

Ultrasonic cleaning equipment: Industrial ultrasonic cleaners typically operate at 20-80 kHz with substantial acoustic power levels. Equipment near these cleaners may experience interference.

Ultrasonic welding: Plastic welding and metal welding processes use ultrasonic vibration that can propagate through structures to nearby electronic systems.

Ultrasonic sensors: Proximity sensors, level sensors, and flow meters use ultrasonic transducers that emit energy that may affect nearby sensitive circuits.

Switching power supplies: Although primarily electromagnetic noise sources, some switching converters produce mechanical vibrations at their switching frequencies that may extend into the ultrasonic range.

Gas discharge: Certain gas discharge phenomena, including corona and arc discharges, generate ultrasonic acoustic emissions along with electromagnetic interference.

Susceptibility to Ultrasonic Interference

Systems particularly susceptible to ultrasonic interference include:

MEMS devices: Microelectromechanical resonators, accelerometers, and gyroscopes have mechanical resonant frequencies that may coincide with ultrasonic interference frequencies, causing spurious outputs.

Ultrasonic receivers: Intended ultrasonic sensors can be jammed or confused by unintended ultrasonic sources in the environment.

Crystal oscillators: Quartz crystal resonators can be affected by ultrasonic vibration at or near their resonant frequency or mechanical vibration modes.

Audio equipment: Although ultrasound is inaudible, it can intermodulate with audio signals in nonlinear circuits, producing audible artifacts.

Electromagnetic Emissions from Speaker Systems

Speaker systems can generate electromagnetic interference through several mechanisms:

Speaker voice coil fields: The voice coil in a dynamic speaker is essentially an electromagnet that generates alternating magnetic fields proportional to the audio signal. In high-power systems, these fields can extend considerably beyond the speaker enclosure and induce voltages in nearby sensitive circuits or magnetic storage media.

Amplifier emissions: Audio amplifiers, especially switching (Class D) designs, generate significant electromagnetic emissions at their switching frequencies and harmonics. These emissions can couple into speaker cables, which then radiate as unintentional antennas.

Cable emissions: Long speaker cables carrying high-current audio signals can radiate electromagnetic fields, particularly at higher audio frequencies where cable lengths become a significant fraction of a wavelength.

Crossover network emissions: Passive crossover networks in speakers contain inductors that generate stray magnetic fields. Active crossovers contain electronic circuits that may radiate or conduct interference.

Electromagnetic Susceptibility of Speaker Systems

Speaker systems can also be affected by external electromagnetic interference:

RF interference: Radio frequency energy can couple into audio cables and speaker wires, where it is demodulated by nonlinear junctions (such as oxidized connections or semiconductor junctions in active speakers) to produce audible artifacts. This commonly manifests as reception of radio broadcasts or buzzing synchronized with mobile phone transmissions.

Power line interference: Magnetic fields from power wiring can induce hum in audio circuits, particularly when ground loops exist or when audio cables run parallel to power cables.

Switching transients: Fast switching transients from nearby equipment can couple capacitively or inductively into audio circuits, producing clicks or pops.

EMC Design Practices for Audio Systems

Good EMC practice in audio systems includes:

• Using shielded cables for low-level signals, with proper shield grounding
• Implementing balanced (differential) signal transmission for long cable runs
• Filtering RF interference at cable entry points and at sensitive circuit inputs
• Avoiding ground loops through careful system grounding design
• Separating power cables from signal cables
• Using ferrite chokes on cables to suppress common-mode interference
• Shielding sensitive circuits and transformers
• Locating power amplifiers away from sensitive source equipment

Types of Electromagnetic Interference in Microphones

Microphones and their associated circuits experience several forms of EMI:

Magnetic field pickup: Dynamic microphones use a moving coil in a magnetic field and can pick up external magnetic fields that add unwanted signals. Similarly, ribbon microphones use a thin metal ribbon in a magnetic field and are very sensitive to external magnetic interference.

Electric field pickup: Condenser microphones and electret microphones have high-impedance capsules that can couple capacitively to external electric fields. The long, high-impedance cables sometimes used with these microphones exacerbate this susceptibility.

RF interference: Radio frequency energy can be demodulated by nonlinear elements in microphone preamplifiers, producing audible artifacts. This is particularly common with GSM mobile phones, whose pulsed transmissions produce a characteristic buzzing sound when demodulated.

Ground loop hum: When microphone and recording equipment are grounded at different points, ground loop currents at power line frequency and its harmonics can appear in the audio signal as hum.

EMI Mitigation in Microphone Systems

Reducing electromagnetic interference in microphone systems involves multiple approaches:

Shielded cables: Using properly shielded cables with complete shield coverage and good shield terminations reduces both magnetic and electric field pickup. The shield should be connected at both ends for RF shielding, though this may require ground loop breaking techniques at audio frequencies.

Balanced connections: Balanced (differential) microphone connections reject common-mode interference that couples equally to both signal conductors. Professional microphones universally use balanced XLR connections for this reason.

Hum-canceling designs: Microphones with dual-coil designs arranged for opposite polarity pickup can cancel magnetically induced interference. This is similar to the humbucking principle used in guitar pickups.

RF filtering: RF interference can be reduced by adding filtering at the microphone preamplifier input. This typically involves shunt capacitors and/or series ferrite chokes to attenuate RF energy before it can be demodulated.

Active ground lift: Active ground lift circuits can break ground loops at audio frequencies while maintaining RF shielding by providing a high-frequency bypass around the isolation.

Phantom power considerations: Condenser microphones using phantom power should be immune to power supply noise. Proper phantom power circuit design includes filtering and rejection of any noise on the phantom supply.

Source Reduction

Reducing acoustic noise at its source is often the most effective approach:

Component selection: Choose components with inherently low electromagnetic-acoustic coupling. This includes low-magnetostriction core materials for transformers, low-electrostriction dielectrics for capacitors, and mechanically stable construction for all components.

Operating point optimization: Magnetic components operated at lower flux densities produce less magnetostrictive noise. Capacitors operated at lower voltage gradients produce less electrostriction. Balancing size and efficiency against acoustic performance is a key design tradeoff.

Waveform control: Smooth waveforms with low harmonic content generate less acoustic noise than distorted waveforms with significant harmonic energy. In power electronics, waveform shaping and harmonic filtering reduce both electromagnetic and acoustic emissions.

Frequency selection: Moving excitation frequencies away from mechanical resonances reduces acoustic output. When possible, operating at frequencies above the audible range (above 20 kHz) eliminates direct audibility, though ultrasonic excitation can still cause problems through harmonic generation and beat frequencies.

Path Interruption

Preventing acoustic energy from reaching sensitive locations or human ears involves both acoustic and mechanical isolation:

Acoustic enclosures: Sound-absorbing enclosures around noise sources reduce airborne sound transmission. Effective enclosures combine sound-absorbing materials (to reduce internal reverberation) with sound-blocking materials (to prevent transmission through the enclosure walls).

Vibration isolation: Resilient mounts between noise sources and supporting structures prevent structure-borne sound transmission. Isolation mount selection must consider the frequency range of concern and the static load to be supported.

Damping treatments: Viscoelastic damping materials applied to vibrating surfaces dissipate vibrational energy as heat, reducing both radiated sound and transmitted vibration.

Mechanical decoupling: Flexible connections between components and enclosures prevent direct mechanical coupling. This includes flexible mounting of transformers, resilient cable routing, and isolated mounting of fans and motors.

Receiver Protection

When source reduction and path interruption are insufficient, protecting the receiver (whether human ears or sensitive equipment) may be necessary:

Active noise control: Active noise cancellation uses microphones to sense noise and speakers to generate anti-phase sound that cancels the unwanted noise. This approach is most effective for low-frequency, periodic noise sources like transformer hum.

Frequency masking: In some applications, adding background sound (such as white noise or pink noise) can mask objectionable tonal noise, making it less noticeable even if not reducing its absolute level.

Component hardening: Sensitive electronic components can be protected from acoustic interference through mechanical mounting that attenuates vibration and through circuit design that rejects microphonically induced signals.