Any industrial (aerospace, medical, automotive, consumer, telecom) products utilizing sensing, optics, DSP, technologies are more challenging to design due to their susceptibility to noise coupling (external & internal) and crosstalk's. A noise is an unwanted electrical signal interfering/coupling into sensitive circuits disrupting the product functionally and performance. For example, let us take any RF communication based product like a cellphone. Some external electromagnetic noise interference (EMI) from some other aggressor can cause your voice to drop or cause disturbance. Another example let us consider a pacemaker (medical device) which regulates your heart beat. If the product not well designed, external EMI could cause the product to fail which could be life threating. Similarly the performance (data transfer speed rate) of the optical telecommunication products which utilizes widely tunable laser to transmit data via fiber cable can be limited due to high noise coupling into the laser from its subcircuitary. The noise coupling can be divided into two categories i.e internal to the product and external to the product.
Internal Noise Coupling
The common aggressors in the internal coupling are ferrite beads, switching inductors in the power supplies, PLL clocks, capacitors, coils etc. The E-field (capacitive coupling) and B-field (inductive coupling) emitted from these components couples into the victim circuits inducing electrical noises.
Inductive Coupling due to Magnetic Field (B-Field)
The inductive crosstalk's are due to the magnetic field (B-Field) coupling which can happen over the air from one chip/part to another or inside the PCB from the via to via, trace/shapes to traces/shapes.
These noise coupling can be reduced using high permeability ferrite material shielding the victim circuits. These ferrites provide low reluctance path to the B-fields generated by the aggressors and bend/deflect those fields away from the victim circuits. Another way to reduce the coupling is increase the spacing between aggressors and victim traces/via/shapes. One another way I created is to increase the the victim side AC coupling path impedance.
Capacitive Coupling due to Electric Field (E-Field)
The capacitive crosstalk's are due to the electrical field (E-Field) coupling which can happen over the air from one chip/part to another or inside the PCB from the via to via, via to trace, trace/shapes to traces/shapes .
Reducing capacitive coupling can be done by shielding, increasing distance, avoiding overlapping circuits or certain connections to ground, reduce dv/dt, lower rise time, minimize parallel paths, reduce trace length.
There are application which requires to maintain constant temperature for proper operation of the product example quantum computers or optical telecom product using laser. In order to main constant temperature of a module which holds some sensitive components, thermoelectric cooler (TEC) are used. If the electric load inside the module suddenly fluctuates it affects the temperature of the whole module, that fast thermal change cannot be handled by the TEC controller. This cause temperature instability. The way to reduce this crosstalk to provide good thermal isolation between different blocks inside the module.
External Noise Coupling
These are the noises coming from external environment in the form Electromagnetic Interference EMI or ESD pulses or conducted noise injection getting into the product. The aggressors of these external noises are everywhere starting from your Cellphone, Microwave oven, Bluetooth headset, Wifi, Speakers, TV, Radio, Laptops, Power Lines, etc. So any product going into the market in an controlled (like hospitals) or uncontrolled (residential or commercial) environment are supposed to be compliance to the regulatory standards like FCC, IEC, ISO, CE etc. The products has to be designed to pass these EMI/EMC/ESD interference.
Electromagnetic Interference EMI due to Electromagnetic Wave Radiation (far field)
An example would be when you use a wireless headset near some microwave ovens, you may notice some interruptions or interference in your audio. The reason behind this is any electronic gadgets or products are vulnerable in picking up electromagnetic waves just like a radio. There are three different methods to help reduce or eliminate EMI: filtering, grounding, and shielding. Common RF shielding materials include aluminum, copper, tin, epoxy and ferrite powders, gold fabric, nickel, nitrile and forms of polyester. The shielding acts as faraday cage by grounding the picked up RF signals ensuring it doesn't reach the critical circuits.
Conductive Interference due to external noise injection
Any device using power outlet can be affected by conducted noise. The conducted noises are in the form surge current, fast transients, harmonic currents, flickers, dips, etc. and are generated by other products plugged into the power outlet. Since all the power outlets are interconnected to the power grids, these noises travels thru the transmission lines and gets into other devices and causes damages or service interruption to the products. So the products internal power supply are designed to be immune from these conducted noises. Usually the power supplies are designed with common mode chokes, ferrite beads and other filter circuits to suppress these noises.
Electrostatic Discharge (ESD)
Electrostatic discharge is a sudden and momentary flow of electric current between two electrically charged objects caused by contact, an electrical short or dielectric breakdown. Usually the human body gets charged over the time and creating static electricity and it gets discharged to the devices we touch. The discharge voltage can range from 1kV to 20kV at 10nsec to 60nsec. The ESD pulse could damage your product or cause temporary interruption. Usually the products are designed with ESD diodes inside the user accessible points. These ESD diodes suppress the pulse, thus protecting other circuitry.