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Noise reduction basics | types of noise reduction techniques

This article covers noise generation sources and mentions noise reduction basics and types of noise reduction techniques used in device and circuit board design encompassing RF and Digital parts.

Introduction:
As we know, noise or interference can not be completely eliminated but can be reduced or minimized. A system is electromagnetic compatible if it satisfies following three criteria.
• It does not cause interference to other nearby systems.
• It is not susceptible to emissions or radiations from other systems.
• It does not cause interference to itself.
Refer EMC compatible system design>>.

Noise Interference Sources

Interference occurs when receiver or receptor receives energy which causes it to behave in undesired manner. There are two main types of interference direct and indirect. The examples of direct interference are common impedance coupling, through the conductor etc. The examples of indirect interference are crosstalk, radiation etc. The figure-1 depicts these sources of noise or interference.

Noise Generation Sources

There are two major terminologies viz. EMI and EMC. The sources of EMI include following.
• Common sources of electromagnetic emissions are lighting, motors, lights, relays etc.
• Cables, AC Power conductors, metal cabinets and other circuits receive or radiate undesired signals.
• Clock circuit used in digital circuit, generates wide band noise. The other elements are RESET lines, CONTROL and INTERRUPT lines.
• Open wire such as power supply leads pick up noise into the circuit. This is example of conductive EMI.
• When the two circuits are connected with common impedance path, noise signal is coupled from one circuit to the other and vice versa due to flow of ground currents.
• When there are two nearby conductors, the flow of current in one conductor causes radiation to the other conductor and vice versa. This is known as crosstalk.
• Die size, pad layout and packing can also affect EMI.
• Improper PCB layout for different sections of the circuit (e.g. analog, digital, high frequency) can also cause EMI.
• Power supply lines should be properly connected to the circuit.

The sources of EMC include following.
• High supply voltages will have greater voltage swings and hence will have more emissions.
• High frequency value as well as periodic signals cause more emissions.
• Noise are created due to current spikes. In digital system, current spikes are created during switching ON/OFF of the transistors. The load current variation causes current spikes in analog systems.
• Improper grounding is another source of noise. There are different types of grounding for different types of circuits. They are single point grounding (below 1 MHz), multipoint grounding (high frequency digital circuits) and hybrid grounding. Hybrid type uses both single point for low frequency and multipoint for high frequency. The proper ground and power plane layout should be made for PCB after segregating digital, analog and high frequency noisy parts.

Following are the major techniques to reduce the noise in the device or PCB. These techniques will help in improving performance of noisy system. There are many more ways to improve the performance based on various applications or products.
➨Try to suppress emission at its source itself.
➨Make coupling path most inefficient.
➨Make receiver less susceptible to radiation or emission.



Noise Reduction Techniques at device level

Following noise reduction techniques are commonly used at device level.
➨Use multiple power pins and ground pins.
➨Use as few clocks as possible.
➨Try to reduce output buffer drive.
➨Eliminate race conditions.
➨Keep more space between low speed traces and high speed traces. Run high frequency signal traces next to the ground bus.
➨Keep clock lines off when they are not in use.
➨Eliminate charge pumps if possible to do so.
➨Keep loop area as small as possible inside the chip.
➨Make use of low power methods
➨Reduce internal impedance of power trace and ground trace.
➨Provide good ground imaging for long traces as well as traces carrying high speed signals.

Noise Reduction Techniques at Board level

Following noise reduction techniques are commonly used at PCB or board level.
➨Make use of ground plane and power plane on the board.
➨Minimize use of surface conductors on PCB.
➨Maximize plane areas in order to provide lower impedance for decoupling of power supply.
➨Make use of narrow traces (about 4 to 8 mils) to increase high frequency damping and to reduce capacitive coupling.
➨Segregate circuits on the PCB based on frequency (e.g. low/high) and type (e.g. analog/digital)
➨Segment GND/Power for analog, digital, transmitter, receiver etc.
➨Do not notch or cut PCB. This often creates unwanted loops due to nearby traces.
➨Avoid large open loop structures during PCB design.
➨Design multi-layer board so that traces between power plane and ground plane are enclosed properly.
➨Use chassis ground. This shielding helps to prevent radiation at circuit boundaries.
➨Use proper grounding type such as single point for low frequency and multipoint for high frequency portions on the PCB.
➨Keep ground leads shorter than (1/20)th of λ in order to prevent radiation and to maintain lower impedance.
➨Do proper routing of traces e.g. use 45 degree instead of 90 degree for trace turns, route clock signal perpendicular to I/O lines etc.
➨Use appropriate filters at different places on the PCB. Filter power line and all signals entering into PCB. Use appropriate decoupling capacitors at power pins. Use ferrite beads at power entry inputs. Use bypass capacitors at appropriate places on the PCB. Make use of multistage filter to attenuate multiband power supply noise. Also refer Power supply noise reduction techniques>>.

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