NoiseReductionTechniques

Source : Noise Reduction Techniques in Electronic Systems – Second Edition. Henry W. Ott, 1988.

Noise Reduction Techniques At High Frequencies (>10MHz)

Noise sources

- When two circuits share a common ground

- ground loop

- ground with high impedance

- When two circuits share a common power supply

- Electric and Magnetic fields

- Galvanic actions : use of dissimilar metals

- Bending of the cable : triboelectric effect

- Conductor motion

Methods to eliminate interferences

- shielding

- grounding

- balancing

- filtering

- isolation

- separation and orientation

- circuit impedance level control

- cable design

- cancellation techniques

*Cabling

- Capacitive coupling : space the cables of 40 times the diameter of the cables.

- Inductive coupling : B factor reduced by physical separation or by twisting the cables

Difference between inductive coupling and capacitive coupling between two cables :

- First : voltage source+ resistance

- Second : one resistance R and voltmeter and variable resistance R2

- If as R2 decreases the measured voltage decreases, electric

- If as R2 decreases the measured voltage increases, magnetic

*Shielding

- A shield grounded at one or more point shields against electric fields.

- Usually at high frequency ( >10MHz), the shield is grounded at both ends (complication for the magnetic field : a loop is formed).

- At high frequency a coaxial cable works as a triaxial cable (skin effect), removing the noise current flowing from the current path.

- For magnetic shielding : decrease the area of the loop.

- At high frequency, absorption loss is the primary shielding mechanism for both electric and magnetic fields.

Refers p 167 for the skin depths, or characteristic attenuation length, of various metals depending on the frequency.

- At high frequency any solid shield thick enough to be practical provides more than adequate shielding for most application.

- The shielding effectiveness is determined by the leakage at seams and joints.

- For high frequencies, it’s better using a magnetic material than a non-magnetic material.

*Grounding

- At high frequencies, use of a multiple ground system.

o Low ground impedance

o Short connections between circuit and ground plane (-> PCB)

- Ground Loops

o Remove one of the connections to the ground

o Isolation of the two remote ciruits

§ Transformer

§ Common Mode Chokes

§ Hybrid grounds

§ Optical Coupling

§ Balanced Circuitry

- Guard shield : around amplifier

o Rule : The guard shield should always be connected so that no command mode current can flow through any of the input resistance

As a consequence, we connect the ground of the guard shield to the low impedance terminal of the voltage source

o Every measurement instrument is now equipped with a guard shield -> Connect it to the right place

*Balancing

- Balance the system allows cancelling in the load the common mode noise voltage spreading through the cables.

- Balance can be preformed by two means

o Symmetry in the circuitry

o Use of transformers to decouple the source from the load

- Balance is additional to shielding

*Filtering

- Use of feed through capacitors when the conductor passes through the shield

- Connect mica or ceramic capacitor with short leads between the conductor and the ground at the circuit end

- Use of a RF choke after the feed through capacitor

Passive components:

- All capacitors are self-resonant at some frequency.

- Mica and Ceramic Capacitor are good high frequency capacitors

- Air core inductors generates more noise fields than do closed magnetic material core inductors, but the later are more susceptible to interfere with magnetic fields.

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