Source : Noise Reduction Techniques in Electronic Systems – Second Edition. Henry W. Ott, 1988.
Noise Reduction Techniques At High Frequencies (>10MHz)
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
- separation and orientation
- circuit impedance level control
- cable design
- cancellation techniques
- 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
- 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.
- 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
Â§ Common Mode Chokes
Â§ Hybrid grounds
Â§ Optical Coupling
Â§ 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
- 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
- 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
- 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.