Interference Flashcards
What is meant by interference?
Interference refers to the unwanted coupling of signals from outside the system
What is meant by crosstalk?
Crosstalk refers to the unwanted coupling of signals from within the system
What is meant by noise?
Noise refers to a random process which may originate internally or externally
What are two words that may describe unwanted circuit elements?
parasitic or stray
What is a parasitic element?
A parasitic element is an unwanted but inevitable electrical element associated with a component.
What is a stray element?
A stray element is an unwanted electrical element which exists between circuit components
What is a primary cause of interference?
capacitive or inductive coupling
What is the equation for capacitance between two parallel wires?
PI epsilon
C = ———————–
ln(d/a)
What is a ground plane?
A ground plane is a grounded plate on a different layer of pcb which helps reduce capacitive coupling
If the load impedance has an inductive element what effect does this have in capacitive coupling?
Coupled voltages will often appear as positive and negative spikes accompanied by delaying oscillations
What effect does capacitive couple have when it manifests due to a break in a wire?
The effect often manifests as a distortion of the waveform, especially for digital signals
What is the typical coupling capacitance of strips and power rails in breadboard?
3pF for general strips and 30 to 40 pF for the power rails
Is capacitive coupling worse of better at high frequencies?
worse
What is inductive coupling?
Where two current-carrying conductors are in proximity, there will be a mutual inductance between them
What is effect of mutual inductance?
Emf induced in affected circuit. emf=M(di/dt)
What is a guard trace?
A grounded guard trace can be put between rails on a pcb where crosstalk may be a problem to avoid it
What is the best defence against inductive coupling?
maximise the separation distance between the likely sources of interference and the circuits which might be affected
What is an effective way to transfer both analogue and digital signals with minimum interference?
Balanced cables/lines
What is a balanced line?
system of two conductors with same impedance. Sometimes have third conductor which is grounded at one end to act as shield
What are two requirements to take advantage of balanced lines?
Must have same exposure to interference.
Differential signals must be used.
How do balanced lines prevent issues due to interference?
The differential signal will not be affected since both conductors should be affected by interference in the same way
Professional audio systems use balanced lines. What connector is used to terminate these?
XLR connector
What is the issue caused by shared ground paths and how is it overcome?
If multiple circuits share a ground path the path produces multiple voltage drops due to the resistance of the ground conductor. This is an issue for sensitive circuits. Can be fixed by having separate ground paths. Sometimes will see analogue ground and digital ground on pcb.
What is a ground loop?
Two pieces of equipment that are grounded, when connected together by a signal cable produce a loop of ground line parallel to the mains protective ground.
