These victims of EMI. The EM noise

These
days, electrical and electronic devices aren’t anymore considered unfamiliar.
With these globally used devices comes circuitry with of course operate with
current flow or as we say, flow of charges. These charges flowing, will induce
electrical fields which is partnered up with magnetic fields. The existence of
these two fields, which we call as electromagnetic fields, have effects to its
source system and may even interfere other devices/systems. Unwanted fields generated
by these system, which affect other systems by radiated and conducted
electromagnetic waves, is known as Electromagnetic Interference (EMI). . It
unintentionally affects the way a circuit works or even bring faults/malfunctions
to an electronic device.

 

Systems
may act as the source or victims of EMI. The EM noise may also be intentionally
or unintentionally spread. Hence, when designing these electrical/electronic
circuits many consideration and action must be taken before beginning. The
circuits should be designed in a way that it can work/perform well and do what
it functions as with taking consideration of its’ compatibility. If the
designing is done in a proper manner, all malfunctions regarding EMI may have
lower cost of fixing and also lets other devices work well without being
interfered.

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This
assignment report encompasses the study on the circuit designed in a Low Cost
Spectrometer in a previous Capstone project on its’ Electromagnetic
Compatibility where it does not cause interference within its’ own system, does
not cause interference to other systems and is immune to possible emissions.
This is done by studying the circuitry and applying all that is learned in this
course, also providing probable mitigation methods like shielding and grounding.

 

 

 

 

 

 

 

 

 

 

 

 

 As per mentioned previously, the current or
the flow of charges in the circuitry and the response between components
produces EM fields which its features is governed by voltage, frequency and
current values. The higher the voltage and the current (also power), the more
noise emission it may source out, same condition applies for frequency values
of the circuit. But if these values are low, the circuit is will be more
susceptible to interference.

Figures
below show lines of E field and H field which act on a current carrying wire.

                         

                        FIGURE 1                                                                  FIGURE
2

 

The
noise sources, noise victims and coupling paths that is most probable will be
recognized and categorized beforehand in order to investigate the circuits’ EMI
characteristics. The way components are connected on the PCB of the Low Cost
Spectrophotometer are the key concern in the report, knowing that EMI
interference is most likely to occur within the system.

The
figure below shows the PCB layout of the spectrophotometer design.

 

 

 

 

 

 

 

 

 

 

 

 

 

                                                            FIGURE
3

As
shown above, the main circuit can divided into 3 subparts which the power
supply, the digital circuit and the input/output module. This can be clearly
shown in the below figure.

 

 

 

 

 

 

 

 

 

 

 

 

 

The noise voltage of the power supply obtained from 7805 is in the frequency
range of 10 Hz to 100 KHz, which is considerably insignificant to cause effect
to other circuits, however the op-amp, an analog circuit, within the IC is susceptible
to radio frequency (RF) emissions coupled through the power supply input which cause
an offset DC voltage when it’s near the op-amp unity gain frequency

 

The Atmega 328
hosted by the Arduino, is a 16MHz chipset that works at a 5V, this digital circuit chipset is a main emitted noise
concern that has a wide spectral content. Signals clocking in the IC are mostly
of a high freq pulse coded information kind, will cause radiation and also
crosstalk between interconnected circuits where the source being the Atmega
328, the victim being the LCD display and this is done through inductive &
capacitive coupling between parallel data traces

 

 

The servo
motor also
has EMI effects on it. The current taken in makes transients and instability in
power supply making it a main noise source. The interconnections between the
driving circuit and the motor forms a current loop which emits EM waves.

                                                                                      

 

Next,
is the infrared sensor. This component is known to be very sensitive and
has a narrow acceptable noise range making it not just a possible victim but
actually a victim of EMI. The noise is capacitively coupled to the IR receiver
from close electrical fields (Atmega 328 of Arduino)

 

 

Areas concerned with EMI on the PCB:

 

Cross-coupling
amongst loops in PCB. A current towards the load is brought by high freq.
signal line, this current has to go by a ground line as its return path. A current
loop is formed from the incident and return path that makes noise coupling increase
as the area of the loop increases. This can be seen in the figure below, where
the long wire having a large area of loop.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Grounding: Single grounding: Throughout
the ground trace, the current level isn’t the same and these currents gather in
the trace from each circuit connected to ground of a given impedance, creating
difference in potential from point to point.

 

 

 

 

Right
Angles: Considered as a necessity, we must decide
if an impedance mismatch through the trace will cause a signal trueness problem.
We know that there will be a decrease in the characteristic impedance of the trace.
The capacitance of the trace increases at corners while the inductance decreases.

 

 

 

Other possible PCB layout
issues include:

Differential
signals traces aren’t put in parallel

Separation
between placement of Analog and Digital circuits

Circuit
type leads crossing next to their circuit types

  b.

 

                  b.

 

  b.

 

  b.
 

 

Below
shows the problems in the designing of the PCB that causes EMI.