paper introduces how to analyze the curve of the testing result, dig out the
reasons of exceeding, and come up with effective solutions so that the
equipment can meet EMC standard limit requirement if they exceed the EMC
Key Words: Exceed
EMC standard limit, EMC enhancement
Example 1: Radar Equipment
Nowadays the digital
circuits are very intensive and their function is complicated, as a result the
electromagnetic interference is inevitable, meanwhile the digit circuits are
affected by interference from other components too. So it is necessary to
consider EMC at the very beginning of product design so that effective measures
could be taken to eliminate interference signals. Take the EMI problems that
may exist for the design of certain radar equipment as an example.
result according to GJB-151A RE102 of the equipment is shown as figure1, we can
see interference at 30MHz and most frequency points between 30MHz and 90MHz
exceeds the limit. In order to find out the reasons of exceeding we use a
spectrum analyzer to identify the leakage position.
Since the structure and
circuit of the equipment is fixed and cannot be changed, we can only improve on
the basis of the original.
As we all know the
structure of the chassis has a great deal with radiated interference. The
components, integrated blocks, wiring of the printed circuit boards inside the
cabinet and where there is signal currents may radiate electromagnetic energy.
The higher the frequency it is most likely to generates interference signals.
If the cabinet is unshielded, the electromagnetic energy will affect the
outside devices. If it is metal cabinet or the inner surface of the non-metal
cabinet is metal-plated, the electromagnetic energy could be restricted only
inside the cabinet.
Testing result according to GJB-151A RE102 of certain radar equipment before
Figure2 Picture of certain radar
1. To improve the
In order to judge if the
radiated interference is mainly caused by chassis leakage, we disconnect the
cable and control lines, only leave power lines for which interference control
measures already taken to keep the equipment being able to work, then we test
the radiated interference again, still interference at most frequency points go
beyond the limits, this means there is obvious leakage from chassis or power
lines. In order to further identify if it is power line interference or cabinet
leakage or both. We moved a near field magnetic field probe( connected with
spectrum analyzer) along the chassis slit or holes to fix the leakage point and
observe the leakage situation at different frequencies. We found comparatively
high field strength at the slit near power supply, and we pasted one conductive
metal belt which was well connected with the chassis metal surface to assure
good conductivity, then there was obvious decrease of the radiated field
strength. The result shows there is leakage from the chassis.
Measures taken could be
adding conductive strap to make the chassis slit meeting the requirement, or
adopting wave-guide design or reducing the interval space of the stud used to
assembly the chassis( along the slit). Once the chassis is well shielded,
connect the shielded cable and control lines, retest RE102, we can see there is
obvious improvement, shown as Figure3, but still some points go beyond the
Testing result after cabinet improvement measures taken
2. Adding EMI filters
electromagnetic interference of the shielded unit coupling to the wires
connecting I/O interference and power lines generates interference current and
transferred to the outside of the shielded unit, causing transferred
interference and radiated interference to outside devices. It is the same that
outside EMI interference could enter into the unit through wires connecting I/O
interference and power lines and cause radiated interference to the inner
equipment of the unit.
Low-pass filters can
effectively suppress and prevent interference and reduce the electric level of
For power line, its
interference frequency are far higher than its working frequency, as a result
low pass filters could well eliminate the interference frequency while let
working frequency pass without any attenuation.
For I/O interface wire,
its interference frequency are also higher than its working frequency, so low
pass filters could well eliminate the interference frequency while let working
frequency pass without any attenuation.
But since the working
frequency of I/O interface wire is much higher than that of the power line, the
insertion loss characteristics of filters matched with them respectively are
different, so are the magnetic materials adopted. The former is
ferro-nickel and the latter is mn-zn
2.1 How to improve the
cable and control lines
a) Replace the current
used connector with filtering connector.
b) Reset the input and output
lines of EMI filter to prevent coupling between them, so as to secure the
c) Increase grounding
points of shielding layer of cable.
d) Insert all the pins
of connector to the ground potential to prevent antenna effect.
2.2 How to improve the
For shielded metal cabinet,
we should choose filters with separate metal cover and mounted at the input of
the power line. And make sure the filter metal cover keeps good electric
contact with the cabinet. The ground wire of filter should be fixed with the
common grounding metal part at the cabinet or cable output. Shown as figure4.
4 Right way of EMI filter grounding
After taking the EMC
enhancement measures above, the testing result is perfect, see figure5.
5 Testing result after EMC measures taken
To sum up, the
aforementioned measures are all good for EMC improvement, but the most commonly
used is to change ground wire structure and cables are enhanced respectively
because it is not only cost saving but also the most effective. Though shielding
will increase the cost but its shielding effect is preeminent above all other
method. So in practice we should mainly consider changing ground wire
structure, enhancing cables respectively and improving shielding, also other
measures as support.
Example 2: Digital circuit
Crystal oscillator is
absolutely essential for digit circuit design. Oscillator provides reference
signal for digit circuit, the higher its oscillation frequency, the richer
harmonics generated, in other words, the more serious the interference. For
most digit circuits, cover of the oscillator isn’t grounded, nor the PCB board
providing efficient grounding area.
More and more electric
engineers notice that cover of the oscillator grounded could effectively reduce
interference, so keep a grounding area correspond to oscillator and remove the
solder mask when wiring the PCB, and solder the cover of oscillator there.
Digital circuit can work
according to certain timing sequence only when oscillation level meet the
amplitude requirement. However, frequency multiplier harmonics generated by
oscillator bring interference, and even disturb the sensitive circuit.
Figure6 shows the
frequency multiplier interference amplitude of 25MHz oscillator, we can see
there is high spike at 2, 3, 4, 5, 6...times frequency multiplier, it is
typical frequency spectrum gram for 25MHz. This gram indicates that the
oscillator circuit or ground wire is not properly handled and we must reduce
oscillator harmonics radiation.
1. Oscillator grounding
Make sure the grounding area
large enough when deal with the grounding issue. If it still exceeds the limit
after grounding, but only the amplitude value decreases, then we should also
reduce oscillator harmonics radiation.
2. Suppress the
Figure6 Typical frequency spectrum gram before
a) Add attenuators while
keeping the sensitivity and noise-signal ratio unchanged. For example,
oscillators for VCD, DVD make the equipment hard to meet EMC requirement,
reducing oscillator frequency is a feasible solution to solve the problem, but
not a sole method.
We can also suppress the
amplitude value of multiple oscillator frequencies.
b) To observe if the
oscillator waveform is sine wave using a high frequency oscilloscope, if not,
change the line-ground capacitor of oscillator to adjust.
c) Add a series of
ferrite beads with low reactance and high impedance to the pin connecting
oscillator and decoder. The impedance of the ferrite beads is above 50MHz and
the higher the better while keeping the reactance not increasing too much.
d) Check if the loop area of
power supply of decoder, decoder oscillator clock, high speed signal and the
wiring around oscillator is too large, if yes, try to minimize the area.
e) If you already take
some of the measures above and the rest are hard to implement, the only thing
we can do is to add ferrite core or ferrite beads to the output lines, but this
is only a matter of expediency. Shown as figure7:
spectrum gram after EMC enhancement