█ Electromagnetic Interference (EMI) Solutions
EMI is generated from various portions of crystal oscillation circuit. Those portions should be clarified, andEMI level can be lowered by designing printed circuit board (PCB) which includes solutions to decrease it.
1. EMI generating points(1) The inside of IC*5

EMI is also emitted from the inside of IC.
LSI(2) PCB traces*1*6

Long PCB trace which connects IC and components
such as quartz crystal units and capacitors behaves as anantenna and the degree of EMI level increases, and highamount of EMI is radiated from trace *2 of the OUT pin ofthe inverter. Negative resistance will also unfavorably*7*2
DC power*3*4

decrease if traces of IN and OUT are lengthened insupplyparallel as shown in the figure.
(3) Quartz crystal units
The sign waveform of oscillation frequency appears on
the electrode of an oscillating quartz crystal unit. GenerallyIC

the waveform on the IN side of the oscillation circuit isclose to sine wave and EMI level is quite low. On the OUT
side, however, noise level is rather high since thewaveform close to square wave which contains a lot ofharmonics.
(4) The ground line of PCB
If the impedance of the ground line is high, radiated EMI

Arrowed PCB pattern
becomes antenna of EMI

level will be rather high.
(5) The power supply line of PCB
A power supply line is at the same potential with theground line in alternating current signals. If the impedanceof the power supply line becomes high, it will become anGND

antenna which radiates EMI.
(6) The output port of LSI
The data output port of LSI also radiates EMI.
(7) Direct current power supply lineIf high frequency noise is contained in the output of power supply, a crystal oscillation circuit and otherfunctional circuits will be modulated by the noise, and EMI level generated from these circuits increases. Also,
oscillation signal of crystal oscillation circuit will appear on a power line if the impedance of the power line is high,and the power line becomes an antenna and EMI will be radiated.

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Design center2. EMI Solutions
Since the oscillation waveform on the IN side of the Pierce crystal oscillation circuit using a C-MOS inverter
exhibits a waveform similar or equal to sign wave, harmonics generated from this portion are minimal. On theother hand, the waveform on the OUT side of the inverter is square or distorted square wave, it contains a lot ofharmonics. PCB trace connected to this portion becomes an antenna, and radiates EMI.
(1) EMI solution for the noise generated from the inside of LSI

LSI

Ground pattern

By designing a field ground under LSI, it is possible toPCB

decrease EMI radiated under LSI.

Side view(2) EMI generated from PCB traces"Open" end without making a loop.

As shown in the figure, PCB should be designed withthe shortest trace length so that the trace connecting
components such as a crystal unit and capacitors with LSIICTop view

does not become an antenna which radiates EMI.
Furthermore, it is also effective to shield the signal patternarea of oscillation circuit with a ground pattern. However,
one should be careful upon designing PCB, since negative
resistance will decrease if these traces are routed too
close. Also, it is important to design ground pattern not tomake a loop by leaving the trace end open.

GNDThe pattern of OUT terminal needs tobe designed the shortest.

Since high amount of EMI is radiated from the line connected to the OUT terminal of the inverter oscillation
part, it is important to design this pattern length shortest. The waveform of the IN terminal of the inverter is asign wave in most cases, the level of EMI is low.

The increases of EMI arising by PCB pattern of the OUT

terminal becoming longer than the pattern of the IN terminal is much less compared with the case when thepattern of the OUT terminal is lengthened.
To oscillation circuit

Please refer to " Precautions on PCB design".

GND(3) EMI radiated from quartz crystal units
In SMD crystal units with metal cap (CX-2520SB,X-tal

CX-3225SB, KSX-23, CX-4025S, CX-96F, KSX-35,GND

KSX-36, CX-91F), EMI level radiated from the cap of acrystal unit can be decreased, if these terminals areconnected to ground since #2 and #4 terminals areinternally connected to the metal cap.

Top view

As for the leaded type quartz crystal units, oscillation signal appears in a metal case, and they are radiated asEMI.
Please use crystal unit "CX-49L" equipped with a metal jacket to connect a metal case to ground decreasingradiated EMI level.
(4) EMI radiated from the ground line
If the layout of the ground pattern on the capacitor end is long and thin, it will become an antenna for theportion radiating EMI, and PCB needs to be designed so that they should be connected to the ground line withthe shortest length. Please refer to (2) on the above.

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Design center(5) EMI radiated from the power supply lineIt is effective to insert bypass capacitors with several

Power supply line
All capacitors are placed closest tovalues of different self-resonance frequencies so that thethe LSI with the shortest pattern.

impedance of a power supply line shall not become high inwide frequency range. Usually, bypass capacitors are1.0

connected with the shortest length closest to the power

0.10.01

supply terminal of LSI as shown in the right hand side

LSI

0.001 micro F

figure. It is very effective to insert these capacitors inseveral places in PCB with long power supply line.

A bypass capacitor is required for each power supply line when LSI needs several power supply voltages.
Large value capacitors such as several micro farads are usually used for low frequency noise, but the careshould be taken since regulator IC itself may oscillate if capacitors with large values are connected to the output
terminal of regulator IC. Furthermore, it is effective to use EMI filters to suppress EMI radiated from the power
supply line.
(6) EMI radiated from the data output port
Line filters may be used depending on the form of data. If it is not possible, it is effective to shield the area ofdata transmission line, and shield the other side of PCB by the ground pattern. Unused ports should beterminated with adequate impedance indicated in the LSI manual.
(7) EMI radiated from the power supply unit
In order to lessen EMI radiated from a direct-current power supply circuit, it is effective to shield the circuit witha metal cover or equivalent.
3. Oscillation circuit structure and selection of circuit constants to decrease EMI.
(Measure to lessen harmonics)
The amount and level of EMI change depending on oscillation waveforms, and EMI decreases the most withsine wave. Since the oscillation waveform of the IN terminal of the inverter is close to sign wave, harmonics
generated from this terminal are little in the C-MOS inverter crystal oscillation circuit. However, square wave ofthe OUT terminal which the sign wave of the IN terminal is amplified contains many harmonics. It is possible todecrease the distortion of the oscillation waveform of the OUT terminal and to decrease EMI by the following
method.
(1) Decrease the amount of the quantity of the electric charge which is charged and discharged in C2 using Rdand Rx. It is necessary to check that it satisfies the target value of design by circuit examination while selecting
proper resistance values, since negative resistance of oscillation circuit decreases and oscillation starting time
becomes long.
(2) Use extremely small value compared with C1 and C2.

Rf =1M ohm

If the capacitance of the OUT side is made small
compared with the IN side, values such as C1=22pF andC2=5pF for example, the distortion of the waveform of
the OUT terminal and EMI level will decrease. However,Sine wave

X-tal

Rx

Rd

if the capacitance of the OUT side is made excessively
small, the amplitude level will become low, and it isnecessary to confirm that the OUT level is sufficient todrive the circuit of the following stage.
KYOCERA KINSEKI Corporation

Design center

C1

C2

Square wave orsine wave
with distortion