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High-speed clock signals generate high-frequency noise, and in these
conditions, the noise frequency is quite close to the signal frequency.
Therefore, an EMI filter with high suppression ability and steep attenuation
characteristics is used, such as the NFW31S Series (Chip "EMIFIL®"
for signal lines), or the BLMxxB Series (Chip Ferrite Bead for high-speed
signal lines).
Noise caused by transient current is also generated on the power supply
line. Therefore, a chip ferrite bead is installed, as well as a by-pass
capacitor. |
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Noise suppression on the clock line using the NFW31SP506X1E4
(Chip EMIFIL® for signal lines, cut-off frequency: 50 MHz)
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Before installing
the EMI filter |
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The 16-MHz clock signal output from the gate array emits high
frequency noise by using the circuit pattern on the board as
an antenna. When the NFW31SP506X1E4 (Chip EMIFIL® for signal
lines, cut-off frequency: 50 MHz) is installed on the clock
signal output, noise level is suppressed by approximately 5
dB at 176.1 MHz.
To prevent the waveform from ringing, a chip resistor is installed
in series with the filter.
Using a filter with a large constant, however, greatly distorts
the clock signal waveform, adversely affecting circuit operation.
Therefore, it is necessary to check the waveform and select
a filter with an appropriate constant. |
After installing
the EMI filter |
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