EMI Suppression Filters (EMC and Noise Suppression)Points of common mode choke coils selection

For the noise measures of high speed differential signal line, it is important to keep the signal quality.

Points to keep signal quality

Improper filters, whose differential mode insertion loss at signal frequency range is high or which is not matched to transmission line impedance, will damage to signal. Please select the filter which does not influence the signal quality before insertion of the filter and after insertion of the filter.

This reduces the loss due to characteristic impedance mismatch.

Matching the filter characteristic impedance
→When the characteristic impedance of the transfer path and of the coil do not match, signal reflection or other phenomena occur and affect the signal quality.
Guideline for signal lines that require characteristic impedance matching: When performing high - speed transfer of 1Gbps or more

It can be confirmed whether characteristics impedance of device is matched by measuring TDR (Time Domain Reflectometry) characteristics of device.

The signal on differential transmission line is transmitted as differential mode.
To keep signal quality, (like voltage amplitude or jitter) , it is important to select the common mode choke coil whose differential mode loss in signal frequency is small.

Most of serious noise is transmitted as common mode.
To prevent noise, it is important to select the common mode choke coil whose common mode insertion loss is large in noise frequency.