Points of common mode choke coils selection

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

  • •Matching of device characteristics impedance to transmission - line.
  • •Optimization of cut - off frequency for differential mode insertion loss characteristics.
  • •Optimization of insertion loss characteristics for common mode insertion loss characteristics.

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. 

- Selection point1: Characteristic impedance matching -
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.

- Selection point2: Differential mode insertion loss characteristics -
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.

<ex.>

* Define the frequency whose insertion loss
  become -3dB as cut - off frequency.

- Selection point3: Common mode insertion loss characteristics -
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.

<ex.>

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