Solution to avoid antenna desense

AN4356

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DocID025260 Rev 1

The key points on the selection criteria for a common mode filter are:

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High common mode rejection in the targeted frequency range; but at the same time,

high common mode bandwidth to be invisible in case of embedded signals (clock

signal for example in MHL standard)

•

High differential bandwidth to drive the high data rate signals without distortion

•

Low DC resistance to ensure lossless signal and impedance matching (TDR)

Another interesting characteristic is integrated ESD protection. The common mode filter is

often close to digital devices sensitive to ESD surges or close to connectors in direct

relationship with potential ESD strikes. This solution allows easier routing, space saving and

reduced assembly cost.

STMicroelectronics provides a wide range of products dedicated for all high-speed links

including all these features.

As an example, Figure 11 shows the common mode rejection offered by an

ECMF02-3HSM6. This device is dedicated to the MHL link and offers an integrated

protection for the ID pin. The better rejection area is centered on 900 MHz

Figure 11. ECMF02-3HSM6 common mode rejection versus frequency

Other devices offer a higher common mode bandwidth and a wider rejection band

(Figure 12). This one is suitable for example if there is some antenna desense issues at the

GPS frequency (1.575 GHz).

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