EMI and noise issues

AN4356

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DocID025260 Rev 1

To evaluate the sensitivity losses due to this kind of noise, a TIS (total isotropic sensitivity)

measurement on a standard cell phone has been done. Figure 7 shows the sensitivity of the

phone in the P-GSM 900 band and Figure 8 in the WCDMA band (band VIII). Two measures

have been done, one without USB traffic on the link and one with USB activity.

Figure 7. TIS measurement on P-GSM 900 band with and without USB activity on cell

phone

Figure 8. TIS measurement on WCDMA band VIII with and without USB activity on cell

phone

We can observe an important loss of sensitivity from 3 dB and up to 15 dB in the worst case.

Sensitivities better than -102 dBm to -105 dBm decrease to -90 dBm. These values are

enough to disqualify the phone by an operator.

This phenomenon is called antenna desense.

The first way to prevent it is to shield all the subsets able to generate EMI. Most of the time

it is not sufficient because unwanted frequencies are not precisely targeted.

-80

-90

-100

-110

-120

No USB traffic

USB traffic

P-GSM 900

935.2 MHz

947.4 MHz

959.8 MHz

-80

-90

-100

-110

-120

927.4 MHz

942.4 MHz

957.6 MHz

No USB traffic

USB traffic

-90 dB

Induce disqualification of the handset

WCDMA Band VIII