AN3141

Link between baseband processor and LCD or camera

Doc ID 17004 Rev 1

5/14

We choose to perform a measurement at 940 MHz, which is near the middle of the GSM RX

band.

The measurement setup is presented in Figure 4. Due to the spectrum content of a digital

signal, band-pass filter is needed to avoid saturation of the spectrum analyzer input stage.

Figure 4.

Harmonic measurement test setup

Measurement results is shown in Figure 5 (DUT short-circuited).

Figure 5.

Spectrum at 940 MHz of a 1.8 ns rise/fall time, 20 MHz signal

The result shows that the emission of this signal is significant regarding the required

sensitivity of a mobile phone.

This measurement was performed on one line, while digital links integrate several lines in

parallel. In addition, measurements have been performed on 50 environment, which is not

the case in the application (link is not impedance matched, drivers are low impedance and

receiver high impedance). Consequently, radiated field can be very high.

Without any filtering, this signal can drastically decrease the sensitivity of a mobile phone.

Signal generator

50

Ω output

Cavity band-

pass filter

Isolator

Spectrum analyzer

DUT