The measurement results of noise received by antennas indicated coupling of broadband noise.
Then we evaluated the adjacent electromagnetic field distribution to identify noise source locations and examined them. As a result, we consider that noise is coupled with antennas due to the noise on the data line and the power line near the flexible cable of the LCD data line.
We selected an L-type filter suited for high-impedance circuits. Since multiple signals are arranged in parallel in the signal line, we selected an array type of 4-circuit configuration with high mounting effect.
After anti-noise measures have been taken, we measured and evaluated the noise level received by antennas with a spectrum analyzer.
As shown in the green graph of the measurement result, the broadband noise value is close to the floor level, indicating that noise suppression exerts an effect.
We found that the noise source is the switching noise of the power supply. Therefore, we selected a ferrite bead to be inserted into the power line of the flat cable that propagates the switching noise.
As a result of measurement and evaluation of the noise level received by antennas after measures were taken, 1.5MHz-cycle noise is eliminated as shown by the yellow data and a value close to the floor level is obtained, indicating that noise was reduced by the effect of the ferrite bead.
After we measured the antenna's receiving sensitivity before and after measures and implemented noise suppression, we evaluated whether the improvement effect was produced.
The measurement result shows that the improvement effect of 3.3 dB (main antenna) and 6.6 dB (sub antenna) was produced.
The adjacent electromagnetic field distribution evaluation result also shows that the electromagnetic field distribution near the flexible cable of the LCD data line is reduced.