Countermeasures other than using noise filters are also taken to further enhance noise levels.
USB cables perform differential transmission of signals using a twisted pair structure that does not easily radiate noise.
However, noise entering the cable shielding may be radiated from the shielding, so the GND was improved as a countermeasure.
In the measured device, the USB connector and the board were originally connected using a single screw and multiple finger gaskets.
The USB connector frame and the smartphone board GND were connected using copper foil tape to create a strong connection and reduce the impedance. Fig. 6 shows the results.
Fig. 6 Effect of GND improvement
Improvement of the GND reduced the noise level by approximately 6 to 8 dB.