One cannot usually control the power level (~600 mW maximum) or frequency (~900 MHz or ~1800 MHz) of a mobile phone without special equipment as those parameters are set by the mobile network base station the phone is connected to. In this case, the phone was operating in the US ~900 MHz band. The indicated signal strength was low (two of five bars) so the phone should have been near, but not necessarily at, its maximum power.

Figure 3 shows a side view of the test setup. The phone was placed over the path crossing the ground break so as to maximize the EMI generated in the path. Remember Murphy's Law! If the phone protective cover had been removed to allow closer spacing to the board, the induced EMI would have been larger than that observed.


Figure 4 shows the mobile phone next to the board to get a clear view of both the phone and board.


Figure 5 shows the resulting scope plot. The period of the sine wave was a little over one nanosecond with a peak amplitude of over two Volts!  An amplitude of two Volts peak into the 50 Ohm termination in the scope is certainly enough to cause effects on real circuits. There is about 40 mW of power delivered to the scope and also to the 47 Ohm termination, a total of about 80 mW. This is a significant fraction of the phone's transmitting power.


Mobile phones are not high powered devices, but they can be very close to some electronic products. When they are very close, as in this example, significant EMI can be generated in the nearby electronic device. There are many cases where mobile phones can be in physical contact with other small electronic devices, such as in a pocket or purse.