This paper proposes a method for improving Electromagnetic (EM) attack sensor precision for countering a wider range of EM attacks on cryptographic modules. During an attack, an EM attack sensor determines the proximity of a probe to an LSI chip by detecting a change in the mutual inductance between the probe and the LSI chip from the shift in the oscillation frequency of the LC oscillator of the sensor. The subsequent would-be attack is thwarted by instantaneous detection of the proximity of the probe. We show that smaller oscillation frequency shifts can be detected by extending the time required for the detection process, and demonstrate that extending the time enables attacks to be detected even when they are coming from the back surface of the LSI chip, which was earlier difficult to achieve. We then examine the possibility of operating the crypto core and sensor simultaneously as a method for reducing the performance overheads of the proposed system. Through evaluation experiments, we show that the simultaneous operation of the crypto core and sensor has no significant effect on the precision of probe detection. Furthermore, we discuss an alternative method for improving the detection sensitivity by a time-to-digital converter without extending the detection process time.