# Protecting Data at Risk of Unintentional Electromagnetic Emanation: TEMPEST Profiling
## 3 Sources of Electromagnetic Leakage
### Piezoelectric Sensors  
The ability of a material to generate an electrical charge in response to an applied mechanical stress is called the piezoelectric effect. Piezoelectric sensors use piezoelectricity to measure physical stimuli such as pressure, force, temperature, and strain [75]. An electric charge arises at the crystal surface when a force is applied to the piezoelectric diaphragm. Since the output signal is generated by the piezoelectric element itself, it is inherently a low-power device. Piezoelectric sensors are sensitive to dynamic pressure changes over a wide frequency and pressure range. These sensors generate output signals directly from the applied strain instead of using an external voltage source. They are robust, shock-resistant, small, and compact. Robustness, high frequency, and fast response make them useful for security alarms, smartphones, smart watches, accelerometers, aviation, and medical devices. It should be noted that piezoelectricity has also been found in bones, indicating the presence of important signals involved in tissue function [76]. Flexible piezoelectric biosensors applicable to the skin have been developed in recent years, making them promising candidates for use in Internet-of-Things (IoT) technologies [77]. In [76], the authors presented a self-monitoring analysis and reporting technology (SMART) implant containing PES. Monitoring the mechanical response of the external fixation device reveals the relative translation and rotation of the external fixation pins and determines the state of long bone healing [78]. SMART implants not only provide but also analyze the data, independently recognize normal or abnormal patterns, and proactively alert the physician [79]. A PESfrequency response decreases at low frequencies because the generated charge cannot be held. At high frequencies, there is a peak at the piezoelectric element resonant frequency. Sensors are usually used within the flat region of the response curve between two extreme values (see Figure 2) [80].  
Piezoelectric sensors generally have a wide frequency range and are not affected by EM radiation. Storage, processing, and wireless data transmission in SMART systems containing these sensors, however, are susceptible to EM interference and inductive coupling [9]. In [39], the authors describe a simple wireless piezoelectric strain measurement using the FM technique. The sensors use a piezoelectric transducer that generates an electrical voltage under mechanical stress. After connecting the PES to a single-transistor FM circuit, the output voltage of the sensor induces a shift in the frequency of the modulation circuit. Subsequently, the modulated signal is transmitted wirelessly to the remote location, where the demodulation circuit acquires the data in real time. This type of sensor finds application in various fields, especially in stress measurement in moving or rotating structures. It is advisable to increase the protective distance and apply EM shielding to reduce the risk of EM leakage. Wireless connectivity within a shielded space can be influenced by absorbing and reflecting RF waves, resulting in a reduction in signal strength; they become attenuated, while the extent of attenuation is determined by the thickness and composition of the shielding material. Shielding can reduce signal diversity, i.e., the number of available paths for the wireless signal to reach its intended receiver. Shielding is more effective in blocking lower frequencies than higher frequencies because of longer wavelengths, which are more easily absorbed and reflected. Additionally, the type of shielding material also affects the attenuation rate. Moreover, the thickness of the shielding material influences blocking RF signals. However, achieving effective protection measures can be a challenge due to the compact structure and dimensions of these systems [81].