The resonance frequency shifts with these LC sensors have been previously recorded with a coupled inductor connected to benchtop vector network analyzers (VNA),[5, 31, 34, 35, 38] or impedance analyzers. These readers are not suitable for wearable applications due to their bulky nature (even downsized VNAs) and cannot be integrated into daily living.
low-cost Colpitts oscillator-based frequency reader (fReader) to wirelessly read out the strain-induced capacitance changes in a convenient way by placing it comfortably in a garment's pocket (Figure 1a). Unlike a VNA that relies on a frequency sweep, the circuit strategy employed in fReader is simple with detection based on a shift in the oscillator frequency (Figure 1b).
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