Energy harvesting technology has advantages as the power supplying for devices, compared to the grid and batteries, which decreases the cost for long-term maintenance. As one of the natural renewable energy sources, it is convenience for vibration energy to supply power for devices manufacturing in small size. The present vibration energy harvesting technologies always work in narrow bandwidth around the resonance frequency, which is considered as shortages in application. Research on mechanical and electrical methods extending the frequency response capability has been done in this paper and there is an impedance analysis on the new developed interface circuit for vibration energy harvesting, called phase variable synchronized switch harvesting on inductor (PV-SSHI). PV-SSHI has wider range in frequency response than synchronized switch harvesting on inductor (SSHI), which results in higher energy harvesting efficiency when the natural vibration frequency is away from the resonance frequency.