Fig.7. UUT resonance pickup
ii). Stabilization of Reference source
a). Mechanical Stabilization
Vibration isolators (dampers or special class of rubber) are used to isolate reference source from UUT in which PLL system resides inside. It gives free floating stress less assembly to reference module which makes reference source to experience lesser level of vibration in comparison to vibration level experienced by UUT. Fig. 8 shows an implemented arrangement of reference source assembly. Selection of vibration isolator is very critical as these are vibration profile dependent special class of rubber; also make sure that it qualifies other environmental test profiles requirements i.e. temperature, shock, acceleration, humidity, etc. To check the effective performance improvement with vibration isolators, vibration sensors has been used to access reduction in intensity level (PSD) or damping effect of random vibration on reference source. Fig. 9 shows a very significant improvement in damping effects by mechanically stabilizing it [6]-[8].