IEC 60444-6:2013 applies to the measurements of drive level dependence (DLD) of quartz crystal units. Two test methods and one referential method are described. This edition includes the following significant technical changes with respect to the previous edition: a) DLD measurement with oscillation circuit had the traditional method to detect the DLD abnormal modes at present time. Therefore, this method made the transition to the Annex B. b) High reliability crystal unit is needed to use for various applications at the present day, in order to upgrade the inspection capabilities for DLD abnormal modes, the multi-level reference measurement method was introduced into this specification.