A hard disk drive with a voice coil motor coupled to a head. The head
moves relative to a disk. The voice coil motor receives a biasing voltage
and has a back emf voltage. A test circuit is coupled to the voice coil
motor to determine the back emf voltage. The back emf voltage can be used
to analyze low frequency vibration in the disk drive. The test circuit
can be constructed so that the back emf voltage is proportional to a pair
of test resistors and/or a pair of test inductors. The biasing voltage
allows the back emf voltage to be measured and analyzed while the head is
at any location from the ID to the OD of the disk.