Minelab X-Terra 50 Manual page 5

Transmit
SECONDARY MICRO
(IN COIL)
SENSING + TRANSMITTING
COIL TYPE, SIZE, FREQUENCY
Analog Signal
Coil
Receive
Standard VLF Signal Precision VFLEX Signal
(Without VFLEX)
For the X-Terra user, this precision means
dependable performance and improved immunity
to environmental conditions such as ground
mineralization, electromagnetic interference
and temperature variations.
VFLEX requires coils that are accurately constructed
and calibrated. Every time the detector starts up,
the micro-controllers in the control box and the
coil establish communication through a digital
data link.
Information about the coil is sent to the control box,
so the detector 'knows' what type of coil is attached
and can set the correct operating parameters.
This optimizes the performance, and also makes
the detector 'future proof', in that it will be capable
of operating at different frequencies* depending
on the electronic properties of the coil.