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Spectra VX
3
User’s Guide
reaction. A so-called “induction-balanced” coil arrangement
prevents the receive coil from being overwhelmed by the trans-
mit signal, allowing it to see very small target signals.
Phase & VDI
Practically all VLF-IB detectors operate as phase discrimi-
nators. The received signal is converted to phase, and the phase
is a strong indication of what the target might be. The particular
phase of a target can vary with the frequency of the transmitted
signal, so different detectors designed to use different frequen-
cies can report completely different phase results.
To keep users from having to learn all these different phase
response scales, White’s has chosen to normalize them all to a
standard “VDI” scale. VDI stands for
Visual Discrimination
Indicator
and is simply a consistent numerical value assigned to
targets regardless of the frequency being used. Therefore, a US
nickel detected with a 6kHz detector will have the same VDI as
with a 15kHz detector. For historical reasons, the standard VDI
scale is based on a 6.592kHz detector and is shown below.
The ability to separate targets by VDI is what allows a
detector to discriminate. Targets with a negative VDI are usu-
ally ferrous (iron), and targets with a positive VDI are usually
non-ferrous. Small gold tends to have low VDI’s while thick sil-
ver coins have high VDI’s. Other targets like cupro-nickel
coins, brass and bronze relics, and aluminum trash can have
wildly varying VDI’s depending on their alloy, size, and thick-
TX
RX
