Instruction manual
82Understanding your X-TERRA - Technology
Q
A
In sweeping the 3 kHz low frequency (LF) and 7.5 kHz medium
frequency (MF) 9-inch Concentric coils over an old U.S. silver
dollar (90% silver), the TID displays a value of 48 at distances
less than 2-inches from the coil. At distances greater than
2-inches, the TID is 46. When using 18.75 kHz high frequcey
(HF) Double-D coils, (6-inch, elliptical and 10.5-inch round) the
same U.S. silver dollar registers a 48 regardless of the distance
between the target and the coil. What causes this TID variation
between coils?
This is a somewhat surprising eect, because one would expect
the TID from MF and certainly LF to be more stable for a high
conductivity target than that from the HF coil. Therefore, the
question requires discussion on two fronts before I can oer an
answer:
How TID’s are aected by coil frequency
As mentioned in the previous answer, the TID is calculated
based on the phase angle of the received signal with respect
to the transmitted signal. Each TID corresponds to a range of
phase angles (a “bin”). For example, the X-TERRA 705 has 24
non-ferrous TID’s in 90 degrees, so about 3.75 degrees for each
“bin”. The accuracy of TID for a particular target is dominated
by whether the measured angle is close to the centre of a TID
“bin” or if it is close to the boundary between two “bins”. If the
target phase angle is close to the boundary, its TID is more
likely to icker between two values, depending on orientation,
proximity to the coil etc.
Q & A with Laurence
3 kHz - modied TID
“bin” separation
18.75 kHz - modied TID
“bin” separation
7.5 kHz- standard TID
“bin” separation
48
2 2 2
4
4
4
48
46
48
46
46
48