GDP-32
II
INSTRUCTION MANUAL
October 2002 Section 12, Page 22
12.11 FIELD SETUPS AND MAGNETIC ANTENNAS
The figures at the end of this TEM manual provide examples of receiver connections using the
optional REFERENCE ELECTRODE or REFERENCE POT connected to both analog ground
(COM) and case ground (CASE GND).
The CASE GND to COM connection is recommended for receivers making only magnetic
measurements, since this tends to reduce noise in most environments. The REFERENCE
ELECTRODE is optional for magnetic measurements.
Another consideration is protection from static discharge and nearby lightning strokes. This
protection is maximized by connecting the case ground to the REFERENCE ELECTRODE.
TEM/3 magnetic coils with an active, differential (3-wire) output, should have the two signal
wires connected to the red and black input terminals and the ground wire connected to COM on
the analog side-panel.
Magnetic coils with a single-ended (2 wire) output (older TEM/3), should have the signal wire
connected to the red input and the ground wire (or shield, if it is a coaxial cable) connected to the
black input. If the coils have a separate power supply, you may want to consider connecting a
common line from the power supply to the GDP COM connector, to avoid common mode
problems.
Passive magnetic coils such as the TEM/1 cable-coil should be loaded with a 1000 ohm resistor
across the two output signal wires, and these two wires connected to the red and black signal
inputs on the GDP. The shield wire should then be connected to the analog ground (COM) on the
analog side panel.
12.12 ESTIMATING RAMP-OFF TIME
For those clients that do not have the ramp turnoff meter for the GGT-Series transmitters, the
following equation will provide a good estimate of the ramp turnoff time:
L = length of loop side (meters)
R = loop resistance (ohms)
T = ramp turnoff time (microseconds)
2 * L
1.25
T = ————
5 + R
For example, the turnoff time for a 100 x 100 meter loop with 4 ohms total resistance would be:
T = 2 * 100
1.25
/ (5 + 4) = 70 µseconds
If you are using a rectangular loop, use the average length of a side for L. Example: if you are
running with a 300 x 600 meter loop, use (300 + 600)/2 = 450 meters for L.
To Next Page
