Geometrics G-882 - Appendix C: G-882 with Larmor Output

Geometrics Inc. G-882 Cesium Marine Magnetometer Page 110
Appendix C: G-882 With Larmor Output
If you did not purchase this special version of the G-882 magnetometer then the following does
not apply.
This version of the G-882 Magnetometer is modified to provide the analog Larmor signal out a
coaxial TNC connector at the white junction box so that it can be counted externally, as well as
being internally counted and output via the serial port.
To get the analog Larmor signal from the magnetometer electronics to the outside world, a
special tow cable is used to bring the Larmor signal to the white junction box through a coax
cable inside the tow cable. In addition there is extra wiring added inside the G-882 fish and in
the white junction box to pass the Larmor signal.
The G-882 is powered normally using a 28-volt supply through the white junction box. The
serial magnetometer data (internally counted) is sent to the 9-pin connector on the white
junction box as with a standard G-882. The only external differences on the system are the
addition of the TNC analog Larmor output connector, a bigger connector (more pins) to
interface the white box to the tow cable, and the special tow cable itself.
The analog Larmor signal is a sine wave whose frequency is proportional to the earth’s
magnetic field, and whose amplitude is related to the angle of the sensor axis to the earth’s
vector. The Larmor frequency is 3.498275 times the total field value. Thus for a 50,000 nT
field the Larmor output frequency would be approximately 175 KHz. The amplitude of the
Larmor signal is approximately 2 volts peak to peak at optimum orientation (sensor axis at 45
degrees to the earth’s vector) and diminishing from there as the sensor is rotated toward either
the polar or equatorial dead zones.
Unlike many magnetometer installations, the output Larmor signal is not superimposed on the
DC power, nor can the magnetometer be powered through the TNC connector. The power to
the magnetometer must be supplied through the power connector on the white junction box.
The Larmor output is AC coupled through a 0.1uF capacitor inside the G-882.
At power up it is possible that a 28-volt positive transition may couple through to the TNC
Larmor output connector. The input to the external counter should be protected against this
transition (see the attached power/Larmor wiring diagram TD99873 for a system level look at
how the Larmor signal is routed).