SAM-III Construction Manual
See last page for document and copyright information, File: SAM-III Magnetometer Construction.doc, Page 60
Crosstalk problems in an existing installation with a
single cable, where it may be impractical to replace the
cables or install a differential interface, may be reduced
by using a low-pass filter in each sensor signal lead. The
filters should be installed close to the SAM-III processor
module. The filter characteristics are best determined experimentally, but a simple LC filter (above, by
John DuBois) has worked well in some installations.
Radio Frequency Interference (RFI)
The SAM-III is susceptible to RFI from nearby amateur radio transmitters. The interference generally is
picked up by the sensor cable. The sensor output is a PWM waveform with a maximum frequency of
about 130 kHz. The SAM-III processor operates at 16 MHz and the sensor multiplexer (SN7400 quad
NAND gate) operates at several megahertz. The interference usually manifests itself in the SAM-III as a
geomagnetic storm because the SAM-III detects and counts the transmitter frequency or
intermodulation products and not the sensor frequency. Here are some mitigation techniques that may
help reduce faulty operation due to interference:
1.
Different configurations of sensor cable shield bonding and grounding (bond to Earth at one or
both ends, mid-point bond to Earth)
2.
Burial of sensor cable
3.
Low pass filter on the signal lead. (the low pass filter should have a cutoff frequency of 1 ~ 2
MHz)
4.
Single-ended to differential transceiver on sensor signal lead (information available at:
http://www.marsport.org.uk/observatory/magmod.htm)
5.
Use the radio transmitter PTT to activate the input voltage detector. When +12 V dc is applied to
pin 1 with respect to pin 2 of terminal block X1 the SAM stops measuring the sensor output. In
this way, the SAM-III will ignore the interference during transmission. When the PTT is released,
and +12 V dc removed from pin 1, the SAM-III resumes sensor measurements
Sensor testing
If power is applied to the SAM-III processor module but no sensors are connected, the display will
appear similar to below. Other characters may be displayed on the right side of the display and
additional characters or numbers may be shown on the bottom line.
X:____________
Y:____________
Z:____________
With no sensors connected, placing the processor module in Calibration Mode by pressing F2 will result
in a display similar to below. Press F2 again to exit Calibration mode.
