Fluke TRACKERTM 90100 - Manual (Page 31)

Theory of Operation
Overall Functional Description of the Model 35360A Tracker Display
3
3-3
3.2.2 300-VOLT Electronic Bias Supply
The 300 volt ionization potential is generated internally by an electronic bias supply circuit. The supply
eliminates the need for the 35360A TRACKER Display to contain high voltage batteries. As explained
below, the ion chamber collector and guard are maintained at the same potential as the chassis of the
35360A TRACKER Display, and the ion chamber bias plates are negatively biased at 300 VDC. A
readout of the bias potential is facilitated by a divided bias signal generated within the bias supply block
and connected to the A-D converter.
The output of the electronic bias supply is highly regulated and filtered. In addition, the output energy and
current have been limited should the operator make direct contact. Since the bias voltage at the detector
input connector is automatically discharged at power down, the operator should always turn the
instrument off before attempting to make or break a connection involving the detector interface cable.
During operation, the Bias Supply output level is continuously measured and verified. Any abnormal level
will be indicated by an annunciator on the VFD.
3.2.3 Front Panel Switches
The nine front panel switches are of the momentary contact type, and are continuously monitored by the
microcontroller for detection of contact closure. At the detection of any contact closure, the
microcontroller scans the switch set to determine which key has been pressed, and then takes the
appropriate action. The dedicated function of each key is described in Section 2.
3.2.4 Processor
The Processor block is based on the Motorola 68HC11 microcontroller and includes 256 KBytes of factory
programmed memory (EPROM) containing all of the executable code (firmware). The revision date
associated with this code is displayed during the power up sequence on the VFD. Also contained within
the Processor block is 32 KBytes of RAM memory which is used to hold all volatile variables (those
whose values are lost when the instrument is powered down).
A 512 byte block of nonvolatile memory (EEPROM) has also been provided. A portion of the EEPROM is
automatically reprogrammed during each power down sequence to hold all of the user's front panel
selections. Thus the units, front panel gain, temperature, and pressure settings in effect at power down
can be automatically restored at the next power up. The EEPROM is also used to hold all of the internal
electrical calibration factors for the unit, as well as all of the customization selections installed by the user
over the RS-232 via the Customization program.
Besides its role as a controller of the other hardware blocks, the Processor Block performs several
computational functions. It applies a number of correction factors to the displayed dose and dose rate
values including the air density correction factor, the ion chamber calibration factor, the display units
conversion factor, and the front panel gain settings. It also performs the ratio calculations necessary to
display the percentage-of-center measurement values for the peripheral ion chambers.
3.2.5 VFD Display
All display functions are handled by a 4 line by 20 character alpha-numeric VFD module. The module is
under the direct control of the microcontroller and is capable of displaying all of the user's front panel
selections as well as the numerical measurement results along with their appropriate units. Other
informational items including the battery and bias voltage, calibration date, and ion chamber calibration
factors may be displayed. The VFD has a distinct advantage over other display technologies, such as
LCD, in that it provides good visibility, and a wide viewing angle in all lighting conditions.