Nautel NX15, Troubleshooting Manual

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NX25/NX15 Troubleshooting Manual Detailed Circuit Descriptions
Issue 5.0 2016-05-06 Page 2-17
Fan Tach A and B:
Each power module monitors the tachometer signals from both fans that are
operating within its respective column. The fan tachometer output is an open collector output that
will switch on and off in direct relation to the fan speed. Pullup resistors R88 and R73 provide
switching signals from 0 to 5 V. The tachometer signals are then applied to the
T1 and T0 timer
inputs (pins 9 and 2) of U22, allowing for fan speed monitoring.
Thermistor:
A thermistor is mounted directly on the RF power module heatsink. A voltage sample
is generated with resistor R87 and the parallel combination of R101 and the thermistor. Resistors R87
and R101 have been selected to provide the most linear voltage response for the operating
temperature range of the RF power module. The voltage sample is filtered through R79 and C72 and
buffered through U21:C. The temperature sample is monitored by the
ADC7 input (pin 22) of U22.
U22 disables the module if the heatsink temperature exceeds 90°C.
Analog Reference:
An analog reference of 4.46 V dc is generated by a voltage divider formed by
resistors R77 and R82. The dc value is buffered through U21:D and applied to the AREF input (pin
20) of U22. U22 uses this reference for all of its internal analog to digital converters.