Nautel NV5, Troubleshooting Manual

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NV5/NV3.5 Troubleshooting Manual Detailed Circuit Descriptions
Issue 3.2 2014-12-10 Page 2-5
PA fail detection
The voltages across resistors R88, R90, R93, R96, R98, R99, R101 and R103, representative of the
individual PA currents, are applied to differential amplifiers U25, U27 through U29, U31 through
U33 and U35. Their output voltages are applied to input channels of multiplexer U30.
PA 1 CUR
through
PA 8 CUR
test points are provided at the module's front panel for monitoring. Multiplexer
U30's output is fed to microcontroller U12. Microcontroller U12 outputs data to 8-bit shift registers
U5 and U6.
During normal operation, when a PA is drawing current, the logic 'low' output at U5-7 causes FET
Q2 to turn off. If no current is flowing through a PA, the U5-7 output will switch to logic 'high'. This
turns on FET Q2 and causes a ground potential to be applied to the
Module Fault output, which
causes the
FLT
(fault) LED on the module splitter PWB (A11) to turn.
PA and IPA bias voltage
Microcontroller U12's memory contains PA bias voltage settings for analog (FM), hybrid (FM+HD)
and digital (HD) modes of transmitter operation. Communication with the control/monitor stage,
through a serial interface (TX/RX connections at P1-24 and P1-26) determines which bias voltage
information is loaded into 8-bit DACs U8 and U9 (for PAs) and U39 (for IPA). The corresponding
analog outputs of U8, U9 and U39 are applied through operational amplifiers (U19:A through U19:D,
U22:A through U22:D and U20:A) to the
PA BIAS 1
(TB3-1) through
PA BIAS 8
(TB3-8) and
IPA BIAS
(TB3-9) outputs for application to the power amplifier PWBs. If microcontroller U12's PA inhibit
output (pin 10) becomes active (logic low), NAND gate(s) U13:A through U13:D and U14:A through
U14:D will switch to logic high and operational amplifiers U19:A through U19:D and U22:A through
U22:D will apply a negative bias voltage to the PA PWBs. A PA failure condition will also cause a
negative bias voltage to be applied to the associated PA PWB.
RF drive sample
A short section of micro-strip transmission line is in close proximity to the active exciter's RF drive
output. Due to the proximity and orientation to the micro-strip, an RF voltage representative of the
RF drive is induced in a power probe circuit. The RF drive signal is peak detected by CR26, C131,
R112 and associated components and applied to the
RF Drive Sample output (P1-18). The sample is
applied to the control/monitor stage's monitoring and protection circuits.
Temperature sensors
Thermistor RT7 on the module splitter PWB (A11) monitors the temperature of the RF power
module’s intake air. Thermistor RT1 on the combiner PWB (A12) monitors the temperature of the
RF power module’s exhaust air. They are negative temperature coefficient thermistors that have a
nominal resistance of 200 k

at 25°C. When a thermistor temperature reaches 100°C, the resistance
will be 11 k

or less. The voltage applied through 8-channel multiplexer U2 to microcontroller U12
will exceed U12's internal alarm threshold. A logic 'high' Pwr Supply Inhibit A (P1-31) or Pwr Supply
Inhibit B (P1-19) signal will be generated, which is applied to the ac-dc power stage to inhibit the