Nautel XR6, Руководство по устранению неисправностей, Страница: 96 / 291

XR6/XR3 Troubleshooting Manual Detailed Circuit Descriptions
Page 2-28 Issue 3.0 2009-07-15
Normally, the RF drive currents will be balanced, there will be minimal voltage across T1's secondary
and the voltage applied to Q1's gate will not be sufficient to forward bias it. Q1 will be turned off and
the
RF Amp Fail signal will be logic false since 24 V is being applied from a pull-up resistor in the RF
drive control PWB's crowbar switch circuit.
When the RF drive currents are significantly unbalanced, the voltage across T1's secondary will be
sufficient to forward bias Q1 and Q1 will turn on. A current-sink-to-ground applied to its cathode
from the RF drive control PWB will be applied to the RF drive control PWB's crowbar switch circuit
as a logic true
RF Amp Fail signal. This status will be maintained until the RF power module is
removed from the transmitter or the transmitter's ac power source is turned off.
RF amplifiers
See electrical schematics Figures SD-19 and SD-25.
There are eight identical RF amplifiers in an RF power module. Refer to Figure SD-19 to determine
which are associated with a specific RF power block and then to figure SD-25 for the electrical
schematic of an RF amplifier.
Each RF amplifier uses four parallel-connected pairs of MOSFETs (Q1 through Q8) to produce
unfiltered, modulated RF output. The power transistors are connected as two parallel, high efficiency,
push-pull, class D RF power amplifiers which switch the
PA Volts at the RF drive frequency
( Figure 2.2 on page 2-29 ). RF drive splitter transformers U1T1 and U1T2 split the RF drive signal
and apply it to transistors Q1 through Q8 with the required phase relationship. Diodes CR1 through
CR4 prevent the output of the RF amplifier from going negative.