Nautel XR12, Troubleshooting Manual, Page: 96 / 283

XR12 Troubleshooting Manual Detailed Circuit Descriptions
Page 2-28 Issue 3.0 2009-07-28
Modulator filter
See electrical schematic Figure SD-20.
An inductor, L/C components on the modulator filter and the input capacitance of two RF
amplifiers form a low pass filter for the modulator's
PDM (B+) output. The filter is connected as a
multi-pole, tuned filter that shunts the PDM frequency component of the PDM (B+) input to
ground. The filter output ( PA Volts ) will be a positive voltage that is the sum of a dc component (RF
carrier level) and an ac component (modulating audio). The ratio of the PA Volts amplitude in relation
to the B+ voltage is the same as the on/off ratio of the PDM (B+) input . When the on/off ratio is
constant (carrier level only present), the PA Volts output will be a constant dc voltage. When the
PDM (B+) on/off ratio is varying (carrier level plus modulation present), the PA Volts output will be
a dc voltage that varies from the carrier level voltage at the modulating audio frequency. The
amplitude of the variations is determined by the amount the PDM (B+) on/off ratio varies.
B+ decoupling assembly
See electrical schematic Figure SD-20.
The B+ decoupling assembly provides decoupling and transient suppression for the B+ inputs on the
modulator input/output PWB.
PA input/output pwb
See electrical schematic Figure SD-19.
There are four identical PA input/output PWBs, one for each of RF power block. They:
• distribute PA volts to both RF amplifiers associated with the power block.
• split and apply the RF drive to both RF amplifiers associated with the power block.
• monitor RF drive current to both RF amplifiers and produce an RF amp fail output if they
are not balanced (indicates a failure in one or both of the RF amplifiers).
• apply the RF output of both RF amplifiers to individual primary windings of an RF combin-
ing transformer.
RF amplifier failure detector
The RF drive is split and applied to the anti-phase wound primary windings of transformer T1. The
voltage transformed to T1's secondary is directly proportional to the difference in the RF drive
current flowing through its primary windings. Any voltage transformed to the secondary winding is
half-wave rectified and applied to the gate of thyristor Q1.