Nautel XR6, Troubleshooting Manual, Page: 77 / 291

XR6/XR3 Troubleshooting Manual Detailed Circuit Descriptions
Issue 3.0 2009-07-15 Page 2-9
The carrier ref input at J1-2 is a dc voltage directly proportional to the expected RF carrier level
squared. When the transmitter is operating at its rated carrier level, the expected carrier ref input at the
output of buffer U2A (TP5) is a nominal 4.7 V. The resulting current flow through R13 is U6's I2
input.
The B+ ref (1) input at J1-3 and the voltage at the output of buffer U3D (TP3) is a dc voltage directly
proportional to the B+ voltage being applied to the transmitter's RF power stage. When the B+
voltage is 350 V (for high power operation), the voltage at TP3 is a nominal 6.1 V. The resulting
current flow through R20 is U6's
I4 input.
The gain of U6 and, in turn, the current flowing from its I3 output, is determined by the following
formula:
I3 = I1 x I2 / I4
When the carrier ref input (I2) is 4.7 V (transmitter's rated output power) and the B+ ref (1) input (I4)
is 6.1 V (B+ voltage of 350 V), the gain of U6 is 0.76 for the modulating reference applied to I1. U6's
I3 output current is 0.76 times its I1 input current. U3B is connected as a less than unity amplifier and
its gain is summed with U6's gain. The end result is the total voltage gain of the circuit, relative to the
voltage at the output of U2D (1.4 V when no audio is present), is a nominal 0.67 when
GAIN TRIM
potentiometer R31 is set to the centre of its range. When there is no modulation present, the carrier ref
+ audio output of U3B (TP8) is 0.94 V (1.4 V x 0.67).
The gain of U6 will change in direct proportion to changes in the carrier ref voltage. If the carrier ref
voltage is set to 0 V, or it is clamped to ground because Q1 is on, U6's gain is zero. In turn, the
modulating reference's multiplication factor is minimum (zero). The transmitter's RF output is set to
0 W. The gain of U6 will change in inverse proportion to changes in the
B+ ref (1) input. This feature
eliminates the need for sophisticated filtering of the transmitter's B+ power supply and maintains the
transmitter's RF output at the original level for B+ voltage variations of ±10%.
GAIN TRIM
potentiometer R31 provides a nominal 10% adjustment in the carrier ref + audio output of
U3B. It is adjusted in dual exciter applications to compensate for tolerance differences in the PDM
generators of exciter A and exciter B. When it is set properly, the transmitter's RF output level is the
same when either exciter is selected, provided the same desired ouput level is being applied.
PDM divider
The PDM divider circuit divides the 2f
PDM
input frequency (J1-7) by two. The resulting f
PDM
output
(J1-10) is a 0 V to 15 V square wave at half the 2f
PDM
frequency. See “N divider” on page 2-7 .