Nautel XR12 Troubleshooting Manual

Also see for XR12: Installation manual Operation and maintenance manual Preinstallation manual

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XR12 Troubleshooting Manual Detailed Circuit DescriptionsIssue 3.0 2009-07-28 Page 2-9The carrier ref input at J1-2 is a dc voltage directly proportional to the expected RF carrier levelsquared. When the transmitter is operating at its rated carrier level, the expected carrier ref input at theoutput of buffer U2A (TP5) is a nominal 4.7 V. The resulting current flow through R13 is U6's I2input.The B+ ref (1) input at J1-3 and the voltage at the output of buffer U3D (TP3) is a dc voltage directlyproportional 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 resultingcurrent 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 followingformula:I3 = I1 x I2/I4When 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'sI3 output current is 0.76 times its I1 input current. U3B is connected as a less than unity amplifier andits gain is summed with U6's gain. The end result is the total voltage gain of the circuit, relative to thevoltage at the output of U2D (1.4 V when no audio is present), is a nominal 0.67 when GAIN TRIMpotentiometer 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 refvoltage is set to 0 V, or it is clamped to ground because Q1 is on, U6's gain is zero. In turn, themodulating reference's multiplication factor is minimum (zero). The transmitter's RF output is set to0 W. The gain of U6 will change in inverse proportion to changes in the B+ ref (1) input. This featureeliminates the need for sophisticated filtering of the transmitter's B+ power supply and maintains thetransmitter'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 ofU3B. It is adjusted in dual exciter applications to compensate for tolerance differences in the PDMgenerators of exciter A and exciter B. When it is set properly, the transmitter's RF output level is thesame when either exciter is selected, provided the same desired ouput level is being applied.PDM dividerThe 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.