Nautel NX15, Troubleshooting Manual

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NX25/NX15 Troubleshooting Manual Detailed Circuit Descriptions
Issue 5.0 2016-05-06 Page 2-13
Fast RF drive loss detection is accomplished using XOR gate U25:A and comparator U18:A. The
XOR gate monitors both RF drive signals and, during normal operation, delivers a short low pulse
during the dead time between the two drive signals. If one of the drive signals is lost the XOR gate
produces a longer pulse equal to half of the carrier period plus two dead times. This will cause the
sample voltage applied to U18:A’s non-inverting input below the threshold on the inverting input
(2.5 V). In this case, U18:A’s inverting input will switch high and latch itself on for a minimum of
332 us.
The inverting output of U18:A ( #RF_Dri ) is applied to the INT0 input of microcontroller
U22. When this signal switches high, U22 will immediately (within 5 - 10 us) produce an interrupt to
disable the module.
Power Amplifier
The class ‘D’ power amplifier consists of four switching devices (Q7 through Q10) arranged in an H-
bridge configuration (see
Figure 2.5 ) to produce unfiltered, modulated RF output. The switching
devices are controlled such that each diagonal pair turns on simultaneously, while the second diagonal
pair remains off. This creates two opposite power strokes through the load which is connected in the
center of the H-bridge.
The amplifier will switch at the desired carrier frequency and generate an RF signal that varies
between ± PA volts. Dc blocking capacitors are connected to one side of the bridge to ensure that dc
current does not flow through the RF transformer connected to the module.
Figure 2.5: Power amplifier H-bridge
Q2
Q1
Q3
Q4
Q2
Q1
Q3
Q4