NX25/NX15 Troubleshooting Manual
Detailed Circuit Descriptions
Issue 5.0 2016-05-06
Page 2-11
The modulator filter’s output voltage is across capacitors C116 and C117, but is not ground
referenced. The output (PA voltage) applied to the RF power amplifier will be positive and is the sum
of a dc component (carrier level) and an ac component (modulating source). The output voltage is
related directly to the PDM positive duty cycle. If the duty cycle is constant the modulator filter
output will be a dclevel. If the duty cycle is varying the modulator filter output will be a dc value plus
an ac component.
Diodes CR8 and CR9 are connected in series between the modulator filter output and ground. They
prevent the modulator filter output from going negative, and possible RF MOSFET damage due to
overvoltage.
Modulator Current Sense
The modulator output passes through a hall effect current sensor (U17). The sensor provides a dc
voltage sample that is indicative of the modulator output current. Microprocessor U22 monitors the
current sample and shuts down the power module during an overcurrent event. This provides short
circuit protection for the modulator circuitry.
The output of the current sensor (U17 pin 3) has a zero current offset of 0.6 V dc. Its output
sensitivity is 65 mV/A.
RF power amplifier circuit
The RF power amplifier circuit contains a Class D, resonant switching amplifier. The switching
MOSFET's have very low on resistance, minimizing conduction losses. This allows for very efficient
operation of the amplifier. The amplifier is capable of operating from 530 to 1710 kHz into a tuned
load. Different load impedances and dead times are required for different frequencies to ensure
proper switching of the MOSFET's. Each MOSFET has its own gate drive circuit allowing
asymmetrical drive signals to be used. This allows a dead time to be introduced between each
switching cycle of the Class D amplifier.
RF Drive Inputs
The RF drive signals are applied to the RF power module through the card edge connector (P1).
There are two separate signals at the desired carrier frequency:
RF Drive 1
and
RF Drive 2
. They are
operating 180° out of phase with each other, with a pre-determined dead time inserted. The dead
time is what allows the zero voltage resonant switching to occur within the class D amplifier. The
amount of required dead time depends on many factors such as operating frequency, MOSFET
device, gate drive circuit, and load current.
The balanced RF drive input signals are converted to TTL signals using RS422 receivers U27:A and
U27:B. These receivers are always enabled allowing for a constant RF drive sample to be monitored
by microcontroller U22.
Summary of Contents for NX15
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Page 8: ...NX25 NX15 Troubleshooting Manual Page viii Issue 5 0 2016 05 06...
Page 78: ...NX25 NX15 Troubleshooting Manual Responding to alarms Page 1 70 Issue 5 0 2016 05 06...
Page 96: ...NX25 NX15 Troubleshooting Manual Detailed Circuit Descriptions Page 2 18 Issue 5 0 2016 05 06...
Page 104: ...NX25 NX15 Troubleshooting Manual Parts Lists Page 3 8 Issue 5 0 2016 05 06...
Page 216: ...NX25 NX15 Troubleshooting Manual Wiring connector lists Page 4 12 Issue 5 0 2016 05 06...
Page 222: ...NX25 NX15 Troubleshooting Manual Reading Electrical Schematics Page 5 6 Issue 5 0 2016 05 06...
Page 250: ...Issue 5 0 2016 05 06 MD 1 Figure MD 1 NX15 NX25 Transmitter Front View...
Page 251: ...Issue 5 0 2016 05 06 MD 2 Figure MD 2 NX15 NX25 Transmitter Rear View...
Page 254: ...Issue 5 0 2016 05 06 MD 5 Figure MD 5 NAE96C 01 Control Exciter Panel...
Page 257: ...Issue 5 0 2016 05 06 MD 8 Figure MD 8 NAPI106 Remote Interface PWB...
Page 261: ...Issue 5 0 2016 05 06 MD 12 Figure MD 12 NAPI98 RF Drive Distribution PWB...
Page 263: ...Issue 5 0 2016 05 06 MD 14 Figure MD 14 NAX243A B Distribution Assembly...
Page 265: ...Issue 5 0 2016 05 06 MD 16 Figure MD 16 NAPA34A Modulator Power Amplifier PWB Front View...
Page 267: ...Issue 5 0 2016 05 06 MD 18 Figure MD 18 Fan Tray Assembly 207 8133...
Page 268: ...Issue 5 0 2016 05 06 MD 19 Figure MD 19 Voltage Probe 207 6111...
Page 269: ...Issue 5 0 2016 05 06 MD 20 Figure MD 20 Current Probe 207 6213 04...
Page 270: ...Issue 5 0 2016 05 06 MD 21 Figure MD 21 NAFP106B 02 Directional Coupler A1 DETAIL...
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