Nautel NX15, Troubleshooting Manual

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NX25/NX15 Troubleshooting Manual Detailed Circuit Descriptions
Issue 5.0 2016-05-06 Page 2-9
RF power module ( NAP39A )
See electrical schematics Figures SD-24 and SD-25.
The RF power module is effectively four wideband RF amplifiers for the AM broadcast frequency
band (530 - 1710 kHz). It operates from a 400 V dc bus with a carrier output level of 2500 W and can
deliver peak power levels of 13.25 kW. The module contains a three phase modulator section that
feeds directly to a highly efficient class D RF amplifier. All circuitry has been integrated into a single
PWB (NAPA19A Modulator/Power Amplifier PWB) eliminating the requirement for interconnect
wiring between stages. This simplifies troubleshooting, and serviceability of the entire power chain.
The RF power module contains three main operating blocks, which are described in the following
paragraphs:
• Modulator
• RF amplifier
• Protection/monitoring circuitry
Modulator circuit
The modulator circuit is a voltage regulating stage between the B+ voltage and RF amplifier. The
modulator output will vary with any change in PDM duty cycle. The PDM duty cycle is determined
by the carrier level plus the envelope information. For an envelope peak (maximum duty cycle) the
modulator output will be approaching the B+ level; for an envelope trough (minimum duty cycle) the
modulator output will approaching zero.
The modulator stage receives three separate PDM signals that are operating 120° out of phase from
each other but with the same duty cycle. This acts to provide cancellation of the fundamental
switching frequency.
PDM Inputs
The three-phase PDM signals are applied to the RF power module through the RJ45 connector (J1)
at the front of the module. The PDM signals are received as differential signals and converted to TTL
signals through individual RS422 receivers U1:A, U1:C and U1:D. A PDM sample is generated by
summing the TTL outputs using resistors R25, R20, and R21. This sample allows for onboard
monitoring of the PDM duty cycle. Each PDM signal is also fed to a FET driver IC (U12:A, U14:A
and U15:A).