Nautel NX5 AM, Operation & Maintenance Manual

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NX5 OPERATION & MAINTENANCE MANUAL DESCRIPTION
VERSION 0.1 2015-08-01 PAGE 3.1.9
VSWR Protection
The transmitter uses an advanced DSP based VSWR protection system. Circuitry in the RF output
network (see
3.1.2 on page 3.1.8 ) samples the RF voltage and RF current at the input to the harmonic
filter. These current (I) and voltage (V) samples are applied to ADCs on the digital AM exciter PWBs. The
digitized I and V signals are used to calculate the impedance (Z) at the combiner output. An FPGA
performs high-speed calculations, so there is minimal response delay.
There are several types of VSWR protection, which continuously operate:
Peak Reflected Power from Directional Coupler
Fast VSWR protection is used to detect transient faults when the reflected power quickly increases due
to arcing, lightning or short circuits. The protection circuit is set at 1.5:1 at 5 kW plus 100% peak
modulation using the reflected power sample from the directional coupler (see
3.1.2 on page 3.1.8 ) and
triggers the controller’s Fast SWR Shutback alarm, designed to shutback PDM (reduce power to zero)
and disable RF drive in less than 100 ns. This protection ensures that the RF drive to the RF power
modules is disabled in less than one RF carrier period, preventing the amplifier from operating into a
potentially harmful load.
Calculated Reflected Power from I and V Samples
The peak reflected power calculated by the FPGA is supplimentary to the fast SWR protection, allowing
the exciter to detect and control the repsonse to the event. The peak reflected power limit - based on a
VSWR of 1.5:1 at rated power plus 100% peak modulation - is 16 % of the ransmitter’s rated power
(i.e., 800 W). If this limit is exceeded, the transmitter’s output power instantly reduces to 0 W. This
triggers the exciter’s
SWR Shutback
alarm.