Nautel NX300, Руководство по эксплуатации и обслуживанию

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NX300 Operations and Maintenance Manual Description
Issue 3.0 2012-11-01 Page 1-7
VSWR protection
The transmitter uses an advanced DSP based VSWR protection system. Circuitry in the RF output
network (see
Figure 1.3 on page 1-6 ) 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 set at 1.5:1 at 300 kW plus 100% peak modulation using the reflected power
sample from the directional coupler (see
Figure 1.3 on page 1-6 ) designed to shutback PDM (reduce
power to zero) and disable RF drive in less than 300 ns.
Calculated reflected power from I and V samples
The peak reflected power calculated by the FPGA is used to detect transient faults when the reflected
power quickly increases due to arcing, lightning or short circuits. The peak reflected power limit -
based on a VSWR of 1.5:1 at rated power plus 100% peak modulation - is 16 %. If this limit is
exceeded, the transmitter’s output power instantly reduces to 0 W. This is called a
Shutback (see
description below).
Shutback:
During a single shutback event (see Figure 1.4 on page 1-8 ), the transmitter reduces the out-
put power to 0 W, and remains in that state for 300 ms. This allows time for an arc or tran-
sient fault to clear. After 300 ms, the output power exponentially ramps up (to the last power
set point or the current ALC value, whichever is lower.
After 30 s, the transmitter returns to full power under control of the normal ALC. When a
shutback occurs, an accumulator value is set to a normalized value of 1. This value decays at
a rate of 1/60 of a second (0.0167 s) so after 60 s, the accumulator value returns to 0 and the
shutback event is no longer in memory.
Cutback:
A cutback occurs after multiple shutback events. When three shutback events occur within a
15 s period, the shutback accumulator exceeds the cutback threshold (2.5). The transmitter
responds by limiting the output power to 85% of the previous power set point or the power
at which the shutback event occurred. When multiple cutback events occur, the output
power is reduced by 85% of the current level after each event. The minimum cutback power
level is 2% of full carrier power. Further cutbacks will not decrease the power beyond this
point. When the transmitter is recovering from a cutback event, the accumulator must first
reach 0, which takes 60 s. After that, transmitter power is gradually restored to full power
according to the curve shown in
Figure 1.5 on page 1-9 .