VSWR Protection in Nautel NX-100 MW Transmitter:

PAKISTAN BROADCASTING CORPORATION Forums High Power Transmitters Multan HPT VSWR Protection in Nautel NX-100 MW Transmitter:

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    Ch. Zeshan Yousaf
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    VSWR Protection:

    The transmitter uses an advanced DSP based VSWR protection system. Circuitry in the RF output network 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 100 kW plus 100% peak modulation using the reflected power sample from the directional coupler 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.

    Shutback:

    During a single shutback event, the transmitter reduces the output power to 0 W, and remains in that state for 300 ms. This allows time for an arc or transient 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.

    Average reflected power foldback:

    If the average reflected power, calculated over a nominal 5 s period, exceeds a limit of 4% of rated power, the output power is gradually reduced in order to maintain a reflected power that is less than this limit. This gradual reduction in power into high VSWR allows the transmitter to continue to operate into VSWR levels as high as 1.5:1 at full carrier power and even higher VSWR levels when the carrier power is reduced. This VSWR protection is intended to protect against short or open circuits when the transmitter is turned on, as well as non-transient faults such as a poorly tuned antenna system.

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