AR 10012514, Руководство по эксплуатации и обслуживанию

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The error produced by this indication is not significant when operating near rated TWTA power levels, but may
cause difficulty when trying to operate high power TWTAs at low output power levels.
For example, consider a hypothetical typical NPD of –76 dBm/Hz, from a 4 GHz bandwidth amplifier. A
broadband detector might see the NPD as [-76 dBm/Hz + 10 (log 4 x 10
9
) BW factor = -76 + 96=] +20 dBm, or
0.1 watts. This power is insignificant for a user operating at 200 watts (+53 dBm), but may be very noticeable to
a user trying to operate below 1 watt (+30 dBm). [One watt is 0.5% of (23dB below) rated power for a 200 watt
amplifier.] A field probe user who obtains a 200 V/M field with 200 watts, may see a field as high as [53dBm –
20dBm = 33dB below 200 V/M=] 4.5 V/M due to this hypothetical NPD.
For these applications the use of a lower power amplifier is highly recommended, especially when considering
safety issues. Alternatively, additional power loss in the form of an added high power microwave attenuator, or
preferably an increased space loss for radiated fields, may be used to lower the noise received by the broadband
measurement device.
Explanation of Limiting the Time in Standby mode and of Repeated Operate Selection.
Traveling wave tubes tend to get “gassy” if they are left in a Standby mode for extended periods of time. In this
Standby mode, the heater (filament) is on but there is no high voltage applied to the collector (or high voltage is
applied to the collector but the grid is off). This is the normal state after a tube’s warm up time, just prior to
entering the Operate mode.
In this state the cathode end of the TWT is heating up but the electron “Beam” is off. In other words, there is no
cathode current. As the cathode heats up, gas trapped in the structure of the tube can be released, thus corrupting
the vacuum of the tube. If the tube become too “gassy”, arcing may occur when the high voltage is fully applied
in the Operate mode. Another possible failure mode is a body over-current fault when the beam is turned on and
the tube is “gassy”.
Occasional arcing is normal for a TWT. The support components are designed to handle this, protecting both the
TWT and its support circuitry. However, if the tube arcs two or three times in rapid succession, or worse yet
repeatedly, a fault will be sensed that will shut the high voltage off, thus removing the unit from Operate status.
The remedy usually recommended is to repeat the selection of the Operate mode until the unit remains in Operate.
It as been found that most of the faults that can be cleared by this method will be cleared within 25 attempts to
enter the Operate mode.
Once the tube is operating normally, gas will continue to evolve at a slow enough rate that the TWTA will not
fault. This happens because the gas in the tube will interact with the beam and become ionized. As the electrons
in the beam hit the gas molecules they ionize the gas, at which point it is accelerated into the collector structure
and “buried” deep enough so that it ceases to be a problem.
To preclude this gassing problem, and thus reduce the need for repeating the Operate selection, it is recommended
that the time in “Standby” be limited – to about one hour. Extended periods in Standby may result in an inability
to clear the fault by this method. In this case, service measures may be needed to correct the unit. Thus, users
should reduce the likelihood of occurrence of this problem by limiting the amount of time in the Standby mode.
The service measures involve pulsing of the tube beam current and gradually increasing the duty of the pulsing
until the unit will operate continuously. Note that a similar condition can exist for tubes with grids when the
TWTA is in the Operate mode (high voltage is on) but gating (control) input is set so that the grid turns off the
TWT beam current. Operational procedures should also limit the time in this mode.
Rev A 27