AR 10027190, Operating And Service Manual

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Models 200T18G26z5A/200T26z5G40A
Rev – 24
if needed up to 25 times, until the Operate mode is actually set. If this condition persists, contact AR Service
for additional assistance. (See
Explanation of…, below)
Noise Power Density (NPD): TWTAs produce RF noise over their operating frequency range, as specified
by the Noise Power Density (NPD). This noise is significantly higher than the noise produced by typical
solid state amplifiers, and is inherent in present TWTAs. The noise may surprise users new to TWTAs when
it accumulates and results in a significant indication in a broadband measurement device – such as a power
meter or field probe. 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