Amplifier Research Manual page 28

Model 500T1G2
if there is no other contraindication, the Operate mode may be selected again. This procedure may be
repeated, if needed up to 25 times, until the Operate mode is actually set. If this condition persists, contact
Amplifier Research 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
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.
Manual Text
9
) BW factor = -76 + 96=] +20
3-20
REV A