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Model 200T8G18A/250T8G18

Rev D

In the pulse mode it is expected that the user

will

connect a pulse generator to the pulse input. If a generator

is connected and the PULSE mode is selected, the unit will operate in the pulse mode; it will amplify the rf
only when the pulse input is set to high level, into the pulse input’s approximately 50 ohms input impedance.
This impedance to signal ground has been selected to reduce reflections. For best response, the user should
make the connection to the generator using a 50-ohm coaxial cable.

NOTE: If the user does not have a pulse generator connected to the pulse input, there will be no RF output

(when the pulse mode is selected).

3.10 TWTA GENERAL CONSIDERATIONS

This section is intended to offer some guidelines regarding operation, storage and use of Amplifier Research
TWTAs.

Storage:

TWTAs, as with other electronic equipment, are best stored in a benign environment at reasonably

constant temperature. Service life is not improved by periodic operation.

Availability:

For critical missions, and after long periods of storage, it is recommended that TWTA

operation be checked sufficiently in advance of the mission to permit repair if required. Though service life is
not improved by periodic operation, users experiencing amplifier trip due to body over current may benefit by
periodically operating a unit with high voltage and grid on, but no rf drive. Such operation for about one hour
on a weekly basis should effectively reduce nuisance tripping. Since the cathode structure has finite life,
extended periods of non-functional operation of TWTAs is not recommended. An alternate approach, if
periodic trip off has been observed, is to operate the unit without rf input for 1-2 hours before planned
functional operation, resetting the unit after occasional trip off.

Cooling during Operate Mode:

AR TWTAs have their air outlets and inlets on the rear panels. It is

important to prevent the heated air, which is expelled from the TWTA’s air outlets, from being recycled into
the air inlets. Applications should have a clearance behind the TWTA of at least two feet for single bench top
units and at least three feet for the higher power units, or the heated air should be ducted away.

Operation in Standby Mode:

Standby mode for TWTAs readies the unit for operation. In this mode the

filaments are on but the high voltage is off. TWTAs should not be left in this Standby mode for extended
periods. Where practical, operational procedures should limit the time on Standby mode to less than
approximately one hour. (See

Explanation of….,

below)

Operate Turn on:

When selecting the Operate mode, when high voltage is first turned on, there may be

some internal TWT arcing which can cause protective circuits to deselect the Operate mode, thereby returning
the unit to the Standby mode. There may be a report of body over-current fault. In either case, 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 Limiting the Time in Standby mode

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

) BW factor = -76 + 96=] +20 dBm,