AR 10009284, Operating And Service Manual

Page: 37 / 62

Model 40T26G40A
Rev A 23
A small sample program that can send commands and receive the strings returned by the TWTA is included
in section 5.5. It is written in Hewlett-Packard’s “Rocky Mountain” BASIC. The program assumes that the
IEEE-488 bus is at address 7 and that the address of the TWTA is 01.
Remote operation is determined by the application (software) program in the system controller. This
application program will aid the user in generating the Command Codes and displaying/monitoring the Status
Codes. Consult the application program users instructions for Remote operation procedure.
The application program should issue only one string at a time. After each functional command is issued the
status should be checked to ensure that the command has been properly executed. The application program
should allow sufficient time for the function to be completed before checking the status.
The application program should facilitate checking the status just prior to issuing a command, since the status
could have been changed by a fault condition of the amplifier or by operator activation of the amplifier.
Periodic checking of the status is also recommended.
3.9 TWTA GENERAL CONSIDERATIONS
This section is intended to offer some guidelines regarding operation, storage and use of AR Worldwide
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: 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 AR Worldwide
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