DSCS II LLTWTA Helix Current Anomaly

The DSCS II satellite uses a low-power TWTA as a driver stage for the power output tube. The tube is operated in a highly backed-off mode for linearity, giving an output of the order 23 dbm (maximum output power is 28 dbm). The preceding stage is a limiter, and the system is set up with attenuator p...

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Bibliographic Details
Main Author: Fowler,P H
Format: Report
Language:English
Subjects:
CATHODES(ELECTRON TUBES)
COMMUNICATION SATELLITES
Electrical and Electronic Equipment
GAIN
Helix current
HELIXES
LIMITERS
LOSSES
OUTPUT
POWER
POWER AMPLIFIERS
POWER LEVELS
RADIOFREQUENCY AMPLIFIERS
SATURATION
TELEMETER SYSTEMS
TRAVELING WAVE TUBES
Unmanned Spacecraft
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Summary:The DSCS II satellite uses a low-power TWTA as a driver stage for the power output tube. The tube is operated in a highly backed-off mode for linearity, giving an output of the order 23 dbm (maximum output power is 28 dbm). The preceding stage is a limiter, and the system is set up with attenuator pads so that the maximum limiter output just saturates the output TWTA. Forty-four of the driver amplifiers have been operated in orbit, for intervals ranging from a few days (orbital testing redundant chain) to over nine years. Several of the driver tubes have caused problems of varying severity by losing gain, to the point the output tube could no longer be driven to saturation. To date no service has been cut short of its design life by this problem as there has always been a working backup to complete the mission. The immediate cause of the gain loss is a rise in helix current to as much as 20 percent of the beam, accompanied by a slow loss of about 20 percent of the cathode current. Several investigations have been made into the mechanism underlying this effect, leading to some tentative conclusions.