1、Lessons Learned Entry: 0414Lesson Info:a71 Lesson Number: 0414a71 Lesson Date: 1996-06-26a71 Submitting Organization: JPLa71 Submitted by: J.A. RobertsSubject: Exciter RF Output Degradation on Mariner Mars 71 Caused by Emitter-to-Base Zenering in RF Transistors (1971) Abstract: Because two RF transi
2、stors were overdriven, Mariner 9 exciter RF output showed slow and continual degradation throughout the mission. Short-term tests of RF circuits may not be sufficient to rule out zenering, and testing may be complicated by “healing” during the off period.Description of Driving Event: (Relevant Histo
3、rical Lesson(s) Learned)Telemetry data of the Radio Frequency Subsystem Exciter RF output on Mariner Mars 71 (Mariner 9) showed slow degradation after launch. The degradation continued at a very slow rate throughout the interplanetary cruise and the primary mission phases of the mission. By one year
4、 after launch, the exciter RF output had degraded sufficiently that the traveling wave tube amplifier power amplifier output had decreased 0.6 dB. The exciter output had decreased 2.6 dB.Investigation, including a review of the circuit design and laboratory testing, determined the exciter output deg
5、radation was caused by two RF transistors (2N3364 and 2N3375) being overdriven. Overdriving results in the permissible emitter-to-base voltage being exceeded, causing the emitter-to-base junction to be forced into a zener condition. The zenering causes the beta of the transistor to degrade, which re
6、sults in a reduction of circuit gain.Reference(s): JPL IOM 3365-72-199, “Investigation of Mariner 9 Exciter Power Degradation,“ R.S. Hughes, 22 May 1972Lesson(s) Learned: Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-The worst-case analysis of all
7、radio frequency circuits must include verification that transistors have sufficient margin to emitter-to-base zenering. Zenering can also occur in circuits, other than RF, if the emitter-to-base breakdown voltage is exceeded.Inflight variables, such as temperature and operating mode, make the assess
8、ment of degradation of this type extremely difficult after launch.Recommendation(s): Short-term tests of the circuits may not be sufficient to rule out zenering. The rate of degradation is dependent upon the magnitude of zenering, and therefore changes in RF output can occur very slowly. Such change
9、s may not be noticeable unless the circuit is operated continuously at a constant temperature for several days. Testing is complicated also by the condition of “healing.“ If the circuit is turned off, some of the performance degradation tends to be recovered when the circuit is turned back on. The e
10、xtent of the recovery is dependent upon the off period. Healing also takes place more rapidly at elevated temperatures.Evidence of Recurrence Control Effectiveness: N/ADocuments Related to Lesson: N/AMission Directorate(s): N/AAdditional Key Phrase(s): a71 Hardwarea71 Parts Materials & Processesa71
11、Test & VerificationAdditional Info: Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Approval Info: a71 Approval Date: 1995-10-16a71 Approval Name: Carol Dumaina71 Approval Organization: JPLa71 Approval Phone Number: 818-354-8242Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
