Abstract:
Preferred Practice for Design & Test. Out-of-specification performance may occur during system EMC tests. The risk of higher cost and the impact on the schedule, late in the program, must be weighed against the cost of providing a reasonable design margin during the design phase of a program. Initially, the design requirements for each subsystem are established so that all non-functional emissions will be at least 9 Db below the emission specification limit.
Description of Driving Event:
This Lesson Learned is based on Reliability Practice No. PD-ED-1225; from NASA Technical Memorandum 4322A, NASA Reliability Preferred Practices for Design and Test. By initially selecting a 9 Db margin, the probability of complying with the electromagnetic compatibility (EMC) specification during system test is high. Implementation Method: Radiated and conducted emission specifications established for the customer are based on overall theoretical requirements which will meet the system specifications. The following design practices for subsystems allow for measurement error, combined effects, and manufacturing tolerance. This assures that an adequate margin will be realized when the final system performance is evaluated. In those cases where an outside agency imposes the allowable emissions levels, the included margins must be taken into consideration so that excessive constraints are not applied. When a specific requirement is identified as a major cost driver for design, margin allocations should be reviewed for possible relief. Conducted Emissions: Out-of-specification conducted emission on power, control, and signal lines can usually be controlled by a low-pass filter to reduce or eliminate unnecessary high-order harmonics. On digital circuits, a compromise must be made between the need for square pulses and the suppression of high-order harmonics. Radiated Emissions: Radiated emission can be reduced by cancellation, cross polarization, or by either magnetic or electric field shielding. In some cases, radiated emission can be reduced by using a balanced design so that the external magnetic or electric fields are cancelled or cross-polarized at a particular sensor. Shielding can be added if the additional weight can be tolerated. Otherwise, increasing the separation distance between the emitter and a sensor may be adequate. Low frequency magnetic and electric field emissions must consider the inverse cube relation between amplitude and the separation between source and sensor in order to determine an adequate separation. Technical Rationale: By allowing a 9 Db margin at the design phase, there is sufficient tolerance to provide a high degree of confidence that the equipment will pass the system EMC tests. 9 Db is needed to account for the differences between idealized theory and practice plus the test margin. The test margin covers both manufacturing tolerance and measurement tolerance.
Lesson(s) Learned:
Out-of-specification performance may occur during system EMC tests. The risk of higher cost and the impact on the schedule, late in the program, must be weighed against the cost of providing a reasonable design margin during the design phase of a program.
Recommendation(s):
Initially, the design requirements for each subsystem are established so that all non-functional emissions will be at least 9 Db below the emission specification limit.
Evidence of Recurrence Control Effectiveness:
This practice has been used on Voyager, Magellan, Galileo, and Ocean Topographic Experiment (TOPEX).
Documents Related to Lesson:
N/A
Mission Directorate(s):
- Exploration Systems
- Science
- Space Operations
- Aeronautics Research
Additional Key Phrase(s):
Additional Info:
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