Preamps (PRE) and Sensors

1. Preamps (PRE) and Sensors • Rachel Hochman • John Bonnell • Space Sciences Laboratory • University of California, Berkeley

2. Overview Motivations and Driving Requirements Sensors and Enclosures Resources: Mass, Power Schematics Frequency Response: modeling, bench testing, fully-assembled PWB Layout  and Fabrication Parts Derating and Stress Analysis Radiation Effects Testing: TID and DDD ETU Thermal Qualification Test Plan and Flight Test Procedures Backup Slides

3. Motivation (In Words) The EFW Sensors, Preamp, BEB and EFW-EMFISIS interface represent the primary analog signal path for E-field measurements on RBSP. Measuring 0.1 mV/m DC E-fields required accuracies of 0.1% in the magnetosphere: tens of mV of signal in the presence of tens to hundreds of mV/m of effective common-mode or systematic noise (photocurrents, SC charging), or tens of volts of common mode signal. The non-linear coupling (I-V curve) of EFW sensors to the external E-field can be optimized through current biasing (factor of 100 decrease in susceptibility to systematic error sources and density fluctuations). This current biasing of sensors drives volts to tens of volts floating potential differences between sensors and SC ground. High effective source impedance (plasma sheath, ten of MΩ), and low-noise and low-leakage current requirements (systematic error reduction again) drive use of high-input-impedance, low-voltage preamps in a floating ground configuration. Deflection and collection of stray photoelectron currents prior to impingement upon sensor also reduces DC biases (WHIP/USHER and GUARD surfaces).

4. Board Requirements/Specs Driving Environmental Requirements Radiation (TID and Deep Dielectric Discharge) Temperature (sunlit to eclipse swings) Driving Measurement Requirements 0.3-mV/m accuracy at DC in the spin plane (EFW-49). 4-mV/m accuracy at DC on the spin axis (EFW-52). 400-mV/m signals in the kHz band (large-amplitude EM fluctuations). 400-kHz bandwidth with low noise floor (EMFISIS interface). Reference numbers from: RBSP_EFW_SYS_001H_Requirements.xls

5. EFW – Preamp

6. SPB PreampSensors and Enclosures • SPB enclosure directly derived from THEMIS-EFI (20 units on-orbit 31 months). • Includes 7-mm Al equivalent 4π radiation shields around OP-15. • minor modifications to accommodate DDD-mitigation capacitors. usher surface fine wire OP-15 and rad shield guard surface SPB cable

7. AXB PreampSensors and Enclosures • AXB enclosure is AXB sensor (spherical shell). • Includes 7-mm Al equivalent 4π radiation shields around OP-15. Hinge (Guard) Whip (Stub or Usher) Sphere

8. The Preamps Themselves Each SPB PRE as shown weighs 3 g, with a 10-g Tantalum cover (13 g total). Each AXB PRE assembly masses 15.8 g. POWER Consumption: each preamp draws 3 mA of current per supply (+/-15V; 90-mW) within expected range of inputs/ operational environmental limits.

9. Schematic (SPB)

10. Schematic (AXB)

11. EFW – Preamp Response Model • Sheath impedance is Rs || Cs, and connects to SPHERE. • Output load is Cc || (Rc + RL), connected to Vout.

12. EFW – Preamp Measuring Input Impedance Stray Capacitance – Sphere and Fine Wire To Box Signal Gen PRE SPB • Faraday Box can be Grounded or Driven by Signal Generator. • Measuring AC gain in both configurations allows for estimation of Cstray and Ci.

13. Bench Testing • Little difference in AC gain between Grounded and Driven configurations. • Stray capacitance small. • AC Gain ≈ Cs/(Cs+Ci) → • Ci ≈ Cs*(1/Gv -1) ≈ 5 pF

14. ETU PRE and Cable Response • Sphere and partially-deployed Fine Wire in F-Box, with ETU SPB. • Plasma Sim is 80-MΩ ║ 10-pF (worst case magnetospheric). • Driven AC Gain ≈ 0.7. • Grounded AC Gain ≈ 0.4. • Ci ≈ 7.5 pF, Cstray ≈ 7.5 pF. • Increasing plasma density: • Moves knee to higher frequency (Rs decreases). • Moves AC gain closer to 1 (increases Cs as Debye length approaches dimension of sensor).

15. EFW – Preamp Predicted Frequency Response • Worst case Magnetospheric response, based on measured ETU input impedance (Aug 2009).

16. Layout (SPB)

17. Layout (AXB)

18. Board Fabrication The boards are made of Arlon 85NT, which is a polyimide resin on Thermount (non-woven aramid) to minimize differential CTE over broad temperature ranges experienced by the preamp PWBs: -135 C to + 90 C; THEMIS-EFI experience. -150 C to +70 C; RBSP-EFW CBE. Thermount is hygroscopic, therefore extra care must be exercised in handling and storing to avoid moisture absorption, and the boards must be baked out prior to mounting parts. Assembly instructions covering this already in-place. Coupons for both SPB and AXB PWBs are in-house, and will be sent out for testing in Q4 2009, according to schedule.

19. Derating All preamp components meet the voltage and stress derating guidelines set forth in EEE-INST-002.pdf Temperature range effects taken into consideration. Complete table with values in backup slides.

20. Radiation (TID) Testing As per an I-PDR RFA (REF#), a TID test of the flight lot and date code of OP-15 was performed. At 100-kRad(Si) TID, the only relevant parametric change was in VOS, which rose to as high as 32 mV, but was stable. EFW ConOps includes on-the-fly removal of differential offset voltage effects, so this magnitude of VOS (equiv. to up to .4 mV/m on 80-m antenna) can be tolerated, and still achieve measurement requirements. Test Report: ftp://apollo.ssl.berkeley.edu/pub/RBSP/1.2. Systems/3. Test/RBSP_EFW_TR_005_OP15TIDTest.doc

21. Deep Dielectric Discharge Mitigation The preamp enclosure does not provide 350-mil Al equivalent shielding for the preamp components, so evaluation of DDD susceptibility required. The OP-15 op amp was tested for susceptibility to damage by DDD using the test defined in the RBSP EMECP. Only pins found to be susceptible at the 1500-V test level were the COMP inputs. Mitigation capacitors were added between the COMP inputs and FGND, and were found to have no significant impact on frequency response. One N/C pin was also connected to FGND to implement the “no floating conductors allowed” requirement. Complete test report available on RBSP-EFW FTP site: ftp://apollo.ssl.berkeley.edu/pub/RBSP/1.2. Systems/3. Test/RBSP_EFW_PRE_TR_001A_OP15DDDTest.doc

22. ETU Thermal Qualification One survival cycle, two operational cycles. Survival cycle from -170 to +90 (powered off). Operational requirements are -160 to 80. With and additional 10 degrees at each extreme, the operational cycle tests were run at -170 and +90 (powered on). Power cycle tests performed at extreme high and low limits. Cold limit is met in eclipse, and EFW not required to make measurements in eclipse, and so units must survive cold, but need not stay in spec. Temperature profile for operational cycles

23. Test Flow (example of PRE→SPB) Test Preamp PWB. Bench and Thermal. Load Preamp PWB Integrate Preamp PWB and Sphere to Cable Integrate Cable Assembly to SPB Chassis Adjust limits and setpoints Stow Cable Electrical Functional Test Functional Deploy/ Length Calibration Stow Cable Electrical Functional Test PER Vibration Test Electrical Functional Test Hot TVAC Deploy Stow Cable Electrical Functional Test Cold TVAC Deploy Stow Cable Electrical Functional Test Deliver to Science Cal

24. Test Procedure for Flight THEMIS procedures used with slight modifications; for example: preamp supply is now at +/- 15V rather than +/- 10V. Large amplitude input tests in addition to lower amplitude frequency response. New documents: RBSP_EFW_PRE_BenchTest_Proc.xls for board level checkout, RBSP_EFW_PRE_TVAC_Proc.xls for thermal vacuum testing.

25. BACKUP SLIDES

26. Grounding • the ground on the board is the floating ground from the LVPS and comes into the board on the shield pin.

27. Derating and Stress (Ref. EEE-INST-002)

28. Parts List (AXB)

29. Parts List (SPB)

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