1. SPP-FIELDS Slow Sweeps, Auto Bias, and All That - UPDATE
J. W. Bonnell, UCB SSL
Past experience - back to ISEE - has shown that the correct current and voltage biasing of E-field antenna elements is crucial for making accurate quasi-DC and LF E-field estimates.
Challenges to a successful biasing strategy on SPP FIELDS:
The finite number of perihelion encounters on SPP – few dozen at most.
The limited opportunities for data download and commanding – less than once per encounter, and only pre- and post-encounter.
The large dynamic range in environmental drivers of antenna properties (e.g. factor of 32 in solar flux!).
The significant uncertainties in plasma environment variability (thermal, suprathermal and energetic) during encounters.

2. Sensor Biasing on SPP FIELDS
Why Bias?
FIELDS biasing system capabilities – PAs, AEBs, FSW.
Keeping On Track – AutoBias0 – OML and SCL currents.
Finding Our Way – Slow Sweeps – Implement with RTS scripts.
Responding to Surprises – AutoBias1 – Add “get out of saturation” algorithm.
John includes the derived requirements for the AEB, reflective of discussions at June 2012 and compiled by S. Harris.
-- existing designs that meet or exceed the requirements are noted after each of the requirements, either THEMIS-EFI or RBSP-EFW flight designs.
-- added the requirement to characterize noise at bias, stub, and usher nodes so that it will get considered in the design, and checked and tracked during test and integration.

3. LF Section Details: Biasing
IBIAS sets operating point on non-linear sheath I-V curve to reduce offset voltages due to stray currents.
VSHIELD and VSTUB set up potential barriers between environmental current sources and WHIP (sensor) to reduce stray currents and improve DC sensor isolation from SC.
500 Hz
Biasing is necessary to reduce the effect of photoemissions
Light hits antenna, causing cloud of electrons around antenna
This sheath acts as a barrier to measurements of free space signals
Answer: Drain off cloud of charges by drawing them into bias line and returning them to spacecraft chassis
BUT
We want to do it without affecting the signals we’re measuring
A simplistic approach of a DC bias voltage across a resistor results in Rin = Rbias
Better approach is to track AC voltages with a smaller DC offset
If both ends of bias resistor follow the input voltage no current is drawn
Adding a DC offset presents a lower impedance to the DC charge sheath, draining it, while presenting a high impedance to AC signals, allowing AC measurements without interacting with AC sources
*note: John Bonnell explained some other reasons for current biasing
20 Vpp

4. AEB Requirements (stable since Dec 2012)
Preamp signal characteristics
DC voltage level: ± 60Vdc w.r.t. AGND (± 60 Vdc at full bias offsets; up to +/- 100Vdc at reduced bias current and voltage levels).
AC voltage level: ± 10V w.r.t. floating ground up to 70 kHz (± 13V capability up to several 100 kHz)
Floating Ground Driver
Input: LF Preamp signal
Input filter roll off: 500 Hz (~450 Hz actual, soft requirement due to limited AC dynamic range).
Output voltage level: ± 60Vdc w.r.t. AGND
Floating supply rails: ± 15Vdc.
Bias, Stub, Shield Drivers (Bias and Box on V5)
Reference Input: LF Preamp signal
Reference input filter roll off: 500 Hz (~450 Hz actual, match FGND)
Output voltage level: Vref ± 40Vdc (max, programmable) w.r.t. AGND
DAC resolution: 12-bit (~.025%).
Noise voltages at Bias, Stub, and Shield outputs consistent with noise floor requirements, downstream filtering and processing, and predicted coupling to antenna (TBD).
John includes the derived requirements for the AEB, reflective of discussions at June 2012 and compiled by S. Harris.
-- existing designs that meet or exceed the requirements are noted after each of the requirements, either THEMIS-EFI or RBSP-EFW flight designs.
-- added the requirement to characterize noise at bias, stub, and usher nodes so that it will get considered in the design, and checked and tracked during test and integration.

5. AEB Requirements, con’t.
Bias, Stub, Shield Drivers (Bias and Box on V5) – Output Currents
Bias ranges (AEB+PA, V5 is lowest range only; 12-bit resolution):
+/- 802 nA (360 nA 1 AU; ~4 bits of next range)
+/- 14 uA (0.25 AU; ~5 bits of next range).
+/- 414 uA (184 uA 9 Rs).
PA1..4 Stub and Shield Currents (max is photoelectron-dominated):
Stub: 60 nA (1 AU), 30 uA (9.5 Rs, nominally shadowed!).
Shield: 200 nA (1 AU), 100 uA (9.5 Rs).
V5 Box Currents (nominally shadowed; max is photelectron-dominated):
Box: ~40 nA (1 AU).
Sunlit surfaces dominated by photoelectron and possibly thermionic electron emission, and so currents tend to be sinking of current from exposed surfaces (sourcing e- to surfaces consistent with sheath I-V curves).
John includes the derived requirements for the AEB, reflective of discussions at June 2012 and compiled by S. Harris.
-- existing designs that meet or exceed the requirements are noted after each of the requirements, either THEMIS-EFI or RBSP-EFW flight designs.
-- added the requirement to characterize noise at bias, stub, and usher nodes so that it will get considered in the design, and checked and tracked during test and integration.

6. AutoBiasing-0 (For Predicted Solar Flux)
Varying the zero-order current (and possibly voltage) biasing as a function of heliocentric distance (RH) will be crucial to maintain DC and LF data quality:
To avoid saturation due to too high of current bias.
To maintain sheath biasing to minimize impacts of stray currents to the sensor.
Implemented in the past with finite number of fixed on-board bias tables (2-4) controlled by time-tagged commands uploaded in batches.
Not very feasible for SPP –
Large dynamic range in solar forcing – lots of tables needed, ~20 to keep within sqrt(2) of nominal “half the photoemission” bias current setting over the ~32-fold change in solar flux from 0.25 AU to 9.5 Rs (closest perihelia).
Significant (?) uncertainty in ephemerides at time of planning (ToP) and time of command uplink (ToCU) – uncertain (?) when to execute the table switching commands.
However Time and Status data distributed to instruments includes CBE ephemerides:
Expected to be more accurate than those available at ToP or ToCU.
Simple FSW algorithm and process can thus determine zero-order bias current (Range Relay and DAC settings) from RH (see diagram, next slide).
PROPOSAL: Develop and implement such an algorithm for DCB FSW.
UPDATE: Implementation still under study - RTS? OML vs. SCL currents?
John includes the derived requirements for the AEB, reflective of discussions at June 2012 and compiled by S. Harris.
-- existing designs that meet or exceed the requirements are noted after each of the requirements, either THEMIS-EFI or RBSP-EFW flight designs.
-- added the requirement to characterize noise at bias, stub, and usher nodes so that it will get considered in the design, and checked and tracked during test and integration.

7. AutoBiasing-0, con’t. Ibias ~ 0.5*Ipe ~ const*(Scaled Solar Flux)
Scaled Solar Flux = (R0/RH)2
Zero-Order Bias Update Waypoints:
Sqrt(2) spacing.
few hours to few tens of hours between updates.
Finer gradiations possible if required or desired.
Toggle at TBD rate between OML and SCL estimates for first few encounters.
Run SDT’s at updates and OML/SCL toggles
OML vs. SCL:
Whip current-voltage characteristics are expected to be orbit-motion-limited (OML), but could become space-charge-limited (SCL) at high fluxes.
SCL currents could be ~0.1 the OML estimates, and so biasing using OML estimates could lead to saturation.
John includes the derived requirements for the AEB, reflective of discussions at June 2012 and compiled by S. Harris.
-- existing designs that meet or exceed the requirements are noted after each of the requirements, either THEMIS-EFI or RBSP-EFW flight designs.
-- added the requirement to characterize noise at bias, stub, and usher nodes so that it will get considered in the design, and checked and tracked during test and integration.
Closest Perihelion (~9.5 Rs)
(Worst Case Number of Changes)

8. Slow Sweeps (Sensor Diagnostic Tests, SDT)
A Slow Sweep, aka. Sensor Diagnostic Test (SDT), is an essential part of the in-flight calibration of the FIELDS LF E-field system.
Such a tool, usually implemented in FSW, has long heritage on E-field instruments (Polar, Cluster, THEMIS, RBSP, etc.)
Conceptually straightforward (example shown on following page, changes from heritage marked in rust):
On a cadence of HOURS (TBR):
For a given RANGE RELAY SETTING, Loop over Bias Current (DAC), Stub Offset Voltage and Shield Offset Voltage on a single or opposed pair of antenna elements (e.g. V1 or V1 and V2), while keeping current and voltage biasing of other antennas constant. Ramps in parameters are linear, with the initial setting, step size, and number steps programmable.
Hold at each step for TBD seconds – e.g. 2N s - (rather than for M spins), collecting single-ended (Vn) and differential sensor potentials (Enm = Vn-Vm), instrument HSK (esp. Bias readbacks) and other diagnostic data (e.g. survey plasma data).
Science Team then evaluates V and E data from swept and un-swept sensors to establish nominal current and voltage bias settings for the given plasma environment, and Ops modifies the on-board biasing tables to reflect the new default settings Upon Delayed Receipt of V, E, and HSK data.
Proposal: develop and implement this algorithm in DCB FSW for SPP FIELDS.
John includes the derived requirements for the AEB, reflective of discussions at June 2012 and compiled by S. Harris.
-- existing designs that meet or exceed the requirements are noted after each of the requirements, either THEMIS-EFI or RBSP-EFW flight designs.
-- added the requirement to characterize noise at bias, stub, and usher nodes so that it will get considered in the design, and checked and tracked during test and integration.

9. Slow Sweeps, con’t.
John includes the derived requirements for the AEB, reflective of discussions at June 2012 and compiled by S. Harris.
-- existing designs that meet or exceed the requirements are noted after each of the requirements, either THEMIS-EFI or RBSP-EFW flight designs.
-- added the requirement to characterize noise at bias, stub, and usher nodes so that it will get considered in the design, and checked and tracked during test and integration.

10. Slow Sweeps (IMPLEMENTATION)
Implemented as nested set of Relative Time Sequence (RTS) scripts:
SWEEP12 sync’s the SDT to a particular cadence, then starts SHLD12
SHLD12 commands bias offset voltage on SUN_SHIELD surface, then starts STUB12.
STUB12 commands bias offset voltage on STUB surface, then starts BIAS12.
BIAS12 loops over set of sensor bias currents, then exits back up to STUB12.
STUB12 then increments to next STUB setting, and starts BIAS12 again. …
SWEEP12 (Start_Time, Step_Duration){
For SHIELD_BIAS = {Start, Delta, Number} do {
For STUB_BIAS = {Start, Delta, Number} do {
For SENSOR_BIAS = {Start, Delta, Number} do {
Set SHIELD, STUB, SENSOR;
Hold for Step_Duration; }
John includes the derived requirements for the AEB, reflective of discussions at June 2012 and compiled by S. Harris.
-- existing designs that meet or exceed the requirements are noted after each of the requirements, either THEMIS-EFI or RBSP-EFW flight designs.
-- added the requirement to characterize noise at bias, stub, and usher nodes so that it will get considered in the design, and checked and tracked during test and integration.

11. AutoBiasing-1 (For Environmental Variations)
Sudden environmental changes (e.g. enhanced energetic electron fluxes) change the major contributors to SC and sensor current balance.
Fixed current bias can then lead to sensor saturation and loss of E- field measurements (DC on up).
Mozer, Harvey, and Bonnell developed and implemented an on-board AutoBias algorithm that determines the need to change the default bias settings using on-board SC_POT estimates, and adjusts the biases on a selectable cadence (32-s nominal for RBSP-EFW).
Core of algorithm is a piecewise linear I-V curve that relates I_BIAS to SC_POT:
Likely additions:
Independent I-V curves for each sensor (V1..V5), possibly with different weights for the different sensor potentials.
V-V curves for each sensor’s stub and shield offset potentials.
Failsafe “detect saturation and reduce bias” algorithm?
John includes the derived requirements for the AEB, reflective of discussions at June 2012 and compiled by S. Harris.
-- existing designs that meet or exceed the requirements are noted after each of the requirements, either THEMIS-EFI or RBSP-EFW flight designs.
-- added the requirement to characterize noise at bias, stub, and usher nodes so that it will get considered in the design, and checked and tracked during test and integration.

12. AutoBias-1 (example, RBSP-EFW)
IBIAS
(V1+V2)/2
Energetic e-
Plasmasphere
(high density)

13. BACKUP SLIDES
There are no Backup slides.

14. Scope of AEB Design Effort
Antenna Electronics Board (AEB, see following block diagram slide)
Full AEB functionality split into two boards:
AEB1 (V1, V2, V5) – primarily controlled by DCB.
AEB2 (V3, v4) – primarily controlled by TDS.
FGND Driver:
1 channel each for 4 forward whips (V1..4).
1 channel for aft antenna (V5).
Sensor bias current, Stub and Shield bias voltage drivers:
3 channels of each for V1..4.
Sensor bias current and “Box” bias voltage driver for subset for aft whip or dipole.
Sensor preamp bias range relay control:
1 channel each for 4 forward whips (3 ranges).
one for V5.
Floating Power Supplies:
2 floaters to support opposing pairs of forward whips.
1 floater to support aft whip or dipole (AEB1 only).
V5 Preamp Power and Heater Control:
FSW-controlled floating power supply and pre-heater resistor.
HF Output Stage Power Regulation:
+/- 6V from LVPS1/2 regulated down to +/- 5V for wide (4x) current consumption (signal amplitude).
Passthrough of LF signal to DFB.
Serial Command and Data (HSK) I/F to DCB/TDS.
The scope of the AEB design effort includes the following elements:
(I’d suggest referring the audience to the two block diagram slides that follow for details)
The DC-LF preamp, including the electrical interfaces to the whip sensors and photoelectron control surfaces (Stub, Sun Shield) , as that portion of the preamp system forms an intimate part of the floating ground generation and current biasing sub-system of the AEB.
The AEB itself, which provides:
-- floating ground and floating power supplies for each axis of the LF Preamp, allowing for a larger LF dynamic range to accommodate spacecraft charging effects and large-amplitude, low-frequency E-field signals.
-- bias drivers for sensor current bias, and Stub and Sun Shield voltage biasing.
-- relay control for the bias resistor ranging.
-- floating and HV power supplies for each axis and for all the FGND and bias driver output stages.
-- serial command and data (housekeeping) interface back to the ICU.
-- associated harnessing between the AEB and the Preamp enclosure, as well as between the AEB and other slices of the ICU (like the DFB).

15. SPP FIELDS PA
3 stages, LF, MF, HF for low, medium and high frequencies
Covers DC to 20MHz
Very high input impedance to avoid interacting with medium
Very wide 140dB dynamic range – from a few nV noise floor (17uVrms) to 60VDC
Why the FET?
High speed amplifiers are usually low input impedance. The FET is a gain 1 buffer to present a high input impedance but drive the HF 20MHz amp input

16. HF Section Details: Input Coupling
Solar Probe has relatively short antennas compared with wire booms (2.3m actual, with a lower effective length)
So there is less coupling to the plasma (low frequencies) or free space (high frequencies)
The coupling can be modeled as a simple RC parallel circuit.
At DC the capacitances are high impedance and can be ignored
The amp input resistance is on the order of 100GΩ (1e11Ω)
The expected plasma environment gives a resistive component that varies between 1M and 10K. This is negligible compared to the 100G.
At higher frequencies capacitances dominate.
All amplifiers have unavoidable input capacitance to ground which forms a voltage divider with the capacitance to the signal sources.
Coax cables and traces on circuit boards have capacitance.
The antenna itself has “base capacitance”, the effect of the spacecraft surface on the antenna pickup pattern.
These are all bad and act to shunt the signal.
Ideally we would have much larger capacitance between the antenna and signal sources than we do from the antenna to ground
But given the shortness of the SPP antennas we have the opposite case at larger distances from the sun. The input capacitance is larger than the source capacitance.
Because of that we’re working hard to minimize input noise and reduce stray capacitance.
Closer to the sun Rplasma is much lower and mitigates this effect.
See the next two slides

17. AEB – FGND Bandwidth and LF Dyn Range
The FGND bandwidth needs to be higher than one expects to allow for low-distortion measurement of large-amplitude LF fields.
The top panel shows a 120-Vpp 100-Hz signal (solid black line) and the response of the Floating Ground Driver (FGND, green dashed) and positive and negative Preamp Floating Supply Rails (red and blue dash-dot).
This example is consistent with the initial RBSP-EFW-BEB FGND design and the OP-15 used in the EFW PRE circuit (~2-V headroom on pos and neg supplies to give harmonic amplitudes < -40dB relative to fundamental).
The FGND is a one-pole RC low-pass filtered version of the input signal, with corner (3-dB) frequency of 300 Hz (three times the input signal!).
The region between the red and blue lines at any give time indicates the range of signal input voltages that will be faithfully reproduced.
When the voltage margin on the pos or neg rail goes negative, the output will be distorted by clipping.

18. AEB – LF->HF Dynamic Range
The dynamic range (maximum amplitude signal measured meeting maximum distortion requirements) varies with input frequency:
At very low frequencies, the dynamic range is set by the AEB HV output stage rails.
At frequencies well above the FGND rolloff, the dynamic range is set by the PRE floating supply rails and the headroom the preamp requires on those rails.
At intermediate frequencies, the dynamic range falls off remarkably fast as the rolloff frequency of the FGND is approached.
450 Vpp
(± 225 V,
~100 V/m),
BUT…
Maximium Amplitude
At FGND 3-dB Frequency
500 Hz:
36 Vpp (± 18 V, ~9 V/m)
26 Vpp
(± 13 V,~7 V/m)
Frequency

19. AEB – LF Response At Different Frequencies
Examples using Flight RBSP-EFW AEB and PRE floating supply designs (500-Hz FGND bandwidth).
Left plot is threshold case for 100-Hz input frequency (65 V amplitude [130 Vpp]).
Right plot is threshold case for 500-Hz input frequency (18 V amplitude [36 Vpp]).

20. AEB – LF->HF Dynamic Range

21. AEB – Biasing Reduces LF Dynamic Range
V_BIAS, V_GUARD, V_STUB, etc.
(up to ± 40 V; typ. ≤ ± 20 V).
The full HV rail-to-rail voltage is not available to the signal at low frequencies if current and voltage biasing is active and one wants stable DC current and voltage biasing.
For typical conditions and designs (current biasing to half possible range, +/- 40 V offset system), this eats up another 20 V or so of the dynamic range in a conservative (worst-case) design.
Similarly, any stable SC potential offset (SC floating potential, VSC) eats up part of the LF dynamic range (few to tens of volts) in a conservative (worst-case) design. Large values of VSC are typically driven by biasing, but will also be driven on SPP by the space charge in the SC wake.