The BIAS unit is a subsystem within the Solar Orbiter’s RPW instrument. The BIAS unit (1) drives bias currents to the three RPW antennas/probes (DC, -60 to +60 μA) and (2) amplifies and distributes analog voltage signals from before-mentioned antennas in the LF frequency range (0 to 10 kHz). Selected analog output signals are then passed on from the BIAS unit to receivers LFR (nominal) and TDS (redundant).

Simplified block diagram for the BIAS unit showing analog input and output quantities. Output channels BIAS 1-5 are sent to LFR, whereas only output channels BIAS 1-3 are sent to TDS.

Signals are either (1) single-probe measurements which can be used for e.g. deriving density and s/c potential estimates, or (2) differential-probe measurements (voltage difference between probes) for deriving e.g. electric field estimates.
In all, the BIAS unit can produce nine different types of analog signals of which a subset are passed on to LFR or TDS and are later downlinked to ground, see below table. At most three of these nine signals are simultaneously downlinked at any given time, depending on BIAS multiplexer settings, BIAS relay settings, and LFR/TDS settings.

BIAS channels
BIAS unit input/output Group of signals Signals and relationship to other signals Comments
Input Antenna signal 1-3 after LF preamplifier V1_LF
V2_LF
V3_LF		 Signal is not downlinked
Output DC single probe channels V1_DC = V1_LF × α
V2_DC = V2_LF × α
V3_DC = V3_LF × α		 Amplified by α=1/17
DC differential channels V12_DC = (V1_LF -V2_LF) × β
V13_DC = (V1_LF-V3_LF) × β
V23_DC = (V2_LF-V3_LF) × β		 No amplification (β=1)
AC differential channels V12_AC = (V1_LF – V2_LF) × γ
V13_AC = (V1_LF – V3_LF) × γ
V23_AC = (V2_LF – V3_LF) × γ		 Amplified by either γ=5 or γ=100. High-pass filtered from 50 Hz. Higher accuracy but reduced range compared to “DC differential signals”.