The Low Frequency Receiver (LFR) is designed to digitize and process signals from the electric antennas (ANT) and the search coil magnetometer (SCM) over a frequency range of quasi DC to ∼10 kHz. Figure 1 shows the LFR overall block diagram. A wide range of time-domain and spectral-domain data products is implemented in the flight software. LFR is also responsible for producing a calibration signal to the SCM, monitoring its own temperature and the temperature of the SCM. The LFR board has furthermore a special area reserved to the SCM heater circuitry.

The signals measured by the three ANT voltage sensors are amplified by high- and low-frequency (LF and HF) preamplifiers. The ANT LF signals (V1LF, V2LF, V3LF) are conditioned and transformed by the BIAS board into five analog voltage signals (V1BIAS, V2BIAS, V3BIAS, V4BIAS, V5BIAS) before entering the LFR. The ANT HF signals (V1HF, V2HF, V3HF) are directly entering the LFR. Three analog voltage signals (B1LF, B2LF, B3LF) also arrive directly from the SCM LF windings. The eleven input buffers of LFR are designed to be 4th order low-pass filters with a cutoff frequency of ∼10 kHz. The input buffers for the V4BIAS and V5BIAS signals are moreover AC-coupled using a single-pole high-pass filter (−3 dB at ∼4 Hz). Eight analog signals are continuously digitized at fi = 98304 Hz through eight 14-bit ADCs (analog to digital converters). The routing strategy is based on two main configurations named BIAS_WORKS and BIAS_FAILS. Nominally, the five first ADCs are used to digitize the BIAS signals producing five data streams sampled at fi: V1_fi, V2_fi, V3_fi, V4_fi, V5_fi. In the case BIAS fails, the ANT HF signals are switched to the three first ADCs while the two next ADC inputs are switched to ground. Whatever the configuration, the three last ADCs are used to digitize the SCM signals producing three more data streams sampled at fi: B1_fi, B2_fi, B3_fi.

As illustrated on Figure 2 and after analog-to-digital conversion, the eight data streams sampled at 98304 Hz are routinely processed within an Actel RTAX-4000D FPGA. Firstly, their cadence is reduced to f0 = 24576 Hz using an anti-aliasing and downsampling 5 second-order stages IIR filter. Then, from this stream of data sampled at f0, three other data streams sampled at f1 = 4096 Hz, f2 = 256 Hz and f3 = 16 Hz, respectively, are produced. Those sampling rates are illustrated on Figure 1. For the generation of the f1 data stream, the f0 data stream is passed through a low-pass (5 second-order stages) IIR filter decimating by a factor 6. For the generation of the f2 and f3 data streams, decimation CIC filters, by factors 16 and 256, respectively, are previously applied to the f0 data stream before passing through the factor 6 decimation IIR filter.

This complex data processing finally provides three kind of data products, waveforms, power spectral densities and basic wave properties. Nominally, V1BIAS is dedicated to single-ended electric potential measurements, whereas the pairs (V2BIAS, V3BIAS) and (V4BIAS, V5BIAS) are dedicated to DC- and AC-coupled differential electric field measurements, respectively. Configurable routing parameters (R0, R1, R2) allow to select either the DC- or AC-coupled differential signals to be sampled at f0, f1 and f2 respectively, and then be processed into science data streams. For the stream of data sampled at f3 it is not configurable and only the DC-coupled pair (V2BIAS, V3BIAS) is processed. The other signals (V1BIAS, B1LF, B2LF, and B3LF) are not concerned by this restriction and are all retained in the data streams sampled at f0, f1, f2, and f3. Nevertheless, the magnetic data B1_f3, B2_f3, and B3_f3 are only optionally transmitted to S/C. Configurable modes allow some additional conditioning on the DC-coupled pair (V2BIAS, V3BIAS). This occur when BIAS operates in a non-standard mode (for instance an undesired antenna preamplifier failure): the differences V2BIAS–V1BIAS and V3BIAS–V2BIAS may digitally be performed and substituted to the stream of data just after the downsampling at f0 of the corresponding data.