EPHIN - electronics
The design of the onboard signal processing electronics is devided into 3 functional groups: the Analog Control Unit (ACU), the Digital Control Unit (DCU), and the High Voltage Power Converter (HVPC). A block diagram is shown in Fig. 1.
- Figure 1: EPHIN Electronics Block Diagram
Analog Control Unit
The purpose of the ACU is to amplify the small charge pulses from 16 semiconductor detectors and a photomultiplier and to convert them for the digital processing. Each of the 16 semiconductor detector signals is capacitively coupled to a dedicated charge sensitive amplifier (CSA) followed by a pole-zero filter (PZ) and then amplified by two serial amplifiers with a common base line restorer. These high gain signals can trigger a discriminator for counting single detector events and coincidence events. To improve the accuracy while allowing a high dynamic range for the input signals (i.e. ≈ 60 dB for the A- and B-channels) both the low-gain and the high-gain amplifier outputs are used for peak detection and 10-bit analog-digital conversion. To reduce the amount of ADC's the 12 amplified signals from the segmented A- and B-detectors are fed into 'Analog-Or Amplifiers' (AOA), whose outputs will follow the highest input level. Additionally the outputs of the A-detector AOA's are connected to four discriminators with higher thresholds that are used to separate electrons, protons and helium nuclei. The photomultiplier tube signal is capacitively coupled to a combined charge sensitive preamplifier, discriminator and pulse shaper. It is used as a veto-signal and is not pulse-height analyzed.
Not shown in the ACU scheme is the housekeeping circuitry and the inflight test pulse generator (IFTG). The following 16 physical quantities are A/D-converted to 8-bit housekeeping values:
- 4 power rail voltages
- 4 power rail currents
- 6 leakage currents of A - F detectors
- 1 high voltage of photomultiplier tube
- 1 temperature inside EPHIN electronics box
In addition a spacecraft powered thermistor is used to record the temperature history of the sensor also when EPHIN is switched off.
For inflight calibration each of the 17 analog channels can be stimulated by generating a charge at the CSA test inputs. There is a predefined sequence of test pulses controlled by the onboard software. Sequence and charge amplitudes can be changed by software upload.
Digital Control Unit
The DCU processes the ACU discriminator signals and A/D conversion results and generates a scientific and a housekeeping data block for transfer to the CDPU once per minute. The DCU also accepts telecommands from the CDPU to control the instrument. The first trigger of an ACU discriminator starts a 2.5 μs coincidence window, during which discriminator signals may set their associated flipflops. At the end of this window the DCU evaluates the coincidence configuration by hardware logic and increments the associated counters. In case of a valid coincidence the DCU starts the pulse height analysis, if the PHA circuitry is not busy with a previous analysis.
There are 56 24-bit counters (see Table 1 in EPHIN - sensor unit). The Column "M" in that Table indicates the multiplicity of counters, i.e. protons are counted in 3 different counters depending on the position in A and B detectors and incident angle of particles: straight in the center sectors A0 B0, straight in the ring sectors An Bn; n=1...5, or oblique, i.e. all other sector combinations. Helium nuclei are counted in 4 different counters similar to protons, whereby the oblique incidence is further distinguished in slightly oblique and strongly oblique. This is needed for 3He - 4He separation.
The EPHIN instrument produces 1290 bytes of scientific data during a 1-minute accumulation interval. In order to adjust to higher data rates, the frame length can be changed in steps of 2 bytes up to 2000 bytes by telecommand. Thus more pulse height information can be transmitted in case bitrate is freed by other instruments within CEPAC.
The data frame is organized in 4 different groups: digital housekeeping, countrates, histograms, and pulse height data.
Digital Housekeeping: 7 bytes at the beginning of the scientific data block are reserved for status information. These bytes contain operational mode, power status, error flags, status of failure mode registers, and a pointer to separate the pulse height data registered in priority mode from those in normal mode.
Countrates: The 24-bit counters are compressed using a 8+4 bit logarithmic compression which allows a decompression with an accuracy better than the statistical accuracy.
Histograms: Histograms provide more detailed spectral information than the countrates though less detailed than the full PHA information. No isotope information can be obtained from the histograms. Histograms are constructed from the pulse height words (PHW) by adding up the energy losses measured in all involved detectors and counting the number of particles versus total energy loss in 64 bins. This is performed separately for the ranges AB, ABC, ABCD and ABCDE with a time resolution of 8 minutes. Thus only 48 bytes per minute are reserved for compressed (8+4) histogram information. This onboard processing provides spectral information for electrons, protons, and helium at a marginal bitrate increase.
Pulse Height Data: In normal observation mode the PHW buffer of 1151 bytes is filled with PHW's in chronological order. A PHW is composed of a 10 bit header and two or more A/D conversion results. The header consists of 3 bits each for the sector information of detectors A and B, and 4 bits to hold the coincidence type. 11 bits are added for each involved channel A, B, C, D and E: 10 bits with the ADC result and 1 bit that indicates the gain range. Depending on the coincidence depth the size of PHW's can vary between 4 and 9 bytes. If the PHW buffer is filled before the accumulation period ends, the buffer is overwritten in reverse direction starting at the end address with priority pulse height data. The priority system is designed to assure transmission of a minimum number of events per sampling interval for each particle type and energy range when the PHW buffer is flooded by overabundant particles. This minimum number which is indicated in column "P" in Table 1 (EPHIN sensor unit) is initialized in the EPHIN onboard software, but can be changed by table upload.
High Voltage Power Converter
For biasing the semiconductor detectors A to F and for operating the photomultiplier tube G there are 7 individually adjustable high voltages provided by 3 independently regulated HV cascades: A (-30 V), B (-60 V), C (+400 V), D (+600 V), E (+600 V), F (-140 V), and G (+900 V). The converter frequency is 64 kHz.
More information about EPHIN
The EPHIN pages are based upon
R. Müller-Mellin et. al.
COSTEP - Comprehensive Suprathermal And Energetic Particle Analyser
Solar Physics 162: 483-504, 1995
