Data Processing Unit

 

The Data Processing Unit (DPU) is developed for recording VLF/ULF signals from the sensor, and processing them in real time. The VLF signal sources are digitized by the DPU at four differential inputs (three components of the magnetic field measured by search coild magnetometer and one component of the electric field). The DC/ULF band vector magnetic field are provided digitally via an SPI-like interface by the SMILE sensor.

The digitized data stream is stored in a large (50 hours long) non-volatile circular buffer memory, from which waveform data can be requested by command. Several data products are generated in real-time:

  • DP1: ULF band waveform is filtered and down sampled to 10Sps
  • DP2: VLF band waveform is filtered and down sampled to 250Sps and from this 256 bin FFT is calculated and the 40-70Hz range is transmitted
  • DP3: From VLF band waveform FFT is calculated FFT bins can be selected to 256, 512, 1024, the result can be averaged in a flexible configurable way.
  • DP4: From VLF band waveform 256 bin FFT generated logarithm calculated and scaled to each bit represent 0.5dBuV
  • DP5: From VLF band waveform 256 bin FFT generated and from a selectable band integrated power is calculated
  • DP6: Onboard event detector output. Event detector runs on a 256 bin FFT and looks for suddenly appearing frequency peaks if such a peak is detected the position, width and height stored in DP6 output frame.

These data products help the user to select the parts of the waveform for downlink. The data products and requested waveform data are transmitted directly to the TCM through two serial interfaces, and status/housekeeping messages are transmitted through I2C bus using CSP protocol. The commands to the unit are received through the I2C bus.


Technical Parameters

Size: 3 PCBs in 96(w)mm x 89(d)mm x 20(h)mm

Mass: 130 g

Power consumption: ~190mA @3.3V + 140mA @5.9V in average 1.5W

Analogue sensor inputs:

  • 4 identical AC coupled differential inputs: BW: ~0.1Hz - 20kHz ; FS: 40kSps

Gain and resolution:

  • for the 40kSps channels(16bit ADC):

Gain

Resolution (uV/bit)

Noise (nV/Hz)

+30dB

10.7

94.8

+20dB

32.8

243

+10dB

104

767

+0dB

317

2340

DSP resources (ADSP BF532):

  • 64Mbyte SDRAM on 100MHz memory bus, 32Mbyte used by OS rest is used for input buffers.

  • 800MMACs (400MIPS with two 16x16bit MAC unit)

  • 64Gbyte CF card for storing waveform data from all channels for 2 days in circular buffer.

  • 3x 16 MByte serial NOR flash in TMR voting schema to store the bootloader, FPGA firmware, Linux kernel and root filesystem.

OS:
Linux with kernel for BF532 which is without MMU(memory management unit)

Timing:

  • External: 1PPS signals and time stamps from GPS from OBC.

  • Internal: TCVCXO ±0.5ppm frequency stability and real time clock DS3231 ±3.5ppm/year.

Digital interfaces:

  • Two identical LVTTL UART interfaces. Both connected to SEAM-TCM. Over one the “raw” waveform data is transmitted when requested, over the other the “intermediate” data (the data products) are transmitted.

  • An I2C interface with CSP. Commands received over this interface and basic status information is sent.

  • An RS232 serial interface for debug and development.

  • 1PPS LVTTL input

  • LVTTL SPI like interface for SMILE data reception.