# Instruments

## Frequently Asked Questions

Q: What are the frequency bands for the Filter bank (FBK) data?

A: For SCM and EDC data, the relevant frequencies (-6 dB) are:

Band | f_{min} (Hz) |
f_{center}(Hz) |
f_{max} (Hz) |
Bandwidth (Hz) |
---|---|---|---|---|

1 | 1390 | 2689 | 5994 | 4604 |

2 | 316 | 572 | 904 | 587 |

3 | 80.2 | 144.2 | 227.4 | 147.2 |

4 | 20.1 | 36.2 | 57.0 | 36.9 |

5 | 5.04 | 9.05 | 14.3 | 9.23 |

6 | 1.26 | 2.26 | 3.57 | 2.31 |

For EAC data, the -6 dB frequencies are similar, except for the maximum frequency and bandwidth of the top band:

Band | f_{min} (Hz) |
f_{center} (Hz) |
f_{max} (Hz) |
Bandwidth (Hz) |
---|---|---|---|---|

1 | 1262 | 2298 | 3672 | 2410 |

2 | 318 | 574 | 906 | 586 |

3 | 80.4 | 144.6 | 227.8 | 147.4 |

4 | 20.2 | 36.2 | 57.0 | 36.8 |

5 | 5.04 | 9.06 | 14.3 | 9.22 |

6 | 1.26 | 2.26 | 3.57 | 2.31 |

Q: What are the frequency bands for the Fourier power spectra (FFT)?

A: The FFT frequency bands depend on the mode in which the DFB has been
commanded to operate. There can be 16, 32 or 64 bands, spaced
pseudo-logarithmically over the range 0-4096 Hz for the SCM and EDC
data, and over the range 0-8192 Hz for the EAC data. There
are tables with the exact frequencies in the Space Science Reviews article
by Cully
et al [2009].

Q: What is the difference between EDC and EAC data?

A: The EDC data is sampled at 8192 samples per second, and has a maximum
range of roughly 400 mV/m for the radial booms at low frequency (below
a few hundred Hz) and roughly 800 mV/m at high frequency. The
EAC data is sampled at 16384 samples per second, and has a sensitivity 6
times that of the EDC data. The EAC data has a high-pass
filter at 10 Hz, and so should not be used below this frequency.

Q: Why are the units for the FBK data |mV/m| or |nT|, while
the units for the FFT data are (mV/m)^{2} or nT^{2}?

A: The FFT data are true wave powers calculated using a (fast) Fourier transform,
and are therefore in power units. The FBK data are
calculated as the mean of the absolute value of the bandpass-filtered
signal, and are therefore in units of amplitude. To convert the FBK
data to wave amplitudes, multiply by pi/2. To convert to
mean-square power, square the FBK data and multiply by pi^{2}/8.

Q: What is the accuracy for the FFT and FBK data?

A: The FBK and FFT data are fully corrected for the known
frequency-dependent instrument responses. The AC response of
the SCM instrument (and hence the accuracy of the SCM FBK and FFT data)
is known from both ground and in-flight calibrations, and the
amplitudes are accurate to within a few percent. The AC response
of the EFI instrument cannot be fully tested on the ground, since the
required test chamber would be prohibitively large. Hence, the EFI AC
response is calculated based on known properties of the instrument
(geometry, input capacitance, etc.), rather than being measured
directly. This response function has been verified in-flight by
cross-calibrating with the SCM data during intervals of well-identified
plasma waves with known E/B ratios. At frequencies above 500 Hz or
below 100 Hz, the EFI amplitudes are accurate to within a factor of
2. From 100 to 500 Hz, the response function changes
rapidly and the EFI amplitudes are consequently only accurate to within a
factor of 4 in this region.

Q: What are the HF quantities in the FBK data?

A: The HF quantities are measurements of the electric wave pseudo-power
(mean amplitude) in the band from 30 kHz to 500 kHz. Two quantities are
reported in the telemetry: the peak value and the average value of the
amplitude in this band during the sampling interval.

Q: What is the smallest (or largest) signal that can be seen in the FBK and FFT data?

A: For both the FBK and FFT data, the maximum range is the full EFI or SCM
instrument range. For the FBK data, the uncompressed dynamic
range is 19 bits. Hence, the smallest amplitude that can be seen is 1/8 of an ADC unit. For the FFT
data, the uncompressed powers are 36 bits wide, so the smallest amplitude is 1/16
of an ADC unit. The relation between ADC units and physical
quantities is frequency-dependent, but at low frequencies, 1 ADC
corresponds to roughly 0.01 mV/m for EDC (radial booms),
0.002 mV/m for EAC (radial booms) and 0.2 pT for SCM.

Q: Where can I find more information on the FBK and FFT data?

A: The FBK and FFT data come from the EFI and SCM instruments. More
information on these instruments can be found in the instrument
descriptions on this site and in the Space Science Reviews articles by
Bonnell
et al [2009] and Le Contel
et al [2009]. The data processing is performed by the
Digital Fields Board, and the Space Science Reviews article by Cully et
al [2009] covers much of the information specific to the FBK
and FFT data.