Mock Spectrometer


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Summary

The Mock Spectrometer consists of 14 FPGA spectrometer boxes covering the 7 ALFA pixels. The 14 boxes consist of 7 primary boxes (Mock 1), which set the LO frequency, and 7 secondary boxes (Mock 2), which have the same LOs as the corresponding primary box but can be set up separately. Each box has two 172 MHz frequency bands, designed so that together they provide good spectral coverage over the whole 300 MHz bandwidth of ALFA.

Setup is carried out at the level of Mock 1 and Mock 2: the same setup is applied to all of the boxes within the set. The same LOs are fed to both boxes, so the center of each 172 MHz band is the same for both boxes. However, if a narrower bandwidth is used this can then be repositioned within the 172 MHz band.

The following capabilities are available:

  • Use both frequency bands or only one (note: both bands use the same set-up but different center frequencies).
  • Bandwidths of 172 MHz (per frequency band)
    • Can be digitally decimated to 172/n MHz (where n = 1, 2, 3, 4, 5, ..., 1024)
    • If decimated, can be repositioned within the original 172 MHz band
  • 16 to 8192 channels (per frequency band) in powers of two steps (e.g. 16, 32, 64, ..., 8192).
  • 32, 16, or 8 bit spectral output (FITS)
  • Choice of polarization products:
    • Stokes I
    • Pol A, Pol B
    • Pol A, Pol B, Stokes U, Stokes V
  • Max data-rate of 60 MBytes/sec per spectrometer box (30 to 40 MBytes/sec per frequency band)
  • Max sample integration time dependant on number of channel — see table below
  • Write either CIMAFITS (standard fits files) or the minimal-header PSRFITS format.

CIMA versions 3.1 and later support ALFA observations with the Mock spectrometer in all observing modes. The Mock spectrometer can be run either alone or in parallel with the WAPPs. See here for details of using CIMA with the Mock Spectrometer.

Single pixel observations with a bandwidth of up to 1 GHz with the Mock spectrometer are supported by CIMA. Only one frequency band is available in this mode (up to 172 MHz bandwidth), and the center frequency can be set independently for each of the seven spectrometers (within the 1 GHz limit of the IF).

Dependence of maximum sample time on number of channels
No. Channels Max dump time (s)
81923.12
40961.56
20480.78
10240.39
5120.195
2560.0975
1280.0487
640.02438
320.01219
160.006095

More details (Phil's page)