Wideband Fast Fourier Transform Spectrometer (FFTS2)

Originally acquired for the CASIMIR instrument intended for SOFIA, a new FFT spectrometer (FFTS2) is now available with the wide band heterodyne receivers. At present, FFTS2 has two IF modules and four FFT modules. Each IF module divides a 4 GHz wide IF band (4-8 GHz) into two 2 GHz sub bands. In turn, each sub band is sampled with 8192 channels by an FFT module. The FFTS2 can be operated simultaneously in parallel with the other FFT spectrometer, FFTS1.

FFTS2 Specifications
Signal processing Non windowed FFT
Sub band width 4 x 2000 MHz
Channels 4 x 8192
Channel spacing 269 kHz [0.350 km s−1 at 230 GHz]
Channel response sinc2


  • Internal oscillators in the IF and FFT modules cause spikes at certain channel positions. These channels are not blanked in the data files. Spikes should be avoided by careful placement of the target spectral lines. For example, because the center channel of each 2 GHz sub band is bad, avoid centering a line, especially a narrow one, at 5 GHz or 7 GHz in the IF band.
  • Although the sub band response is limited to 2.0 GHz, the FFT sampling rate actually corresponds to 2.2 GHz. So the 372 [ = (1-2/2.2) x 8192 / 2 ] channels at the edge of each sub band do not contain useful information and should be avoided. Any lines near the edge of a sub band will be aliased into the edge region and will appear as ghosts.
  • There is little (unfiltered) overlap between adjacent sub bands unless the center frequencies are shifted (see below). (Details of the filter used to limit the sub band response are not available.)

Sub band assignments

  • In the CLASS headers, the TELESCOPE field identify the four sub bands: CSO FFTS 2B1, CSO FFTS 2B2, CSO FFTS 2B3, and CSO FFTS 2B4. The sub bands are connected to the wide band receivers as follows:
Receiver Sub
230 GHz B1 4-6
B2 6-8
460 GHz B3 4-6
B4 6-8
(345 GHz)
B1 4-6
B2 6-8
  • Each FFTS2 sub band has 8192 channels. Collectively, the channels for the entire FFTS2 are numbered from 1 through 32768.
subband channels
B1 1– 8192
B2 8193–16384
B3 16385–24576
B4 24577–32768

UIP commands

  • Start the spectrometer with:
  • The sub band center frequencies, usually 5 GHz and 7 GHz, may be shifted to provide overlap. For example,
    will place the center frequencies at 5.2 GHz and 6.8 GHz. Then the IF coverage will be 4.2-7.8 GHz and there will be 400 MHz overlap near around 6 GHz.
  • To point using the FFTS2, specify the channel range corresponding to the appropriate sub band by using the HALF_WIDTH argument and /CENTER option or by using the /RANGE option. For example, with FFTS2B3 connected to the 460 GHz receiver,
    UIP> five_point 10 /range 16385 24576 /ffts2

Data analysis

  • Flag (blank) bad channels with the draw kill command in CLASS.
  • This script flags channels that are often bad. Use with caution. N is the sub band.
    LAS> @ cso_clean_ffts2 N
cso/instruments/heterodyne/ffts2.txt · Last modified: 2013-12-07 05:24 by sradford
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