RPG Radiometer Physics GmbH

- Fast Fourier Transform Spectrometer

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RPG is now offering a new type of wide-band Fast Fourier Transform Spectrometer (FFTS). The new analyzer boards make use of the latest versions of GHz analog-to-digital converters and the most complex FPGAs commercially available today. These state-of-the-art chips have made possible to build digital spectrometers with coherent bandwidths of up to 1.8 GHz and up to 16384 spectral channels.

To simplify the combination of many analyzer cards into, e.g. an Array-FFTS, the novel boards include a standard 100 Mbits/s Ethernet interface. Precise time stamping of the processed spectra is realized by an on-board GPS/IRIG-B time decoder. The compact FFTS board (100 x 160 mm) operates from a single 5 Volt source and includes a programmable ADC clock synthesizer for a wide range of bandwidth setups (100 MHz - 1.8 GHz). The power dissipation per board was measured to be less than 20 Watt. The boards are available with one or two IF inputs.

The wide-band Array-FFTS equipped with 8 FFTS boards. A single FFTS board provides 1.5 GHz monolithic bandwidth with 8192 spectral channels. The frequency resolution (equivalent noise bandwidth) in this configuration is 212 kHz.

Unlike the conventional windowed-FFT processing, a more efficient polyphase pre-processing algorithm has been developed with significantly reduced frequency scallop loss, less noise bandwidth expansion, and faster sidelobe fall-off.

The superior performance, high sensitivity and reliability of the FFTS has now been demonstrated at many telescopes world-wide, including APEX (Chile), CSO (Hawaii), 30-m IRAM (Spain) and the 100-m Effelsberg observatory (Germany).

  • High coherent bandwidth: 1.5 GHz in field tests, 1.8 GHz in lab tests
  • Very stable: Allan stability of 4000 s are demonstrated routinely
  • 8192 channels per board
  • Easy combination of multiple boards in one unit
  • Superior performance, high sensitivity and reliability has now been demonstrated at many telescopes world-wide, including APEX (Chile), CSO (Hawaii), 30-m IRAM (Spain) and the 100-m Effelsberg observatory (Germany).

We summarize the striking advantages of this new generation of compact FFT spectrometers:

  • FFTS offer high coherent bandwidth (today 1.5 GHz) with many thousand frequency channels, thus offering broadband observations with high spectral resolution without additional IF processing.
  • They provide very high stability by exclusive digital signal processing (Allan variance stability plot).
  • Field-operation has been proven to be very reliable, with calibration- and aging-free digital processing boards, which are swiftly re-programmable for special observation modes.
  • Low space and power requirements -- thus safe to use at high altitude (e.g. APEX at 5100 m) as well as on spacecrafts and satellites.
  • Productions costs are low compared to traditional spectrometers through the use of only commercial parts.

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