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Wideband Spectrometer Development in the Astronomy Department of the University of Maryland

Wideband spectrometers are becoming increasingly important for astronomy at millimeter and submillimeter wavelengths.  Applications include molecular line searches for primordial galaxies at unknown redshifts and extragalactic submillimeter spectroscopy of galaxies, whose spectral lines are very wide because of the Doppler effect's scaling with frequency. 

A review paper (pdf) from the SOFIA/NASA Far-IR, Sub-mm, and mm Detector Workshop provides an overview of different wideband heterodyne spectrometers and some of their applications to astronomy.  Another paper, from the 2002 SPIE Astronomical Telescopes and Instrumentation meeting (pdf), describes broadband analog correlator technology and suitable receiver architectures in more detail. 

Technical information is available below on the WASP2,WASP3, APHID analog lag correlators and their software. These are spectrometer systems that we have been developing in the High Frequency Radio Astronomy Laboratory at the University of Maryland.

WASP2 Analog Lag Correlator

WASP2 is a 3.8 GHz bandwidth lag correlator spectrometer optimized for heterodyne astronomical spectroscopy with moderate resolution. This image shows three of the multipliers and the resistive taps to the transmission lines.

Some useful reference documents are:

• "A wideband lag correlator for heterodyne spectroscopy of broad astronomical and atmospheric spectral lines ," a paper describing the WASP2 instrument.   Copyright (2001) American Institute of Physics. This article may be downloaded for personal use only.   Any other use requires prior permission of the author and the American Institute of Physics.   The article appeared in The Review of Scientific Instruments and may be found here.
• "Spectroscopy with Multichannel Correlation Radiometers," a paper describing cross-correlation spectroscopy with WASP spectrometers.
  
• Images of the insides of the WASP spectrometers.
• Spectrometer and component properties.
  
• Technical details and interfaces for the WASP2 spectrometers (pdf).
• Photograph of WASP2 on the CSO with the SURFER front-end (jpg).  The large wasp in the image is not real.
  
• Passband shapes for the WASP2 spectrometer with internal signal processing for a nominal 4 to 8 GHz input with 0 dB and 3 dB internal attenuation (eps).
• Signal processing mathematics for analog autocorrelation spectrometers (pdf, postscript).
• Assembly steps for the microwave modules (pdf, msword).
• Peter Teuben's amusing description of our observing run at the CSO in April 2000 with many links to many nice pictures.

WASP3 Ultra-wideband Multiplier

WASP3 is a 25 GHz bandwidth Gilbert cell multiplier project for very broadband correlators and spectrometers.   Stephen Maas of Nonlinear Technologies, Inc. designed this custom MMIC for this project, with testing in the High Frequency Radio Astronomy Laboratory of the University of Maryland's Laboratory for Millimeter-wave Astronomy.

A number of memos and images summarize measurements of the chips and associated components:

• An overview is in the most recent progress report (by now a little out of date) submitted to NASA, who funds this project through its Scientific Research and Technology Program with grant NAG59075 (pdf).
• Comparison of the prototype and "production" run devices (pdf)
  
• Dimensioned image of the chip (jpg).
• First comparison of the two prototype multiplier chips, from 2 to 18 GHz, measured in a probe station (pdf).
• More complete characterization of a sample of eight multiplier chips from 2 to 18 GHz, measured in a probe station (pdf).
• Characteristics of a multiplier from 2 to 30 GHz, measured in a microstrip fixture (pdf).
• Characteristics of the multipliers as a function of bias voltage (pdf).
• Measurement of the test amplifier on the prototype wafer. (pdf).
• Measurements of a prototype microstrip 2-way Wilkinson power divider (pdf).
• Measurements of a prototype microstrip 16-way Wilkinson power divider (pdf).

APHID Correlator for Phase Correction

In its current incarnation, APHID uses a single WASP2 board as an auto- or cross-correlator to test ideas for atmospheric phase correction for the BIMA and OVRO millimeter wave interferometers.

Some references:

• A memo that discusses precision total power radiometry.  It includes an exploration of the spectral effects of nonlinearities in a correlation spectrometer (pdf; postscript).
• A document describing APHID's electrical and mechanical interfaces (pdf).
• Mechanical drawing of the APHID module (png).

Software documentation for the spectrometers

Software is key to spectrometer operation.

A preliminary set of references for the spectrometer control, data reduction pipeline, and archiving software:

• Microcontroller commands for firmware version wasp223 (ps). Previous versions: wasp222 in ps; wasp220 in pdf, ps; wasp219 in pdf, ps.
• Notes on frequency calculations for connecting WASP and CSO systems (pdf, mcad).  Also definitions of various common velocity scales and FITS spectral coordinate systems (gzipped ps) (see also Thompson, Moran & Swenson Appendix 10.2) and how to add relativistic velocities (html).
  
• A set of data flow diagrams for WASP spectrometer data processing (pdf, 65 kB; also ppt).
• Kevin Rauch and Amar's slides describing the WASP spectrometer data processing software (postscript).
• The dictionary of FITS keywords for the latest generation of WASP spectrometer data acquisition and storage pipeline.  This page also contains links to references defining the FITS standard and related topics.
• Peter Teuben's spectrometer software reference page.
• Doxygen source code documentation for the latest generation of WASP spectrometer data acquisition and storage pipeline.