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Status and Development Plans for the Ka-band and Q-band Receivers

During science and commissioning observations in early fall 2005, intermittent poor spectral baseline performance was noted with the Q- and Ka-band  receivers. Investigation revealed intermittent, irregular baseline structure with typical ripple periods between 10-40 MHz. These ripples make observations of wide lines, like those from high redshift CO, extremely difficult. Various Q-Band tests have isolated the source of the structure to between the front-end beam switch and the feed vacuum window. The feeds are cooled and reside within the cryostat while the beam switch is at room temperature. The spectral baseline structure at Q-Band is qualitatively different than baseline structure that has been previously studied with most GBT receivers, but is similar to that seen with the Ka-Band. The net effect is that over 1-2 hours integration the Q-Band system only achieves between 2-4 times the theoretical RMS noise with the 50 MHz spectrometer modes performing better than the broadband 800 MHz modes. Still, the baseline structure always remains at some level. Mitigating or eliminating the irregular baselines will be a high priority for the work this summer. To assist, a scaled-down version of the GBT spectrometer for laboratory use is now being completed and tested. Additional details about the Ka and Q receivers’ baseline performance can be found at: 

• Ka Receiver Performance Characterization, Fall 2005
• KA Band Spectral Line Characterisation
• Q-Band Spectral Baseline Performance
• Summary of Q-band Receiver Baselines

In addition, preparations are underway for improvements and new capabilities for the GBT 40-50 GHz (Q-Band) and 26-40 GHz (Ka-Band) receivers, to be accomplished during the summer of 2006. Because of various hardware limitations, the Q-Band performance is less than desired below 41.5 and above 48 GHz. (For a discussion on the current performance, see  http://www.gb.nrao.edu/~rmaddale/GBT/Q-Band/40_52ReceiverPerformance.htm.) During the summer improvements will be made to the mixers, low-noise amplifiers, and other components which should result in good performance from 40-49 GHz. Above about 49.1 GHz, the polarizer axial ratio degrades significantly, and we do not plan to replace the polarizers at this time. When high-frequency observing ends in May we have plans to accomplish several receiver improvements. http://wiki.gb.nrao.edu/bin/view/Projects/QbandUpgrade06 and http://www.gb.nrao.edu/~rmaddale/GBT/Q-Band/QBandScienceCase.htm describes the project and its science justification.

The Zpectrometer (http://wiki.gb.nrao.edu/bin/view/Projects/ZpectProject), an ultra-wideband spectrometer (14 GHz instantaneous bandwidth, 825 spectral channels, two polarizations), is under construction at the University of Maryland and is targeted specifically for use with the GBT Ka-Band receiver. Some relatively minor modifications to the receiver will be accomplished over the summer to provide interface ports for the Zpectrometer. System testing with the receiver and spectrometer is also anticipated before the receiver is reinstalled this fall.

You can find the plans and timescales for these upgrade and repairs at: (http://wiki.gb.nrao.edu/bin/view/Projects/HFsummer06 . We're nearing the completion of two important pieces of equipment: a test dewar for the Q-band upgrade and the Lab Spectrometer, a mini version of the GBT Spectrometer and a very important piece of test equipment for solving the baseline problems. The plans have the receivers being mounted on the telescope in September. Since the receivers will have had a complete reworking, we anticipate the need for a full recommissioning next fall. We're hoping that the previous lab tests with the Lab Spectrometer will reduce the length of the on-telescope commissioning. The receiver commissioning will be in competition with Zpectrometer and Penn Array commissioning for high-frequency telescope time. Thus, we don't expect to finish the recommissioning of the receivers until late November. If the commissioning happens faster than this, we plan on scheduling the over 400 hours of Q- and Ka-band proposals that have been on hold and waiting for these improvements.

Both of these receivers are two-beam, dual-polarization with cooled feed horns, polarizers, and HFET amplifiers. The Q-Band is a conventional heterodyne architecture, while the Ka-Band is a correlation architecture designed for optimum continuum performance (but also with spectral line capability).

R. D. Norrod,  D. S. Balser, and R.J. Maddalena

-- RonMaddalena - 08 May 2006