A low-noise receiver covering the 40-52 GHz band has been on the GBT wishlist for several years. The scientific case for such a receiver has been presented by Al Wootten (GBT Memo 109) and others. In GBT Memo 141, E. Wollack and S. Srikanth proposed a specific configuration for the initial GBT receiver in Q-band. A receiver similar to that proposed is under construction, and nearing completion.
An earlier report described a one-beam version of this receiver and results of tests.
The receiver provides four dual-polarized beams covering the frequency range 40-52 GHz. Cooled to 20 Kelvin are corrugated feedhorns, sloped-septum polarizers (for circular polarization reception), four-stage InP "MAP/VLA" low-noise amplifiers, and balanced mixers with output frequency range of 4-8 GHz. The mixer LO is obtained from the GBT system synthesizer and X4 multipliers (one for each beam). Room temperature IF amplifiers and filters are incorporated in the receiver front-end to provide sufficient signal level to drive the standard GBT IF system.
The receiver is complete and tested. Preliminary commissioning on the GBT was done in the Spring of 2003. Commissioning will be completed in the Fall of 2003.
Four cryogenic RF
assemblies are shown sitting on the dewar 20K station. Each assembly
consists of a profiled corrugated feedhorn, an Atlantic Microwave
polarizer which separates the left and right polarizations, and two
identical channels consisting of: noise cal couplers, an isolator, an
HFET amplifier, an image-reject filter, another isolator, and a mixer.
Also included are power splitters for the noise cal and mixer LO
signals.
Another view showing
the feedhorn corrugations. The feedhorns will illuminate the GBT
subreflector through a low-loss microwave vacuum window which sits
just above the horns. The noise cal and LO signals enter the dewar
through stainless-steel WR22 waveguide for thermal isolation. The
eight 4-8 GHz IF signals exit through 0.085 stainless-steel coax.
The room-temperature
components associated with the front-end mount below the cooled dewar.
Monitor and control of the receiver is via the NRAO serial MCB
interface card located in the shielded box near the bottom of the
photo. Bias cards and other printed circuit boards will mount in the
cardcage next to the shielded box.
Acknowledgements: Engineers who have contributed to this receiver system include E. Wollack, S. Srikanth, W. Grammer, R. Norrod, G. Watts, and G. Anderson. L. Beale did the assembly work on the dewar. The HFET amplifiers were designed and constructed by the NRAO Central Development Lab, under supervision of Gerry Petencin.
