Status of the New Receivers

Eugene Lauria

There has been a lot of activity in the Gregorian feed module lately, and I am pleased to say that on the 1st of November we were able to receive "first noise" from the L-band receiver and obtained a spectrum using the new correlator (see box, next page). We were able to observe the galactic plane pass through the still-unfocused beam of the new reflector system! Since we are installing receivers into the feed module, this is a good opportunity to review the status of the receivers that will be in place for the initial observations with the upgraded telescope. There are six receivers in this first group: 430 MHz, 610 MHz, 1.0-1.73 GHz (L-band), 2.38 GHz (S-band), 4-6 GHz (C-band), and 8-10 GHz (X-band). All of these receivers are dual channel and all except for the 610 MHz are cryogenically cooled.

This hydrogen line emission profile was observed with the Gregorian optics on November 1, 1996, following installation of the tertiary reflector. This is a classic "first photons" result, warts and all. The system was (and still is) two feet out of focus, awaiting final tensioning of the platform support cables. No baseline correction or gain profile has been applied. An interference spike is visible at 1419.8 MHz, just to the left of the hydrogen emission. The L-band receiver was still at room temperature. Nevertheless, it demonstrates the successful operation of the receiver, IF, LO, fiber optics, and correlator on the first day possible; power for the Gregorian electronics was hooked up only that morning.

The 430-MHz receiver is the smallest receiver of the group. Its dewar package weighs about 60 lbs. It is small in size because the orthomode is outside the dewar. The dewar only holds the cryogenic low-noise amplifiers (LNAs). These LNAs were manufactured by Berkshire Technologies, and are specified to have very good return loss, which eliminates the need to cool a cumbersome isolator that would deteriorate the noise performance. The measured equivalent noise temperature of this receiver is 6 K, which is far below the 20-K minimum sky temperature. The feed horn used in this system is a dual-mode type, which greatly reduces its size. Even so, its mouth is 3 ft in diameter and it stands 5 ft tall! The orthomode was designed by Jon Hagen (NAIC), and is a unique design that is much more compact than the standard orthomode turnstile (OMT) used in the carriage house. However, it is not a hybrid type, so its output is a dual linear polarization as opposed to the left and right circular polarization for a hybrid orthomode. The instantaneous bandwidth of this receiver is 10%.

The 610-MHz receiver is the most basic in this initial group. It simply consists of a feed and room-temperature LNAs. The feed, however, is a unique disk-ring type, consisting of a crossed dipole (for both linear polarizations) spaced a quarter wavelength in front of a ground plane, and a ring in front of the dipoles. This arrangement improves the symmetry of the beam in the H-plane. Per-Simon Kildal (Chalmers) initially designed it, and it was tested by Hilmer Reyes (Cornell) as an M. Eng. project. It is a nice compact design, and its bandwidth is no less than that of the dual-mode horn, which is about 10%. The amplifiers for both polarizations work at room temperature and have an equivalent noise temperature no worse than 43 K. The output of this receiver will either go straight into the IF/LO monitor system to monitor RFI, or go through a tunable bandpass filter for astronomical observations. This receiver is basically a prototype. The design of the next 610-MHz receiver will make use of the knowledge gained from the observations made with this system. We plan to install this receiver in December.

The L-band receiver will be one of the most heavily used receivers at the Observatory. It has the largest instantaneous bandwidth of all of the receivers thus far. This receiver was inherited from NASA JPL, where it was used in the SETI program. It employs a quad-ridged, four-wavelength-long OMT, which is strapped to the first stage of the cold head to reduce its insertion loss. This receiver also has a pre-cool circuit that allows the OMT to reach thermal equilibrium in about 8 hours. Without the circuit it takes about 40 hours for the OMT to reach thermal equilibrium. The feed horn for this receiver is very compact. It is only 2 ft in diameter, and weighs only 52 lbs. Its instantaneous bandwidth is about 1.8:1. Noise temperatures were measured in Ithaca and an average equivalent noise temperature of 12 K was obtained when RFI was absent. This receiver has been installed and is ready for operation. We made a quick-and-dirty Y-factor measurement for the warm receiver, which yielded an equivalent noise temperature of 88 K.

Measured receiver noise temperatures for L-band.

The dual-channel S-band maser that was used with the old planetary radar system is being refurbished for use with the new system. This receiver has a bandwidth of approximately 80 MHz centered on 2,380 MHz and an input flange equivalent noise temperature of 4 K. The system temperature should be close to 20 K.

The C-band receiver is virtually identical to the L-band receiver. It uses the same type of OMT and feed horn, and is cooled in the same fashion. The only difference is that a pre-cool circuit is not necessary and the receiver reaches thermal equilibrium in 8 hours. Its equivalent noise temperature was measured to be around 12-13 K at midband, and about 20 K at the low end of the band. The higher noise level at the bottom of the band is due to the marginal performance of the cryogenic isolators at the lower frequencies. The mounting bracket for the receiver is in place and awaits the installation of the receiver. The feed horn has been sent out to be Alodined to prevent corrosion. The receiver should be installed in December.

Measured receiver noise temperatures for C-band.

Lastly there is the X-band receiver. Due to its narrower bandwidth it is able to employ a stepped-septum hybrid orthomode. This allows for left-hand and right-hand circularly polarized signals to be obtained at the output of the dewar. Our initial receiver noise temperature measurements yielded 25 K at the band edges and 15 K at mid-band. We believe that the high variance in the temperature was due to vignetting of the beam from the window of the dewar. We have since revised the design to have the feed horn mounted outside the dewar rather than inside. Noise temperature measurements will be repeated once the changes have been applied to the receiver. These modifications should improve the system performance. This receiver should also be ready to be installed in December.

Future Plans

Currently we have a disassembled 1.7-2.6-GHz (S-band) receiver that is identical to the L-band system. This receiver was also inherited from JPL, and will probably be the next receiver to be assembled. A new 2.4-4-GHz receiver will then be assembled, and finally a dual-beam 6-8-GHz receiver with an internal Dicke switch for continuum measurements will be built. Also in the future we plan to focus on incorporating multiple-beam receivers for the upgraded telescope.

NAIC/AO Newsletter No. 20 - 2 DEC 1996

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