NEW MILLIMETREWAVE RECEIVERS AT NRAO

James W. Lamb', John M. Paynel, and Nancyjane Bailey2

ABSTRACT

A new generation of low-noise, dual-polarisation rece'ivers covering theatmospheric windows between 65 GHz and 300 GHz is nearing completion. Thesereceivers use niobium junction SIS mixers which have proven to be very sensitiveand reliable. A modular approach has been used and each receiver channel isconstructed as an insert which is mounted in a stainless-steel dewar cooled by aclosed-cycle 4K refrigerator. Several frequency bands may be accommodated in asingle package. Currently, a 65-116 GHz receiver with noise temperaturestypically <1OOK SSB, a 130-170 GHz with noise temperatures typically <200K, anda 200-270 GHz receiver with noise temperatures typically <200K SSB areoperational. A 260-300 GHz insert is under construction.

INTRODUCTION

A suite of new millimetrewave receivers for radio astronomy is being completed forthe 12--m telescope on Kitt Peak, Arizona. These receivers use modular insertswhich cover the atmospheric transmission bands, divided up as follows: 65-90 GHz,90-116 GHz (A 3mm band); 130-170 GHz (A 2mm band); 200-270 GHz, 260-300 GHz (A lmmband). Each band has two inserts so that two linear polarlsations may bedetected. Currently the three bands are in separate packages but eventually theinserts for the A 2mm band will be incorporated in the A 3mm package, the "low-frequency receiver", Figure 1. The "high-frequency receiver" is shown in Figure2.

All the receivers may be operated in single-sideband mode, since the density ofspectral lines at these frequencies makes it likely that lines in the unwantedsideband would be folded over into the desired sideband. It also reducesatmospheric and antenna noise in the image sideband.

CRYOGENICS

Large stainless steel dewars (600 mm in diameter) are used to hold theelectronics. A commercial refrigerator is used to cool a radiation shield toabout 70 K. It also pre-cools the He gas in a Joule Thompson 4K refrigeratorbuilt by NRAO, which has an unloaded capacity of greater than 1W at 4.5K. Thereare eight ports on the bottom of the dewar into which receiver units may beinserted. Ultra pure copper straps connect the inserts to the 4K station.

The vacuum windows above each insert are 75mm in diameter and made from 25pm thickMylar sheets supported on a 30mm thick low-density foam backing (1). An infraredfilter is mounted on the radiation shield below each window. These are made fromPTFE which has low loss at millimetre wavelengths but is opaque to roomtemperature thermal radiation. It turns out that much of the infrared blockingis due to the foam window since its inner surface is radiatively cooled to around200K (2).

'National Radlo Astronomy Obser-vatory, 949 N. Cherry Ave., Campus Bldg.65, Tucson, Arizona 85721-0655, USA

2National Radio Astronomy Observatory, 2015 Ivy Road, Charlottesville,Virginia 22903-1797, USA

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The cryogenic systems have proved to be very reliable on the telescope. Cool-downtimes range from 11 hours for the high-frequency receiver with two inserts to 17½hours for the low-frequency receiver with four inserts.

RECEIVER INSERTS

Each insert comprises a complete receiver channel. There are differences indetail between bands, but the general design is the same. The signal is focusedby a high-density polyethylene lens into a corrugated horn. It was found that thelens dielectric constant changed on cooling. A correction based on the Lorentz-Lorenz relation (3) relating dielectric constant and density was applied using theknown contraction between 300K and 4K. This was found to agree with experimentalvalues (4). The change in linear dimensions of the lens was also corrected for.LO power is injected through a waveguide branch line directional coupler betweenthe horn and the mixer.

All of the mixers use arrays of six niobium junctions which have proved to be veryrugged and stable. The low-frequency receiver mixers may be adjusted to rejectthe image frequency, typically with > 20dB rejection. In the high-frequencyreceivers the image rejection is achieved optically, as described later. Eachmixer has two remotely adjustable backshorts to optimize the tuning at therequired observing frequency. The mixers have been described in more detailelsewhere (5-8).

A bias-T provides a D.C. connection and passes the 1.5 GHz (600 MHz bandwidth) IFsignal through an isolator to a cooled HFET amplifier with about 35 dB gain and3K noise temperature (9). All of the above components operate at about 4.5Kphysical temperature, and the 70K station is used only to heat sink waveguide,cables, and infra-red filters.

LOCAL OSCILLATOR

One Gunn oscillator (10) is used for each mixer pair. These are phase-locked tothe telescope frequency standard. For the 65-90 GHz and 90-116 GHz bands, theGunn output is split between the two polarisations and passes through a ferritemodulator which servos the LO power to maintain constant mixer current. In the130-170 GHz band the Gunn output is doubled before being split to two polarisationchannels via ferrite modulators. Since modulators are not available in the 200-300 GHz bands, the Gunn output is first split for the two polarisations andcontrolled by the modulators before being multiplied by a tripler in eachpolarisation. For this band overmoded guide carries the signal in to thedirectional coupler.

OPTICS

Two optical arrangements are used, one for the range 65-170 GHz and one for 200-300 GHz.

Low frequency: A crossed-grid is used to split the orthogonal polarisations tothe two channels. (Fig. 1) The crossed-grid is made in three parts. One sectionhas a rectangular frame with wires wound parallel to the long side, while theother section is made from two U-shaped frames with the wires running across the"U". These are brought up perpendicular to the first grid on either side.

Each polarisation is then focused by an offset paraboloid to produce a frequency-independent beam-waist at the lens in the insert. This lens then matches the beamto the curvature of the horn field. A signal may be injected into the beam abovethe grid via a mirror. The signal is produced by a 3-GHz oscillator and aharmonic mixer which can be adjusted to give a test signal in each sideband.These are used to tune the 65-116 GHz for single sideband operation with typically

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> 20 dB rejection. The complete optics assembly may be rotated to select therequired mixer pair.

High frequency: Again a crossed-grid is used for polarisation diplexing (Fig. 2).Each polarisation then passes through a Martin-Puplett diplexer and down into thereceiver insert in the dewar. When the diplexer is set to zero path difference,the feed is coupled to the antenna in both sidebands. Setting the diplexer pathdifference to AIF/4 allows only one sideband to enter from the antenna, and theimage sideband comes in through the other port of the diplexer. To minimize thenoise injected at the image frequency, this port is terminated by a cold load onthe 4K station of the dewar and a spherical mirror is used to focus the beam fromthe diplexer through the dewar window. Receiver tuning is aided by having arotating chopper with absorbing vanes which is used to give a signal against thecold sky; the noise temperature can be measured at the observing frequency andminimized by tuning.

The sideband filter is calibrated in the lab and a look-up table is used fortelescope observation. A test tone generator based on a digitally tuned 5-GHzoscillator and harmonic mixer is being constructed to allow direct checks to bemade on the telescope. It is also possible, by removing a grid and replacing thecrossed-grid with a plane mirror, to use the diplexer as a circular polariser forone channel. This has been used for VLBI observations.

MONITOR AND CONTROL

All mechanical tuning adjustments are provided wlth D.C. servomechanisms. Controland monitoring of bias voltages, tuning adjustments, temperatures, etc., are donethrough a purpose-built data link communicating on an IEEE-488 bus. The bus islinked to a personal computer in the control room via a fiber optic link. Alladjustments may be done manually through the computer, and automatic tuningprocedures are being implemented.

PERFORMANCE

All the receivers have proved to be very reliable and stable. Noise temperaturesare shown in Figure 3. In the 65-116 GHz band they are as low as 60K SSB, andtypically 85K SSB. Preliminary measurements in the 130-140 GHz band givetemperatures around 60K DSB. Single sideband temperatures are between about lOOKand 280K. The SIS junctions in the mixers are designed for the X3mm band andsignificant improvements are anticipated when A2mm band junctions are installed.For the 200-270 GHz band, we get about 90K DSB up to 230 0Hz, rising to 250K DSBat 265 GHz. In single sideband mode, the noise temperatures are less than 200KSSB to 230 GHz, rising to 500K SSB at 265 GHz. At virtually all frequencies,rejection of the 'image frequency is greater than 20dB, and sometimes as high as35dB, at band centre. Spectral baselines are very good, and deep integrations ofsignals with a few millikelvin antenna temperature are achieved.

ACKNOWLEDGEMENTS

The success of the new receivers is due in large part to the efforts of: J.Cochran and J. T. Fitzner for construction of the JT refrigerator, compressors,and LO circuit; G. Taylor and M. Hedrick for machining the insert parts and dewar;D. Boyd for assembly of the inserts; A. A. Perfetto and E. Kemp for design andconstruction of the electronics; and J. S. Kingsley for much of the mechanicaldesign.

REFERENCES

[I] A. R. Kerr, N. J. Bailey, S.-K. Pan, and D. E. Boyd: "A broadband millimeterwave quasi-optical vacuum window, " National Radio Astronomy Observatory,Electronics Division Internal Report, in preparation.

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[2] J. W. Lamb: "Infrared filters for cryogenic receivers," National RadioAstronomy Observatory, Electronics Division Internal Report No. 250, April1992.

[3] M. Born and E. Wolf: Principles of Optics, Oxford: Pergamon, 1970.

[4] J. R. Birch and K. F. Ping: "An interferometer for the determinatlon of thetemperature variation of the complex refraction spectra of reasonablytransparent solids at near-millimetre wavelengths", Infrared Phys. , vol. 24,no. 213, pp. 309-314, 1984.

[5] S. -K. Pan, M. J. Feldman, A. R. Kerr, and P. Timbie: "Low-noise 115-GHzreceiver using superconducting tunnel junctions," Appl. Phys. Lett., vol.43, no. 8, pp. 786-788, 15 Oct. 1983.

[6] A. R. Kerr, S. -K. Pan, and M. J. Feldman: "'Integrated tuning elements forSIS mixers, Int. J. Infrared Millimeter Waves, vol. 9, no. 2, pp. 203-212,Feb. 1988.

[7] A. R. Kerr and S. -K. Pan, "Some recent developments in the design of SISmixers," Int. J. Infrared Millimeter Waves, vol. 11, no. 10, Oct. 1990.

[8] A. W. Lichtenberger, D. H. Lea, R. J. Mattauch, and F. L. Lloyd: "Nb/Al-A1203/Nb junctions with inductive tuning elements for a very low noise 205-250 GHz heterodyne receiver, " IEEE Trans. Microwave Theory Tech., vol. MTT-40, no. 5, pp. 816-819, May 1992.

[9] J. D. Gallego and M. W. Pospieszalski: "Design and performance ofcryogenically-coolable, ultra low-noise, L-band amplifier", Proc. 20thEuropean Microwave Conference, pp. 1755-1760, Budapest, Hungary, Sept. 1990.

[10] J. E. Carlstrom, R. L. Plambeck, and D. D. Thornton: "A continuously tunable65-115 GHz Gunn oscillator, IEEE Trans. Microwave Theory Tech. , MTT-33, pp.610-619, 1985.

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