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MARS ( MATRIX RADIOMETRIC SYSTEM ) PROJECT

A.B.Berlin, G.M.Timofeyeva
St.Petersburg Branch, Special Astrophysical Oservatory of Russian Academy of Sciences
65 Pulkovskoye Shosse, St.Petersburg, 196140, Russia
E-mail: abb@fsao.spb.su

N.A.Nizhelsky, A.V.Bogdantsov
Special Astrophysical Observatory of Russian Academy of Sciences
Nyzhny Arkhyz, Karachai-Cherkessia, 357147, Russia
E-mail: nizh@rs.ratan.sao.ru

O.M.Pylypenko, V.M.Chmil', Yu.N.Meshkov, A.N.Zdor
"Saturn" Scientific and Productive Open Join-Stock Company
2B, pr.50 Richya Zhowtnya, Kyiv, 252148, Ukraine
E-mail: alr@jssaturn.kiev.ua

MARS ( MAtrix Radiometric System ) project is reported. The instrument is intended to be used as a possible radiometric support to the "Cosmological Gene" project. MARS is planned to be installed in the focal line of the RATAN-600 radio telescope.

An input signal of two orthogonal linear polarizations is transferred via orthomode transdusers, 128 units in line, to 256 radiometric modules. Each module of the system is an independent noise-added, gain-balanced straight-amplifier radiometer. First prototype models of the radiometric module and of the noise generator unit design and test results are presented. MMICs and HEMT chip transistors as active components were applied.

For one radiometric unit, 210K noise temperature, 65 dB gain and 6 GHz bandwidth at 30 GHz center frequency were measured.
1. Introduction

An evaluation was undertaken to choose the appropriate radiometric support solution for the "Cosmological Gene" project[1] for the RATAN-600 radio telescope [2,3]. Two concepts were selected for final comparison: a beam-switched, cryogenically cooled down to 15K radiometer with a quasi-optical feed system and an uncooled matrix concept.

The latter ( matrix ) concept was chosen for a combination of reasons, among them a greater application versatility and future opportunity to enhance sensitivity considerably by applying some kind of cryocooling.
2. Design goals and basic concept
The MAtrix Radiometric System ( abbreviation: MARS ) project is intended to learn what can be done by comparatively simple means to provide a radiometric support for the RATAN-600 the "Cosmological Gene" project.
Main goals of the MARS project are as follows:

• Implementation of radiometric sensitivity, for each of two orthogonal linear polarizations, less than 1 mK ( 1 sec integration time ) for the central frequency 30 GHz and bandwidth up to 6 GHz.
• The focal line up to 1750 mm long can be used.
• It might be as well to use commercially available components only.
• No cryocooling needs to be applied at the first step of design.
• It is necessary to accomplish an interchangeability of the building blocks of the matrix.
• Compatibility with standard laboratory microwave test equipment must be provided to verify all basic parameters of the matrix components ( frequency band, gain, noise temperature, feed pattern ).
• Maintenance and reliability problems have to be taken into account to to ensure the continuous operation of the system during up to five years .

Figure 1. MARSproject building block concept

The idea of the design is shown in Fig. 1. Quasi-scalar input feeds ( 128 units ) and orthomode transdusers ( OMT ) following after then are placed along the RATAN-600 focal line with a 13.5 mm spacing.

Two MARS elementary radiometers are connected to each OMT, 256 radiometric modules ( RM ) in all. Each elementary radiometer is of single beam, noise-added, gain-balanced mode of operation. From the point of view of theoretic sensitivity, this mode of operation may be considered as being quite close to the switched mode and, like the latter, enables the gain instability to be avoided [3].

We can consider as an advantage of the noise-added mode the fact that input feeder is absolutely free and has no lossy active devices. We are not going to use the total power mode to avoid the "knee frequency" problem. The mode of operation accepted is very sensitive to input noise temperature instability ( like the total power mode ), but, it is known from our experience, this problem could be overcome by means of very good power supply and thermostat regulation ( better than 10^-3 ).

Noise-adding and calibrating signals are fed to the input feeds of the matrix by means of open-space coupling from the source placed at the secondary mirror surface at a distance of about 2.5 meters from the focal line. Open-space coupling of the noise-adding signal was successfully applied for radiometry previously [4].

The working cycle of the noise-added, gain-balanced radiometer is locked-in to the LF square wave driving voltage. During one half-period of the driving voltage the noise-adding generator is turned-off and the gain modulator ( which is controlled attenuator actually ) is open ( minimal loss state ). During the other half-period of the driving voltage the noise-adding generator is turned-on and the gain modulator is closed.

In this connection, noise-adding signal at the input feed is of the order of T_SYS, and gain modulator ( attenuator ) has losses of the order of 13 dB. The balance, usually performed before the observing session, means, that there is no difference in the post-detector output signals for these two half-periods, which can be accomplished by fine tuning of the modulator ( attenuator ) loss for "closed" state around 13 dB level.
3. Active modules design

3.1. Radiometric module

Each radiometric module ( RM ), one of 256 identical units, is of straight amplifier design, without frequency conversion. The structure of the RM is shown in Figure 2.


Figure 2. Radiometric module configuration

The RM consists of ( sequentially ): WR28 to microstrip transition, LNA, gain modulator, high frequency amplifiers, band-pass filter, detector and low frequency amplifier. The LNA is of three-stage hybrid technology design with EC2612 United Monolithic Semiconductors PHEMT transistor chips. The matching circuits are made by thin-film technology on a .2 mm thickness fused quartz substrate. The capacitive matching tuners were used for the fine tuning of the amplifier. The LNA parameters: frequency band is 27-33 GHz, gain 23 dB, gain ripple 1.5 dB, input/output VSWR is 1.9/1.5, equivalent noise temperature, including waveguide to microstrip loss and averaged for full bandwidth, is 190 K. The LNA noise budget, taking into account the LNA components contribution ( calculated ), is shown in Table 1.

Table 1. LNA noise budget

	Waveguide to Microstrip Transition	Input Matching Network	First Stage Transistor	Subsequent Stages
Transmission Coefficient (Loss,Gain),dB	-0.3	-0.3	8.0	14.3
Inherent Noise Temperature,K	21	21	110	180
Noise Contribution to the Equivalent Input Noise Temperature,K	21	23	126	33
Equivalent Input Noise Temperature of the LNA,K (averaged for F = 6 GHz)				190

The controlled attenuator MMIC HMMC-1002 of the Hewlett-Packard is used as the gain modulator with TTL driving square-wave voltage and with additional DC ( 0...5 V ) control voltage for the modulation depth trimming ( i.e., balancing of the radiometer ).

The dependence of the wavefront steepness of the modulated signal envelope on the attenuator chip illuminance was detected. To make the fronts of the envelope faster, backlighting of the attenuator chip by IR LED chip was introduced in the design.

Two MMICs, connected in series, were used as the high frequency amplifiers. We tested HMMC-5040 of the Hewlett-Packard and AA038N1-00 of the Alpha Ind. successfully. The band-pass filter incorporates seven half-wave transmission line resonators, made by thin-film technology on a .2-mm thickness fused quartz substrate and placed into the channel of 3 mm by 1.5 mm cross-section. The detector applied is a zero bias beamlead GaAs Schottky diode of Hewlett-Packard.

3.2. Noise generators unit

The noise generators unit serves as a noise power source for the noise-adding and calibration purposes. The unit includes two identical noise generator ( NG ) modules. The NG-1 module provides modulated noise-adding power to the feed inputs of all radiometric modules via open space coupling. The NG-2 module provides calibrating noise power, which can be switched on and off according to the observing session program and is fed to the same emitter, as the NG-1, via a waveguide directional coupler. The structure of the unit is shown in Figure 3.


Figure 3. Noise generators unit structure

The noise generators modules NG-1 and NG-2 are of identical design. Two MMIC amplifiers, connected in-series, ( of the same type, as for radiometric module ) with matched load at the input represent the amplified-noise generator proper. The same attenuator chip, as for the RM, is used as the control device for both modules. It plays the part of a 100%-modulator for the noise-adding module and of an on/off switch for the calibration module with TTL control.

4. Results

The radiometric and the noise-adding modules were composed at a laboratory to form a noise-adding radiometer. The bandwidth-averaged noise equivalent temperature of the device was measured radiometrically using hot ( ambient temperature ) and cold ( liquid nitrogen ) loads; the result is T_N 210 K.

This yields a sensitivity ( = 1 sec, T_SYS = 260 K, F = 6 GHz ) of order of 5.5 mK, and for one polarization with all the matrix about 0.5 mK. We hope that a more sophisticated design of the LNA can reduce its noise. Further progress can be achieved with LNA cooling down to, let us say, the liquid nitrogen temperature.

Photographs of the radiometric module and of the noise generator module are shown in Figure 4 respectively.



Figure 4. Photograph of the radiometric module and of the noise generator module

5. Conclusion

The MARS project concept was proposed and evaluated as a radiometric matrix decision for 1 cm wavelength with a 6 GHz bandwidth. The prototype of the noise-added, gain-balanced radiometer developed from radiometric and noise-adding modules was tested and 210 K noise temperature was measured, which yields one polarization matrix ( 128 elements ) sensitivity of about 0.5 mK.

Acknowledgements

The authors wish to thank Professor Yu.N.Parijskij for first calling our attention to the MARS project and for his encouragement and support. We would like to thank also Yu.N.Konovalov and S.S.Yermolenko for their suggestions concerning the OMT mechanical design and assembly and for their expertise in manufacturing of the OMT mechanical parts.

This work was partially supported by grants of INTAS, # 97-1192, RFBR, # 99-02-17114, CCPP "Cosmion", "Astronomy" program # 2.1.2.7.

References

• 1. Yu.N.Parijskij " New generation CMBA experiment "Cosmological Gene" at RATAN". In this issue. See also: http://www.sao.ru and http://brown.nord.nw.ru
• 2. Yu.N.Parijskij "RATAN-600: the world biggest reflector at the "cross roads". IEEE AP Mag. Vol.35, No.4, pp.7-12, Aug. 1993.
• 3. Yu.N. Parijskij, D.V. Korolkov "Experiment Cold: the first deep sky survey with the RATAN-600 radio telescope". Soviet Scientific Reviews/Section E. Astrophysics and space physics reviews, ed. by R.A. Syunyaev, Harwood Academic Publishers GmbH, vol.5, pp.39-179, 1986.
• 4. A.E. Wright et. al. " A novel noise-adding radiometer". Proc. ASA 6, (4), pp.512-516, 1986.