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A.B.Berlin, G.M.Timofeyeva
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 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. 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:
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].
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
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 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 
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. AcknowledgementsThe 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
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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 
F = 6 GHz
= 1 sec,
TSYS = 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.