*In Proceed. of ESA Workshop on Millimetre Wave Technology and Applications, Espoo, May, 1998
 
 
MMIC SOLUTION FOR MULTI-PIXEL RECEPTION IN RATAN-600 ABERRATIONLESS FOCAL ZONE*
 
  
KEYWORDS: RADIO TELESCOPE, MMIC, MULTI-PIXEL RECEPTION,
FOCAL-PLANE ARRAY.

  1.INTRODUCTION

 
The significant progress in MMIC amplifier technologies in MM band (Weinreb and Chao, 1997) gives us a chance to fully realize an important RATAN-600 radio telescope advantage - wide aberrationless focal zone L in Zenith (Parijskij at al.,1997) and Radio-Schmidt (Khaikin, Majorova, Chukhlebov, 1997) modes. This seems possible even now when MMIC amplifiers are far from being perfect and cannot be widely used in high sensitive radio astronomical receivers.

 
2.MULTI-PIXEL RECEPTION IN RATAN-600 CASE

 
To fully sample the cosmic source distribution we must use a spacing equal to or less than Nyquist sampling interval in the telescope focal plane:

 

where f-focal distance, D- aperture size. In RATAN-600 case f/D is close to or less than 1. For an effective RATAN-600 illumination the minimal receiving element feed size is at least twice more in a waveguide horn solution. An attempt to improve a radiating pattern and decrease side lobe level will extend more the feed size. We can instead consider the dipol type feed which has smaller size and is almost free from spillover effects. However an attempt to achieve a closely packed multi-feed system will reduce the sensitivity of each pixel (Whyborn,1993). Really elements which work well in isolation will not effectively work when placed close together due to "crosstalk" or mutual coupling(Padmen, 1994). To avoid mentioned effects and reach a factor  in sensitivity a multi-feed construction and spacing dL should provide practically uncorrelated output noise in N channels and high level of input mutual feed isolation (not less than -20 dB).

The field of view of a radio telescope is limitted by aberrations as a receiver element is moved to off-axis positions. Aberrations lead to an efficiency loss and a distorted beamshape. The both field of view and sensitivity of a large radio telescope in MM band strongly depends on a number of independent receiver elements which give undistorted elements of a sky picture - pixels. Multi-pixel reception (up to 500-1000 pixels in RATAN-600 case) may significantly (more than one order) increase RATAN-600 sensitivity and the field of view in survey tasks such as investigation of 3 K Cosmic Microwave Background(CMB) anisotropy at subdegree scales S. It is also the best way to suppress variations of atmospheric radiation with the help of wide-scale beam scanning (Parijskij at al., 1997) or two-pixel subtraction at scales:

 

3.RATAN-600 ABERRATIONLESS FOCAL ZONE

 
RATAN-600 includes the Main mirror (576 m in diameter), 4 Secondary mirrors in a form of parabolic cylinder with the horizontal focal line and one conical Secondary mirror with the horizontal focal plane. Both types of the RATAN-600 Secondary mirrors are well adapted for multi-pixel reception along the focal line or at plane. Our calculations show that RATAN-600 aberrationless zone L along the focal line may exceed 3 meters for the biggest Secondary mirror (12 m size) to study 20'-30' scales of CMB in the range of 30 GHz at high elevation angles. Some ways also exist to expand the available focal zone in cross direction by using more narrow illumination angles and optimal turn of the plane from which the Secondary mirror may be illuminated but this would decrease aperture efficiency almost twice.
 
 
 

 
 
 
Fig1. RATAN-600 power aberration curves.
The widest field of view and longest focal aberrationless zone up to 5 meters may be available for multi-pixel reception (up to 1000 pixels) in Radio-Schmidt mode for any elevation angles H and cosmic scales S up to point sources. In this mode which is under development now RATAN-600 Periscope mirror may be used as a Schmidt corrector removing spherical aberration of the Main mirror (Khaikin, Majorova, Chukhlebov,1997). Power aberration curves for Schmidt and Zenith modes are shown in Fig.1.

RATAN-600 beam patterns with undistorted beamshape in the center and at edges of aberrationless focal zone in Radio-Schmidt mode is shown in Fig.2.
 

 
4.MMIC ARRAY SOLUTION
 
  *Tiny size of completed active elements with HEMTs.
*Full identity of characteristics for MMIC LNAs made from one wafer.
*Cascaded MMIC LNAs do not require additional isolation.
*Human factor is excluded in LNA performance and tuning.
*It is cost effective alternative to hybrid assemblies.   Some types of MMIC focal-plane arrays are presented in (Weinreb, 1994). MMIC array solution may be applied in combination with patch antenna technology well developed now for multi-element phased arrays of MM range(Parnes, 1989). Its main advantages are the following:
*Very big (up to 10000) element number is available.
*Very high technological and cost efficiency.
*Tighter packing (minimal dL) of antenna elements in comparison with
   waveguide technique.
*High reproduction ability.  
Modern materials developed for patch antennas have high enough dielectric constants with low dielectric loss in MM range and give a factor 2-3 to reduce the size of antenna elements and increase their concentration (minimal dL) in comparison with the waveguide solution. The same factor 2-3 is lose in a bandwidth for dipole microstrip radiators but it is a rather difficult task to realize an effective high sensitive reception in the whole waveguide bandwidth even for a single horn not for an array.

 
 
5.MMIC ARRAY PROTOTYPE
 

The prototype of uncooled 8-element MMIC focal linear SUB-ARRAY in a range of 26-30 GHz is under fabrication for RATAN-600 at present. Multi-beam Integral Patch ARray (project "MIPAR") which is under development now can include up to 70-100 8-element SUB-ARRAYs. MIPAR is nonphased focal-plane array with independent inputs and beams. The input of each array element corresponds to a separate radiation pattern. The previous attempt to build 32-element focal phased array for RATAN-600 from ordinary hand-made radiometers (Pinchuk at al.,1993) has shown us the need to use new integral approaches and technologies.

In prototype of SUB-ARRAY rectangular patch radiators and MMIC receiver elements are fed by microstrip lines laying in the plane of radiating sheet. We use Rogers Corp. ceramic filled composite materials with loss tangent 0.0013 and constant 3.02 for the dielectric substrate. Microstrip radiators in the first prototype will receive signal of linear polarization but we plan to receive circular polarized waves with MIPAR as well. Input mutual radiator isolation is provided at -20 dB level. The front-end includes 3 GaAs MMIC LMA-422 of Litton SSD with NF=2.5 and gain G=22 dB in a receiving channel for direct RF amplification. Super low noise HP Schottky square low detectors complete VHF parts. Output mutual channel isolation is provided at -70 dB level. We put a microstrip band-pass filter before detector to limit channel bandwidth to 2 GHz at -3 dB level in an agreement with input radiator bandwidth. One loaded LMA-422 is used as a noise oscillator for a communal input channel calibration. Noise injection is produced through special radiators fed by a distributing microstrip line. A radiometric part of SUB-ARRAY is in the thermostat box. SUB-ARRAY is provided with radio transparent radome. Ultra-low noise high precision AD FET monolithic operational amplifiers are applied in the wideband multi-channel back-end.

We expect System temperature to be 270 K for the array directed to zenith and hope to realize 10-15 mK sensitivity per second in a channel in ordinary "Total power" receiving mode. To remove 1/F noise from a recording and decrease dG/G contribution into sensitivity we want to use a "frequent calibration" switched on each 100 msec during 10 msec with noise level of 10 K. PIN-diode modulator will be used for it. An amplitude and envelope of short noise calibration impulses are measured by quick enough A/D converter and used as a "pilot signal" for a real time two-pixel data treatment in DSP of a digital part. We hope to reach a factor 2 in sensitivity in this mode. Other way is to apply more perfect pilot-stabilized MMIC LNAs that gives a factor 2.5 in sensitivity (Weinreb, 1998). Fast switching or a correlation mode is needed to reach more high sensitivity with MMIC HEMT LNAs today (Weinreb, 1998).

 
6.ANTENNA MEASUREMENT APPLICATIONS

Another exciting possible field of applications for multi-pixel MMIC reception at radio telescopes is millimeter wave antenna technologies including holographic antenna measurement techniques. So multi-pixel (up to 4000-8000 pixels) MMIC receiving systems may be used to get moment holographic surface maps with a high resolution on strong cosmic or beacon satellite sources that gives a chance to correct surface errors practically in real time. It may give a powerful impulse to develop an active and adaptive optics technique at radio telescopes including wave front correction but requires at least one more order in MMIC cost reduction.
 

7.CONCLUSION

This seems possible to build 500-1000 beam MMIC focal array for the radio telescope in the nearest future. It looks as an exciting challenge for all those who develop MMIC technologies and apply them in Radio Astronomy today.

 
REFERENCES

1.Yu.Pariskij at al. "Dark Ages" of the Universe. Proceed. of International School of Astrophysics "D. CHALONGE", 1997 in press.
2.S.Weinreb and P.C.Chao. Full-Waveguide Band, 90 to 140 GHz, MMIC Amplifier Module. Proceedings of IEEE 1997 MTT-S Symposium, Denver,1997.
3.V.Khaikin,E.Majorova,M.Chukhlebov. RATAN-600 as Radio-Schmidt Telescope. In proceed. of XXVII Radio Astronomical Conf., V3, p.52, St.Petersburg, 1997.
4.N.Whyborn. Complete multi-feed systems for radio astronomy. Proceedings of NRAO Workshop No.25,p.117, Tucson, 1994.
5.R.Padman. Optical fundamentals for array feeds.Proceedings of NRAO Workshop No.25, p.3, Tucson, 1994.
6.S.Weinreb.MMIC Millimetre-wave focal plane arrays. Proceed. NRAO Workshop No.25, p.189, Tucson, 1994.
7.G.A.Pinchuk, I.V.Berlizev,A.D.Dibizhev, E.K.Majorova, D.V.Shannikov. The SET-UP of Linear Feed Array for the RATAN-600 Radio Telescope. Bull.Spec.Astrophyz.Obs.,36,138,1993.
8.M.D.Parnes. Experience of the automated designing of patch antenna arrays. Ship-building industry, issue 19, 1989.
9.S.Weinreb. VHF Pilot-Signal Stabilized Monolithic Integral Circuit Millimeter-Wave Radiometers. Presented at the North American URSI Meeting, Montreal, July 18,1998.