Receiver System

The GMRT currently operates at 5 different frequencies ranging from 150 MHz to 1420 MHz. Some antennas have been equipped with receivers which work up to 1750 MHz. Above this frequency range however, the antenna performance degrades rapidly both because the reflectivity of the mesh falls and also because of the rapidly increasing aperture phase errors because of the deviations of the plane mesh facets from a true parabola. A 50 MHz receiver system is also planned. Table 18.1 gives the relevant system parameters at the nominal center frequency of the different operating frequencies of the GMRT.

Table 18.1: System parameters of the GMRT

System Properties				in MHz
	50	153	233	327	610	1420
Primary beam (degree)		3.8	2.5	1.8	0.9	0.4$\times$(1400/f)
Synthesized beam
Full array (arcsec)		20	13	09	05	02
Central array (arcmin)		7.0	4.5	3.2	1.7	0.7
System temperature (K)
(1) $T_{\rm receiver}$		144	55	50	60	40
(including cable losses)
(2) $T_{\rm ground}$ = $T_{\rm mesh}$ + $T_{\rm spillover}$		30	23	18	22	32
(3) $T_{\rm sky}$		308	99	40	10	4
Total $T_{\rm sys}$		482	177	108	92	76
= $T_{\rm sky}$ + $T_{\rm receiver}$ + $T_{\rm ground}$
Gain of an antenna (K/Jy)		0.33	0.33	0.32	0.32	0.22
RMS noise in image$^{\star}$ ($\mu$Jy)		46	17	10	09	13

$^{\star}$For assumed bandwidth of 16 MHz, integration of 10 hours and natural weighting (theoretical).

The GMRT feeds, (except for the 1420 feed), are circularly polarized. The circular polarization is achieved by means of a polarization hybrid inserted between the feeds and the RF amplifiers. No polarization hybrid was inserted for the 1420 MHz feed, in order to keep the system temperature low. None of the receivers are cooled, i.e. they all operate at the ambient temperature. The feeds are mounted on four faces of a feed turret placed at the focus of the antenna. The feed turret can be rotated to make any given feed point to the vertex of the antenna. The feed on one face of the turret is a dual frequency feed, i.e. it works at both 233 MHz as well as 610 MHz.

After the first RF amplifier, the signals from all the feeds are fed to a common second stage amplifier (this amplifier has an input select switch allowing the user to choose which RF amplifier's signal is to be selected), and then converted to IF. Each polarization is converted to a different IF frequency, and then fed to a laser-diode. The optical signals generated by the laser-diode are transmitted to a central electronics building (CEB) by fiber optic cables. At the central electronic building, they are converted back into electrical signals by a photo-diode, converted to baseband frequency by another set of mixers, and then fed into a suitable digital backend. Control and telemetry signals are also transported to and from the antenna by on the fiber-optic communication system. Each antenna has two separate fibers for the uplink and downlink.