The GMRT receiver system uses the SUPERHETERODYNE principle for processing the data. Hence cosmic signals are to be converted into suitable frequency band called Intermediate Frequency (IF). The IF serves the purpose of transmission of observed signals from antenna base to Central Electronics Building (CEB). The conversion also helps in the transmission through Fiber Optic Cable. The conversion of RF signals is done with the help of Local Oscillator (LO System) frequencies, which are generated at the antenna base using standard reference generated at CEB and transmitted frequencies. The two channels of IF band are transmitted from antenna base to CEB and are given to  Base Band for further processing.



The Analog Backend system of the GMRT is the penultimate system of processing the data. The Analog Backend System has been divided into Intermediate Frequency System (IF System), Local Oscillator Synthesizer (LOS) and Local Oscillator Reference (LOR) System and Base Band System.



1. Local Oscillator Reference

Max Freq Deviation : +/- 2KHz (10V)
Spurious Levels : -50dBc
Harmonic Levels : -70 dBc
Phase Jitter : 1 nS
Phase Noise : -90 dBc/Hz at 100Hz offset


2. Local Oscillator Synthesizer

Some important features of the LO Synthesizer are mentioned below:

Harmonics: Better than -20dB
Spurious: Better than -60dB

Phase Noise: Better than -60dBc/Hz at a typically 10kHz offset, which corresponding to a peak-to-peak phase jitter of better than 0.1 degrees, in time scales of the order of 0.1 mSecs.

50MHz Modulation: Better than -20dBc

Lock Range: Guaranteed at Very Important Frequencies(VIF) and also from
-1.2*Fmax of the Oscillator


3. Intermediate Frequency System

Important features if the IF system are given as follows:

1. Total Power Dynamic Range (ALC OFF): 28dB
2. Total Power Dynamic Range (ALC ON): 32dB
3. Steps in which gain should be varied: 2dB
4. Total Range of Gain Flexibility: 65dB

The power dynamic range is limited by the voltage controlled amplifier IVA05208 and ALC ON range is limited by MAR-6 amplifier.


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