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Overview

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.

Introduction

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.

Features

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:

• Total Power Dynamic Range (ALC OFF): 28dB
• Total Power Dynamic Range (ALC ON): 32dB
• Steps in which gain should be varied: 2dB
• 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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Block diagram

Overview

The basic purpose of having two different synthesizer systems is to increase the accuracy of choosing the frequency of the LO Synthesizer. The two different LO Synthesizers allow the users to select the frequency to be generated accurately.

Introduction

There are two synthesizers in the LO Synthesizer system namely as SYNTH1 and SYNTH2. SYNTH1 uses three VCO's and the SYNTH2 uses YTO. Each Oscillator is preceded by its own differential OP-AMP active loop integrator filter with a circuit optimized for the range of oscillator. An oscillator is selected depending on the frequency to be synthesized and its output is routed to the input of Power Amplifier through a single-pole four-throw switch. It is ensured that the output of the switch is typically +5dBm over the full range of the synthesizer. The synthesizers SYNTH1 and SYNTH2 accept a reference signal of 105MHz from a Voltage Controlled Crystal Oscillator(VCXO), which is a part of the narrow band Offset Phase Lock Loop in the scheme for Local Oscillator Reference (LOR) Generation. The fifth overtone crystal is used in the circuit of VCXO and provides a gain of about 200Hz/volt. The loop bandwidth is typically about 40Hz. The synthesizer also accepts offset signal at 1MHz which is regenerated in the LOR system at each antenna with an edge jitter of fewer than 2ns. The frequency of reference and VCXO signals are divided into a value equal to step size using synchronous up-counters and dual-modulus pre-scalars using Pulse Swallow Techniques. A high gain (about 300mV/radian) digital Phase-Frequency detector compares the two divided signals and produces a pulse width modulated waveform at the output, whose DC average is proportional to the phase difference between the input waveforms. The error signal is integrated into a loop filter and drives the oscillator in a proper direction so as to achieve and maintain lock.

The 1MHz signal is used for the purpose of synchronization. If the system is operating at a step size of S MHz and relocks at a frequency FLO after a period of unlocking (caused by whatever reason), the phase of the relocked signal could jump by (2* PI* X* FLO) degrees, with a variable X free to take any integer value from 0 to 100/S. This is because there is no retention of "prior" condition in the event of a disturbance. The synchronization scheme aims to prevent this by giving a "time stamp" to the divider chain.

Features

Some important features of the LO Synthesizer 1 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 timescales of the order of 0.1mSecs.

• 50MHz Modulation: Better than -20dBc

Lock Range: Guaranteed at Very Important Frequencies(VIF) and also from

• -1.2*Fmax of the Oscillator.

Block diagram

Overview

The D46 i.e. The VCO 1 & VCO 2 are used to generate the signals whose frequency lies in the range of 100 MHz to 400 MHz. The VCO 1 generates the signals with a frequency range of 100 to 200 MHz and the range of VCO 2 is 200 to 400 MHz. The D47 i.e. the VCO 3 & the YIG generates the frequency in the range 400 MHz to 1750 MHz in which VCO 3 takes care of 400 to 600 MHz and YIG takes care of frequency in the range of 600 to 1750 MHz. The frequency in the range of 600-1750 which is generated by YIG is not used for LO Synthesizer 1. The D48 unit acts as the synthesizer unit. The D49 unit is the control unit for the LO synthesizer System.

D46 :
The D-46 PIU is used to generate the frequency ranging from 100 MHz to 400MHz. The VCO 1 i.e. the first Voltage Controlled Oscillator is used for generating the frequency in the range of 100MHz to 200MHz with a step size of 1MHz whereas the frequencies in the range of 200MHz to 400MHz are generated by VCO 2. D46 PIU consists of two Low Pass Filters of frequency 200MHz and 410MHz, along with a current monitor and monitor multiplexer unit.

D47 :
The D-47 PIU is used to generate the frequency ranging from 400 MHz to 1750MHz. The VCO 3 i.e. the third Voltage Controlled Oscillator is used for generating the frequency in the range of 400MHz to 600MHz whereas the frequencies in the range of 600MHz to 1750MHz are generated by Yig. The frequency generated by this PIU acts as the reference frequency for the D48 PIU i.e. the Synthesizer PIU.

D48 :
The D48 PIU is called as the Synthesizer PIU. It uses Peregrine prescalar for SYNTH 1 and Qualcomm Prescalar for SYNTH 2. It contains Overall Lock Indicator, Temperature Monitor, Isolated Amplifier, Power Amplifier, SP4T switch.

D49 :
The D-49 PIU is a monitor and control unit used to control LO Synthesizer System. The PIU consists of a control card, a monitor card, a MCM card, and a WNG card which are used to keep monitoring and controlling the working of the system.

The basic purpose of the Intermediate Frequency system is to convert the R.F. signals received from Front End system into the intermediate frequencies which are suitable for the transmission with minimum loss and minimum noise insertion. The IF system converts the R. F. signals into 70MHz frequency for both channels (CH-1 & CH-2) which is called first IF Conversion (I IF) conversion. I IF conversion is done in the plug-in unit (PIU) C41A and C41B respectively. This 70 MHz IF frequency again converted in 130 and 175 MHz bands. Second IF conversion (II IF) is done in C42 and C43 respectively for C41A and C41B PIUs. This signal is further converted into 130MHz and 175MHz by means of C42 and C43 PIU respectively. These PIU's are called as 130MHz converter and 175MHz converter respectively.

The PIU C44 is recognized as the Monitoring PIU as it monitors the whole IF system. The C45 PIU is called as the control PIU and it controls the working of the whole IF system.