Version: 1.0
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TESTING TOOLS :

Auto Set LO Control for GAB-LO Synth

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Auto Set LO Control for Antenna FSW-LO Synth

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MCM Control for GAB System

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Attenuation test for GAB-Rx

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LINKS :

Analog Backend Documents Link

    

GMRT Upgradation Status

    

DESCRIPTION:

A versatile Analog Back-end System (GAB) has been developed to process the RF signal received from antennas using high dynamic range circuits. The modifications being implemented in Analog and Digital backend receivers will improve the overall specifications of the Backend receiver.

The major up gradation related to the analog section includes complete processing of the RF signals at the Central Electronic Building (CEB), seamless frequency coverage up to 1600 MHz and an instantaneous bandwidth of 400 MHz (Max).

The main function of the Analog Backend receiver is conversion of the signal at RF frequencies received from antennas through the Optical Fiber cables to Baseband frequencies in the range 0 to 400 MHz for Digitization in high speed ADCs.

The main function of the Analog Backend section of GMRT receiver is to down convert the RF signals received from the antennas and process them so that the digitization and further digital processing is done conveniently.

The incoming RF signal is passed through a variable gain circuit whose gain can be adjusted in steps of 0.5 db so that any variation in signal levels between antennas can be corrected at this stage and the ADC will receive same power levels. The RF signals are then passed through a RF Filter bank which will have same filter as the one used immediately after the feed being used. This filter is used to improve the out of band rejection and provide a clean signal to the later stages.

Since the ADC is operating at 800 MHz sampling rate, we need to down convert all signals above 400 MHz to a lower frequency. So the frequency conversion stage is provided with a bypass stage so that whenever the observation frequency is lower than 400 Mhz, one can use the bypass path and the RF signal will be directly given to the ADC circuits.

For RF bands above 400 MHz a suitable LO signal is to be used to down convert the signal to 0 to 400 MHz range. The Mixer unit is followed by a 400 MHz low pass section to attenuate the high frequency signals due to LO/RF leakage.

The Baseband signals thus generated are provided as two outputs, one directly as a 400 MHz signal (Spare output) and the main output with a Baseband Filter bank which provides a facility for 100, 200, 400 MHz filter selection based on observation requirements. The power levels are adjusted for the ADC linear range.

The Local oscillator for the mixer is generated from a 105 MHz Reference signal provided by the GPS disciplined Rubidium Oscillator which is the Time & Frequency standard used at the observatory. There are two ways in which the LO signal is generated.

A common Signal generator which covers the full frequency range is locked to this 10 MHz Reference. The output of this unit is amplified and distributed to down conversion units for all 30 antennas. This provides a facility for setting a common LO for all antennas, but provides facility to vary the set LO frequency in steps of 1 MHz.

A second LO generation scheme uses individual LO signals generated in the range 600 to 1700 MHz in 0.5 MHz steps. Here individual antennas can be set to different LO frequencies as per requirement.

The system also provides complete control & monitor of the parameters and health of the system through online and also facilities to monitor the signal levels at various stages in the receiver. A facility also exists to inject a Noise or CW to the circuits to check the gain and other system parameter without removing the units from the rack.

ANALOG BACKEND SYSTEM SPECIFICATIONS :


ANALOG RECEIVER :

Input Frequency : 30 - 1600 MHz

Input Power : -24 dBm

Max Bandwidth : 400 MHz

Headroom : 27 dB

Output power : -12 dBm

Noise Contribution to FE : < 0.01 K

Power detector at input and output

Noise + CW cal signal at input

Noise (var corr) cal at ADC input

LOCAL SYNTHESIZER :

Frequency Range : 600 - 1700 MHz

Step Size : 500 KHz

Output Power : + 7 to +13 dBm

Reference frequency : 105 MHz

RF FILTER :

Filter bank - 7 filter selection

BB FILTER :

(3 BB converted signal )
100MHZ , 200MHz , 300MHz BW

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