New Analog System
GMRT ANALOG BACKEND (GAB) : A versatile Analog Back-end System 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 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 10 MHz Reference signal provided by the Active Hydrogen Maser which is the Frequency standard used at the observatory.
The LO generation scheme uses individual LO signals generated in the range 100 to 1500 MHz in 10 KHz steps. Here individual antennas as well as polarisations /channels 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.
Intermediate Frequency System (IF System) converts the signals collected by Front End i.e. R.F. System. IF System converts RF signals in to the appropriate frequency bands which are suitable for transmission through Fiber Optic cables to the Central Electronics Building.
Frequency Conversion or modulation is done with the help of Local Oscillator (LO) Frequency. These LO systems are also situated in the same rack which is called as ABR (Antenna Base Receiver) rack.
IF System means generation of IF frequency as well as amplification of IF signals. The facility of attenuation of IF signal is also available in IF system. By means of this the If signals can be attenuated from 0 to 20 dB in steps of 2dB
There is another facility made available i.e. Band Selection. By this facility the final output can be taken in bands of bandwidth of 6, 16, or 32 MHz.
An important feature of IF system is ALC i.e. Automatic Level Controller. By this feature the output of the IF system can be controlled for particular power level as per the need of the successive stages.
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 preceeded 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 less than 2 ns.
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 in 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 unlock (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.
At the GMRT, the dual polarized RF signals from each antenna are processed through super heterodyne receivers and finally brought to a central location for further processing. The intermediate frequency signals from each antenna are then down-converted to baseband signals and fed to the digital signal processing backend. A baseband system is used for converting the IF signal to baseband signal to get a 32 MHz bandwidth from each polarization.