GMRT TIME & FREQUENCY SYSTEM

Transcription

1 GMRT TIME & FREQUENCY SYSTEM ajith, navnath / (ver1.0) The GMRT Time and Frequency system (FTS) generates the master time and frequency standards for the GMRT Receiver system. The FTS system is located in the New Receiver Room (NRR) at the Central Electronics Building (CEB) in Central Square, GMRT, Khodad. The system generates signals at 10 MHz, 1 pps and 1ppm rate for use as frequency ref and time stamp in the receiver and data acquisition systems. There are two FTS systems installed, one of which is active at any given point in time and the other unit is kept as live spare. Description The block schematic of the FTS unit is shown in Annex-1. The main standard used is a low phase noise Rubidium oscillator PRS-10 (manufactured by Stanford Research Systems SRS) which is disciplined using a TM-4 GPS receiver (from Spectrum Instruments). The technical details of PRS-10 and TM-4 are given in the Annex-2. The PRS-10 is an ultra low phase noise Rubidium oscillator and meets the requirement as a reference frequency standard for GMRT synthesisers. The frequency offsets and long term stability of PRS-10 is further improved by phase locking the PRS-10 on to a TM-4 GPS receiver. The PRS-10 can be phase locked to the 1pps signal from the GPS receiver with a 1 nsec resolution and the results can be read back on a PC through the RS232 interface on the D-type connector. Phase Noise measurements of the PRS-10 unit has been done using the Aeroflex PN9000 phase noise analyser at SAMEER, Mumbai and the results match with the plots given in the data sheets. The measured phase noise is about Hz offset. The results of the phase noise measurement is attached as Annex-3. The TM-4 GPS receiver is available in a 1U rack mountable form with outputs for 10 MHz, 1pps and a D-type connector with 1ppm and timing signal on NMEA protocol for time synchronising a ntpd server PC. The 1 pps signal from TM-4 is used to phase lock the PRS-10 oscillator as shown in Annex-1 and the NMEA data is made available on a D-type connector. All other signals, 10 MHz frequency reference, 1 pps, 1 ppm are derived from the PRS-10 Rubidium oscillator. The 10 Mhz clock signal from the Rubidium oscillator is provided on two output ports on the rear side of the FTS unit using a power splitter. The output 1 ppm signal is derived from the 1 pps output from the Rubidium oscillator using a frequency counter circuit which is synchronised to the 1 ppm of the TM-4 GPS. This provides a 1 ppm signal with 50% duty cycle required in the backend systems. Since these timing signals are to be send over to backend systems over long cables, these are converted to differential signals. Hence a differential to single end conversion will be needed at the receiving end to regenerate the TTL signals. The schematic of the circuit for generating the 1pps and 1ppm at final output is shown in Fig 4(a). Fig 4(b) shows the plots of 1pps and 1ppm waveforms on a DSO. From the plots it can be seen that the time difference between the 1 pps at GPS to the final output can vary between 0 to 25 ns whereas the 1 ppm at final output is delayed by 64nS compared to the GPS signal. Fig 4(c) shows the plots for the 10 MHz clock signal at Rubidium output and the 1 Mhz and 5 MHz TTL signals derived from it.

2 Installation in NRR The FTS units are installed in a standard 19 rack in the NRR (near the LO Master rack). The details are shown in Annex-5. The two FTS units are kept powered on and any one unit can be used as standard based on requirement. These systems need a warm up time of about 30 min or so and also the GPS receiver needs time to lock on to a minimum of four satellites. Hence it is advisable to keep the system powered on. The units are powered from two sources to avoid any chance of power failure first a DC power supply running on the UPS in the NRR and second a 24V battery system which is charged separately. These power sources are wired in such a way that in case of failure in any one, the other source will take over. In such a case the circuitry provides a continuous audio alarm indicating that one of the power sources is not working (the FTS rack will be still powered on and the signals still usable)

3 Annex 1

4 Annex 2a PRS-10 Rubidium Oscillator Specifications

5

6

7

8 Annex 2b TM-4 GPS Receiver Specifications

9

10 Annex 3 PRS-10 Phase Noise Measurements Equipment used : Aeroflex PN 9000 The red coloured lines are 50 Hz pick-up from line frequency Measured Phase Noise : Hz

11 Annex 4a Schematic of the 1 ppm Generation & Distribution

12 Annex 4b 1 ppm, 1 pps Timing Measurements Fig (a) : 1 pps Signal From GPS and Final FTS System Output Yellow : Signal from GPS Unit Blue : Signal at Final output 1 pps Pulse train Timing of 1 pps Signals (varies 0 to 25nS) Fig (b) : 1 ppm Signal From GPS and Final FTS System Output Yellow : Signal from GPS Unit Blue : Signal at Final output 1 ppm Pulse train Timing of 1 ppm Signals (64 ns)

13 Fig (c) : Rb Oscillator signals 10 MHz Sinewave 5 MHz TTL 1 MHz TTL Annex 5 (a) FTS Unit - Internal wiring & Installation in Racks

14 Annex 5 (b)