Aishik Acharya

All digital implementation of dual mixer time difference technique for precise phase and frequency measurement

At National Physical Laboratory India, we have developed a precise phase and frequency measurement system as an alternative to the traditional analog approach in order to measure the time differences between the microwave frequency standards and the ensemble of time scale. We have successfully validated the performance of the system and now we are working towards the complete implementation of fully digitized time scale. In this paper we will discuss about the noise floor performance and the possibility for multichannel expansion of the system without affecting the performance. All digital implementation of dual mixer time difference technique is a step towards more precise measurement and evaluation of frequencies and will provide better stability to the timescale. The Allan deviation of the system noise floor stability so far we have achieved for dual channel configuration is σTotalMod (τ) = 6 × 10−11 × τ−1.5.

A new time dissemination service with an accuracy of ± 10 ms over public switched telephone network

A new time dissemination and synchronization service over public switched telephone network (PSTN) has been established at CSIR-NPL. The time synchronization accuracy achieved using is this service is ± 10 ms. The measurement of round trip delay and its cancellation in the final time code sent to the receiver has been implemented in this service. In this paper, the technical details of this service have been described.

Systematic shifts in the frequency of NPLI-CsF1

At CSIR-National Physical Laboratory, India (NPLI) we have developed first generation of Cesium atomic fountain (NPLI-CsF1) primary frequency standard (PFS). NPLI-CsF1 is now fully operational and its frequency is now being evaluated along with all systematic and statistical uncertainties. The present experimental data conforms that fountain frequency is stable to few parts in 1015 at less than one day averaging time. The fountain frequency is compared with the Hydrogen MASER (H-MASER) which is contributing to TAI already and hence fountain frequency is traceable to TAI. Recent evaluation results were communicated to BIPM and published in circular T. Typical stability of NPLI CsF1 is ~7 × 10-13 τ-1/2. The present performance of fountain is limited by the measurement uncertainty of cold collisional shift.

A novel technique for precise phase and frequency measurement

A novel technique for precise phase and frequency measurement is described in this paper. A primary frequency standard, namely, Cs atomic fountain has already been developed at CSIR-NPL. Six commercial Cs atomic beam clocks along with a H-Maser are continuously inter-compared and constitute a stable time scale. H-Maser frequency is being evaluated with respect to the Cs fountain on regular basis. For evaluating the frequencies of these different frequency sources precisely, a stable, low noise and very accurate frequency measurement instrument is required. All digital implementation of dual mixer time difference technique is a step towards more precise measurement and evaluation of frequencies and will provide better stability to the timescale.

Present status of primary frequency standards at NPL, India

National Physical Laboratory, India (NPLI) has developed its first Cesium atomic fountain frequency standard (NPLI-CsFl). The indigenously developed fountain frequency standard is now fully operational and its frequency is now being evaluated along with all systematic and statistical uncertainties. Seven frequency evaluation have been conducted to estimate fountain frequency with respect to TAI (International Atomic Time) and were reported to BIPM. Recently NPLI CsFl took part in live contribution of TAI evaluation. The fountain frequency is compared with the Hydrogen Maser (H-Maser) frequency which is contributing to TAI (International atomic timescale) and hence fountain frequency is evaluated with respect to the TAI. The fountain frequency is stable to few parts in 1015 at less than one day averaging time. Typical uncertainty of NPLI-CsFl ~2.8×10-15 A second generation Cs fountain is being developed in order to overcome the short comings in NPLI-CsF1.

Precise phase and frequency measurement using all digital dual mixer time difference technique

A new technique for precise phase and frequency measurement is described in this paper. A primary frequency standard, namely, Cs atomic fountain has already been developed at CSIR-NPL. Six commercial Cs atomic beam clocks along with an H-Maser are continuously inter-compared and constitute a stable time scale. H-Maser frequency is being evaluated with respect to the Cs fountain on regular basis. For evaluating the frequencies of these different frequency sources precisely, a stable, low noise and very accurate frequency measurement instrument is required. All digital implementation of dual mixer time difference technique is a step towards more precise measurement and evaluation of frequencies and will provide better stability estimation of timescale, microwave and optical (under development) standards at NPL India.

Comparison of Caesium fountain clocks in Europe and Asia

A remote comparison campaign of Caesium fountain clocks from four National Metrology Institutes in Europe and Asia was carried out in May 2013. Six fountains at Physikalisch-Technische Bundesanstalt (PTB) Germany, Scientific Research Institute of Physical-Technical and Radio Technical Measurements (VNIIFTRI-SU) Russia, National Physical Laboratory of India (NPLI) India and National Institute of Metrology (NIM) China were compared by Two-Way Satellite Time and Frequency Transfer (TWSTFT) and GPS Carrier Phase (GPS CP) techniques. The frequency differences and the comparison uncertainties between the compared fountain pairs were evaluated. The comparison uncertainty (uc) results from combining the uncertainties of the two compared fountain clocks and the uncertainty introduced by the comparison link. All the frequency differences between the fountains agreed within the 1-sigma uncertainty in the low 10-15 level. We present in this paper the measurements and the evaluation of the campaign in details.