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## Coherent Combining of Signals of Hydrogen Masers for VLBI Measurement Improvement

Transactions of IAA RAS, issue 59, 9–13 (2021)

Keywords: coherent combining, synchronization, frequency locking, frequency stability, phase noise

### Abstract

Measurement accuracy in Very Long Baseline (VLBI) interferometry is crucially depend on the frequency stability of reference atomic clock signals. Each observatory of Quasar VLBI network contains several hydrogen masers, therefore coherent combining of independent signals can be used to obtain summary signal with improved frequency stability and reduced phase noise. Frequency stability better than single hydrogen maser should improve the accuracy of VLBI measurements in millimeter band. Experiments on coherent combining of active hydrogen maser signals using passive elements, done in the observatories “Svetloe”, “Zelenchukskaya” and in “Vremya-Ch” JSC are described. Frequencies of masers were automatically adjusted to each other or to a single maser of the ensemble using mutual measurements in multichannel comparator. In case of unidirectional synchronization to a single maser, control loop time constant value was selected more than the maximal averaging time for which frequency stability improvement of summary signal was expected. Frequency stability estimation for summary signal was done using measurements with respect to independent active hydrogen maser. Results of the experiments comply with the results predicted by simple theory and explained by averaging of uncorrelated phase fluctuations of sources with similar statistical characteristics. It is shown that coherent combining of two hydrogen masers CH1-1035 allows achieving frequency stability of about $4×10^{−14}$ at averaging time 1 s in 3 Hz noise band, in case of four signals frequency stability of summary signal is about $3×10^{−14}$. The measurement results of frequency stability and phase noise power spectral density of new active hydrogen masers with a single state selection system promising further improvements of the characteristics are also shown. Estimation of the frequency stability for one maser of this type gives $3.8×10^{−14}$ at 1 s in 0.5 Hz band. Coherent combining of two masers with single state selection system reduces noise level by 3 dB, so power spectral density of phase noise for 5 MHz is around −133 dBc/Hz for 1 Hz offset.

### Citation

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A. V. Vytnov, K. G. Mishagin, V. A. Polyakov. Coherent Combining of Signals of Hydrogen Masers for VLBI Measurement Improvement // Transactions of IAA RAS. — 2021. — Issue 59. — P. 9–13. @article{vytnov2021, abstract = {Measurement accuracy in Very Long Baseline (VLBI) interferometry is crucially depend on the frequency stability of reference atomic clock signals. Each observatory of Quasar VLBI network contains several hydrogen masers, therefore coherent combining of independent signals can be used to obtain summary signal with improved frequency stability and reduced phase noise. Frequency stability better than single hydrogen maser should improve the accuracy of VLBI measurements in millimeter band. Experiments on coherent combining of active hydrogen maser signals using passive elements, done in the observatories “Svetloe”, “Zelenchukskaya” and in “Vremya-Ch” JSC are described. Frequencies of masers were automatically adjusted to each other or to a single maser of the ensemble using mutual measurements in multichannel comparator. In case of unidirectional synchronization to a single maser, control loop time constant value was selected more than the maximal averaging time for which frequency stability improvement of summary signal was expected. Frequency stability estimation for summary signal was done using measurements with respect to independent active hydrogen maser. Results of the experiments comply with the results predicted by simple theory and explained by averaging of uncorrelated phase fluctuations of sources with similar statistical characteristics. It is shown that coherent combining of two hydrogen masers CH1-1035 allows achieving frequency stability of about $4×10^{−14}$ at averaging time 1 s in 3 Hz noise band, in case of four signals frequency stability of summary signal is about $3×10^{−14}$. The measurement results of frequency stability and phase noise power spectral density of new active hydrogen masers with a single state selection system promising further improvements of the characteristics are also shown. Estimation of the frequency stability for one maser of this type gives $3.8×10^{−14}$ at 1 s in 0.5 Hz band. Coherent combining of two masers with single state selection system reduces noise level by 3 dB, so power spectral density of phase noise for 5 MHz is around −133 dBc/Hz for 1 Hz offset.}, author = {A.~V. Vytnov and K.~G. Mishagin and V.~A. Polyakov}, doi = {10.32876/ApplAstron.59.9-13}, issue = {59}, journal = {Transactions of IAA RAS}, keyword = {coherent combining, synchronization, frequency locking, frequency stability, phase noise}, pages = {9--13}, title = {Coherent Combining of Signals of Hydrogen Masers for VLBI Measurement Improvement}, url = {http://iaaras.ru/en/library/paper/2100/}, year = {2021} } TY - JOUR TI - Coherent Combining of Signals of Hydrogen Masers for VLBI Measurement Improvement AU - Vytnov, A. V. AU - Mishagin, K. G. AU - Polyakov, V. A. PY - 2021 T2 - Transactions of IAA RAS IS - 59 SP - 9 AB - Measurement accuracy in Very Long Baseline (VLBI) interferometry is crucially depend on the frequency stability of reference atomic clock signals. Each observatory of Quasar VLBI network contains several hydrogen masers, therefore coherent combining of independent signals can be used to obtain summary signal with improved frequency stability and reduced phase noise. Frequency stability better than single hydrogen maser should improve the accuracy of VLBI measurements in millimeter band. Experiments on coherent combining of active hydrogen maser signals using passive elements, done in the observatories “Svetloe”, “Zelenchukskaya” and in “Vremya-Ch” JSC are described. Frequencies of masers were automatically adjusted to each other or to a single maser of the ensemble using mutual measurements in multichannel comparator. In case of unidirectional synchronization to a single maser, control loop time constant value was selected more than the maximal averaging time for which frequency stability improvement of summary signal was expected. Frequency stability estimation for summary signal was done using measurements with respect to independent active hydrogen maser. Results of the experiments comply with the results predicted by simple theory and explained by averaging of uncorrelated phase fluctuations of sources with similar statistical characteristics. It is shown that coherent combining of two hydrogen masers CH1-1035 allows achieving frequency stability of about $4×10^{−14}$ at averaging time 1 s in 3 Hz noise band, in case of four signals frequency stability of summary signal is about $3×10^{−14}$. The measurement results of frequency stability and phase noise power spectral density of new active hydrogen masers with a single state selection system promising further improvements of the characteristics are also shown. Estimation of the frequency stability for one maser of this type gives $3.8×10^{−14}$ at 1 s in 0.5 Hz band. Coherent combining of two masers with single state selection system reduces noise level by 3 dB, so power spectral density of phase noise for 5 MHz is around −133 dBc/Hz for 1 Hz offset. DO - 10.32876/ApplAstron.59.9-13 UR - http://iaaras.ru/en/library/paper/2100/ ER -