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ALMA Memo #310

Measurements of Allan Variance and short term phase noise of millimeter Local Oscillators

R. Ambrosini
May 25, 2000

Phase stability over rather wide time scales is one of the most crucial requirements for the receivers of a millimeter interferometer like ALMA. The short term part of its spectrum is mainly related to the possibility of multiplying a signal source up to the millimeter region. The long-term part deals with a successful integration of the radioastronomical signal, after the down conversion operated by the many local oscillators (LO), spread over the array.

The SSB phase noise and the Allan Variance are the physical quantities usually measured, with different techniques, to represent the two spectral regions of phase stability, with a crossover around one second.

Here we propose a new data acquisition system, essentially dedicated to the measurement of the Allan Variance, which is built around two programs, written in the LabView environment.

The RF configuration is the same of the one used for phase noise measurements, so it turns out to be very convenient to characterize the wide range of LO frequencies, expected in ALMA, both at long and short term.

The main program, called Time Stability Analyzer (TSA), runs in real time accumulating data and computing Allan Variances from 1s up to 100 000 seconds in 2,5,10 sequence on each decade. Results are displayed on log-log plots for both cumulative statistics (since the beginning of the measuring session) and restricted to single batches of data. All information is recorded on disk and, optionally, hard copied on paper at each batch end.

A simultaneous acquisition of a temperature sensor allows looking for a correlation with the environment (as commonly happens).

A second program, called TSAcal, makes a very accurate calibration of the transfer function of the mixer, used as a phase detector. Its working parameters (Kv and Offset) are retrieved by a non-linear fit on line, even in a very short time, because the frequency of the calibration tone can be as high as 100Hz.

All previous features make the new developed TSA software, together with the RF configuration we propose here, an easy-to-use tool during the design phase of the many LO planned for ALMA, and a convenient aid for their maintenance.

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Last modified: June 01, 2000