Contains reports on two research projects

Badessa, R. S.

Bates, V. J.

Brown, D. A.

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VI. MOLECULAR BEAMSProf. J. R. Zacharias V. J. Bates F. J. O'BrienProf. J. G. King D. A. Brown J. E. SteelmanProf. C. L. Searle R. Golub C. O. ThornburgR. S. Badessa W. D. Johnston R. WeissE. L. KamenA. CESIUM BEAM TUBE INVESTIGATIONThe quadrature component referred to in Quarterly Progress Report No. 66 (page 37),which appeared as a residue preventing a true null from occurring at the output of the100-cps amplifier, has been investigated further. Originally we believed that the quad-rature was the result of second-harmonic distortion in the waveform of the frequencymodulation of the X-band excitation signal. However, after trying several modulationschemes differing widely in principle and theoretically incapable of causing quadraturedifficulties from this source, we have concluded that second-harmonic distortion is notthe primary cause of trouble. We now believe that quadrature will occur even with dis-tortionless modulation whenever the beam-tube resonance characteristic has a slightdissymmetry. The reason is not as obvious as it may appear, since it involves con-sideration of dynamic, rather than static, operation. We conclude that quadrature couldbe reduced substantially by extracting information from the beam tube in a manner thatis not likely to be influenced by transient behavior.During the next quarterly period, a new type of control loop is to be investigatedbased on the transient response studies just completed. Essentially, our method is touse pulse techniques to gate out undesirable transients. If it is successful, it will resultin the attainment of a null at the output of the modulation-frequency amplifier that is afunction only of the degree of detuning of the X-band excitation frequency from the peakof the static cesium resonance characteristic.R. S. Badessa, V. J. BatesB. MEASUREMENT OF FREQUENCY STABILITYWe have ascertained (see Quarterly Progress Report No. 66, p. 38) that a usefuland enlightening criterion for judging the frequency stability of a frequency standardinvolves the determination of the autocorrelation of the phase difference between a refer-ence and the standard that is being measured. In particular, it is quite easy to deter-mine the mean-square frequency deviation for different averaging times by using thiscriterion. In practice, of course, rather than compare the frequency standard to aperfect frequency we can only compare one frequency standard with another frequencystandard. If the two standards are identical and independent, the mean-square deviationfor one standard will be half the mean-square deviation for both.QPR No. 67(VI. MOLECULAR BEAMS)The eventual goal is to compare a large number of frequency standards so as to deter-mine the relative merits of each. In view of the large amount of data that is anticipated,it is desirable that the data gathering and data processing be automatic, to obviate theneed for long data taking, data reduction or computation by hand.We have built the system illustrated in Fig. VI-1 for detecting phase differencesbetween two cesium-beam frequency standards. The voltage proportional to the phase264 MC 264 MCMIXERLOCALX 16 OSCILLATOR X 1630 MC 30 MC16.5 MC GAIN G A IN  16.5 MC FROMFROM X I X II30 MC PHASE DETECTORVOLTAGE PROPORTIONALTO PHASE DIFFERENCEFig. VI-1. Experimental system.is recorded on an Ampex FM tape recorder. The tape recorder is capable of recordingat 30 ips, 3 ips, and 0. 3 ips, and will play back at 30 ips. When a lot of data isrequired, the recording is done at 0.3 ips; this allows a playback speedup of 100:1.Because of the short time available for data recording, we used 3 ips.The analog-to-digital conversion was done at the Electronic Systems Laboratory,M. I. T., with the help of Mr. Robert Paine. The digital output was in the form ofpunched cards, with five samples on each card.Three programs were developed for which Fortran and the IBM 7090 computer atthe Computation Center, M. I. T., were used. The first program computed the auto-correlation by the time-average method. The second program computed the autocorre-lation by the same method, but spaced the abscissa unevenly. The third programcomputed the autocorrelation the same way as the second, but the data were quantizedmore coarsely.QPR No. 67(VI. MOLECULAR BEAMS)The results of the computer runs show that it is very reasonable to automaticallygather and process the phase information received from the frequency standards. Ithas also been shown that the data can be very coarsely quantized and still give goodresults in the autocorrelation.This work is reported on in detail in the author's thesis. 1D. A. BrownReferences1. D. A. Brown, Statistical Measurement of Frequency Stability, S. M. Thesis, Depart-ment of Electrical Engineering, M. I. T., August 20, 1962.QPR No. 67

Molecular Beams

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Molecular Beams

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