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PERFORMANCE OF THE NRC PRIMARY TIME STANDARDS CsV, CsVI A, CsVI B, CsVI C
A. Mungall, H. Daams, J. Boulanger, C. Costain
To cite this version:
A. Mungall, H. Daams, J. Boulanger, C. Costain. PERFORMANCE OF THE NRC PRIMARY TIME STANDARDS CsV, CsVI A, CsVI B, CsVI C. Journal de Physique Colloques, 1981, 42 (C8), pp.C8-233-C8-240. �10.1051/jphyscol:1981828�. �jpa-00221723�
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CoZZoque 68, suppZ6rnent au n012, Tome 42, de'cembre 1981 page C8-233
PERFORMANCE OF THE NRC PRIMARY T I M E STANDARDS CSVI C s V I A r C s V I B r C s V I C
A.G. Mungall, H. Daams, J.S. Boulanger and C.C. Costain National Research Counci 2, Ottawa, Canada.
Abstract.- The 2.1 m NRC primary clock CsV and the three 1 m NRC CsVI primary clocks have estimated limits of accuracy of 1 and 1.5 x 10-l3 respectively, and estimated probable accuracies of 5 and 7 x 10-14. The frequency stabilities of all four clocks are similar, with the value of
Q ( ~ , T ) approaching 1 x 10-l4 for T= lo5 s. Re-evaluations of the
accuracy of CsV, performed at yearly intervals, have shown no
significant change in this estimate. In addition, comparisons with the cesium standard of the PTB, CsI, made during the past four years, indicate long-term agreement to within a few parts in 1014. No seasonal variation between these two standards is apparent, in contrast with that evident between them and TAI.
Introduction.- Prior to 1970, when CsV, the present NRC primary clock was designed, laboratory cesium standards were used only intermittently to
calibrate secondary continuously operating clocks. The design of CsV was such as to permit later continuous operation as a primary clock if this new mode of operation in fact proved feasible. In 1973 and 1974 it functioned
intermittently as a frequency standard, and during this time, improvements were made to the electronics. These, coupled with repeated evaluations which showed that the systematic frequency shifts were both small and stable, showed that reliable clock operation would be practical. Continuous operation began on May, 1975, and since January 1, 1976, the time scales TA(NRC) and UTC(NRC) have been derived from the scale of proper time generated directly by CsV,
PT(NRC CsV), with the only rate correction being that for the gravitational red shift arising from the 103 m elevation of CsV above sea-level.
The success of CsV led to a decision early in 1976 to build three clocks with an interaction length of 1 m. These clocks, CsVIA, -B, and -C, began operation as secondary standards late in 1978. Some difficulties arose from mechanical distortions and from thermo-electric currents flowing in the interior aluminum support cylinder and magnetic shields. The latter were largely overcome by improving the room temperature control and by individually controlling the temperature of each clock. The mechanical distortions were effectively eliminated by using spring supports for the ion pumps and the microwave excitation system. Operation of the CsVI standards as primary clocks began after accuracy evaluations were completed in late 1979 and since then reports have been submitted to the BIH at 10-day intervals.
Accuracy Evaluations.- In the 1975 evaluation of C S V , ~ the accuracy, estimated from the uncertainties of the known frequency shifts, was determined to be 5 x 10-l4 if the uncertainties were considered random, and 10 x 10-l4 if they added linearly. Since that time, a full evaluation has been carried out in the fall of each year, and each time the results have been consistent with those of the original evaluation. The two auto-tuned NRC hydrogen masers were used as reference standards in the earlier evaluations, and since 1979, the three CsVI clocks have provided the required reference frequencies and time scales. For
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981828
C8-234 JOURNAL DE PHYSIQUE
most of the measurements the frequency shifts can be measured with an accuracy of 1 or 2 x 10-l4 in 24 hours, with all four clocks operating under normal conditions. For CsV, when the beam direction is reversed, a longer time is required, from two to four days, depending on the degree of contamination of the hot wire detector to be used for the new beam direction. The time required for the CsVI clocks is much less, about 5 hours, because of improved cesium oven and detector efficiency compared to CsV. Between evaluations, small frequency changes have been observed, and some of these are visible in Fig. 1, as indicated by the dotted arrows for CsV, and by the solid arrows for the CsVI clocks. The frequency instabilities of the latter arose primarily from
malfunctions of the various components of the microwave excitation systems.
These included the 5-90 MHz multiplier, the 12.6 MHz frequency synthesizer, the step recovery diode matching networks, and the Gunn diode phase-locked
frequency source. Such malfunctions apparently affected the spectral purity of the microwave excitation signal. However, in most cases, the impurities were so small that their effects could be detected only by the clock itself by measuring power-dependent frequency shifts and noting whether they agreed with those predicted from calculations of the second order Doppler and cavity phase difference frequency shifts.
Modifications made to the CsVI electronics systems have improved their performance, but further work is necessary to attain the frequency stability and accuracy of which the clocks are capable. Similar modifications have yet to be applied to the electronics used in CsV, which are similar, though not identical.
In spite of these difficulties, it was found that the second complete evaluation of the CsVI clocks early in 1981 gave systematic corrections which were within a few parts in 1014 of those found in the earlier evaluation. The frequency offsets from CsV for CsVIA, -B, and -C were respectively 2.2, 2.4, and 2.0 x 1014 immediately subsequent to the 1981 evaluation. These off sets have, of course, altered since then, but they have remained within the claimed accuracy limit.
Short Term Frequency Stability.- For clock operation, CsV functions at a peak- to-valley resonance amplitude of 1.75 PA, and the three CsVI clocks at 2.75 pA, with the microwave excitation power level set 3 dB below that for maximum
transition probability. At these levels, the expected lifetime of the CsVI clocks is about 25 years with 1 g of cesium in each oven. The cesium ovens in CsV were last charged with 4 g in December, 1974, and after almost 7 years of operation show no signs of depletion.
Intercomparison of the CsV and CsVI clocks has been effected by both phase comparisons of the 5 BBz output frequencies and by time interval measurements of the 1 pps signals. Figure 2 shows o(2,r) plots for values of
T up to lo5 s. It can be seen that the four clocks provide similar performance and that their stability can be represented by the straight line
o(2,r) = 4.6 x 10-I2 T - ~ for any pair, or U(~,T) * ~ ~ = 3.0 x 10-l2 r-Oos2 for each individual clock. This stability is adequate for calibration of present day commercial clocks.
lo-'d""."' '"'""I ' " I " " ' ' " ' " - " '"''.J
10 lo2 lo3 I 0' I oS
r, SECONDS
Figure 2. The short term stability of the CsV and CsVI clocks.
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As a part of the continuing program of partial evaluations, the C field is measured at least every week for the CsVI clocks, and at least every two weeks for CsV. Changes in the corrections seldom exceed a few parts in 1015, and normally, steering via C field adjustments is performed only to maintain the accumulated time scale error less than 20 ns.
The time corrections are made on the assumption that the C field changes linearly between measurements. Figure 3 shows the effect of applying these corrections to the various time scales. The result is to extend the straight line fit to a value for (2,~) of 2 x 10-l5 for r = 21 days.
h;
r with
....
0
-15 1 , , ,,,,,,,
10 o - 4 1 od3 1 o - ~ 0 . 1 1 13 ' ' ; 0 . .+2
Days
Frequency stability of CsVI clocks compared to CsV
Figure 3. The short term stability of the CsVI clocks with the times corrected from the C field measurements.
Long Term Stability.- Confidence in the long term stability of the standards appears justified since the complete evaluations made at yearly intervals show that their systematic corrections remain constant within the estimated
uncertainties. The evaluations are necessary to maintain their status as primary standards. Frequency intercomparisons among the four clocks support these measurements.
An additional measure of the long term stability of CsV can be obtained by comparing the time scale TA(NRC) produced by CsV, with TA(PTB) produced by the primary cesium clock, Csl, of the PTB. This comparison is effected from data provided by the BIH 'D' circulars. The difference between the two scales is plotted in fig. 4, which shows that the values of TA(NRC) - TA(PTB) have remained constant within * 1 us over the past four years. The fluctuations apparent in this plot arise from both Loran C propagation delay changes and from variations in the two clocks themselves. For example, reference to the CsVI comparisons of figure 1 show that there was no significant change in the frequency of CsV between M J D 44200 and 44500, but figure 4 shows a short term change of about 1 x in this interval for the NRC and PTB comparisons.
In spite of such fluctuations, the rms deviations and o(2,r) of TA(NRC) -
TA(PTB) are less than 3.5 x 10-l4 for periods from 150 to 500 days. The NRC- PTB plot also indicates the absence of any seasonal variation during this entire period, in marked contrast to that for UTC(NRC)-UTC(USN0). This behaviour will be discussed in the following section.
MJD - 44000
Figure 4. Comparisons of the NRC, PTB and USNO time scales.
Seasonal Variations.- In the UTC(NRC) - UTC(USN0) plot shown in figure 4, an annual periodic fluctuation with an amplitude of 500 ns is superimposed on a long-term, more gradual variation. Figure 5 shows the comparison between the NRC and PTB time scales, and also the relationship between each of these and TAI, the mean international atomic time scale maintained by the BIH. It can be observed that the seasonal variations apparent in the UTC(NRC) - UTC(USN0) plot of Fig. 4 also appear in TAI-PTB and TAI-NRC, but with reversed sign. Since comparisons with'the CsVI clocks have already indicated that the NRC scale did not change frequency at W D 44250, it is evident that either TAI must have altered by 2 x 10-l3 at that date, or that the means of comparing the time scales (Loran C) introduced an equivalent frequency shift.
In a recent paper the dependence of Loran C propagation delay with temperature is discussed in detail. Figure 6, taken from that paper shows the correlation between the changes in UTC(USN0) - UTC(NRC), the differences in time of arrival at NRC of the Loran C signals from Seneca and Nantucket, and the outside air temperature at Ottawa. From the degree of correlation evident in this figure, it seems likely that much of the seasonal effect observed in various time scale comparisons arises from changes in the Loran C propagation delay.
In summary, therefore, there does not at present seem to be any convincing evidence for significant seasonal variations in the frequencies of the NRC and PTB primary clocks. It is possible that such variations do in fact exist, but are well correlated, since both clocks are located in the northern hemisphere, but if the effects are large, they would probably be visible, particulary in the more precise time comparisons using the Symphonic satellite which will be discussed in the next section.
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-30 -
1979 JAN.1 1980 JAN.1 1981
1 I I 1 I I I
44200 44300 44400 44500 44600 44700
MJD
Satellite T h e Transfer Via Symphonic.- Since February, 1980, time transfer via the Symphonie satellite between Canada and West Germany has made possible a more precise comparison between the primary clocks of the NRC and PTB. The results are shown in Fig. 7 on a larger scale than in Fig. 4.
UTC(NRC1 - TA(N11 o r UTC(PTB1 via Symphonie L ~ g h t l i n e i s a moving a v e r a g e on 5 points
MJD - 44000
Figure 7. The difference between the NRC and the PTB time scales measured using the Symphonie satellite.
Examination of Fig. 7 shows very clearly the consequence of the
different modes of operation of the NRC and PTB primary clocks. For example, a complete evaluation, with beam reversal, of CsV was made at NRC on MJD44512, and a new frequency of CsV adopted. It must be emphasized that each such evaluation is independent of all previous ones, and no reference is made to past values. In effect, CsV is a 'new' clock after each such evaluation.
At the PTB, the direction of the cesium beam is reversed at 40-day intervals, and no corrections are made at the time of the reversal, it being assumed that over an 80-day period the mean frequency is correct and free from any cavity phase difference frequency shift. Between MJD 44580 and 44880, a series of fluctuations with an 80-day period can be seen on the UTC(PTB) time scale. It may thus be inferred that these fluctuations do in fact arise from these beam reversals. It appears that the frequency of Csl changes by about 5 x 10-l4 on beam reversal, and that over the 300-day period ending at MJD 44880 the frequency difference between NRC CsV and PTB Csl is
-2 + 2.5 x 10-14, alternating for the 40-day periods. Any seasonal variation between the two standards appears to be no greater than 1 x 10-14.
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Conclusions.- When the decision was made at NRC in 1976 to construct three more primary cesium clocks, it was evident that, once operating, these clocks could well reveal a number of potentially embarrassing errors and instabilities in the primary clock CsV, hitherto invisible because of the much larger time and frequency fluctuations inherent in any of the inter-laboratory comparison techniques. It has, therefore, been reassuring to find that the four NRC long beam primary cesium clocks agree generally within a few parts in 1014, well within the combined limits of error of 1 x 10-l3 for CsV and 1.5 x 10-l3 for the CsVI clocks, despite the recognized faults of each. It is similarly encouraging to note that for over four years the frequencies of the NRC and PTB primary clocks, CsV and Csl, have similarly been in agreement within a few parts in loi4. Such agreement is of particular significance when one considers that the two standards are quite different both in design and in their method of operation.
With regard to the future, it is not unreasonable to expect that further improvements can be made to the NRC clocks in particular the electronic
excitation systems, which should lead to even better stability than that now attained.
References.-
1. A.G. Mungall, IEEE Trans. Instr. and Meas. I W 2 7 , (1978) 330.
2. A.G. Mungall, H. Daams, D. Morris and C.C. Costain, Metrologia 12, (1979) 129.
3. A.G. Mungall, H. Daams and J-S. Boulanger, Proceedings of the Second International Conference on Precision Measurement and Fundamental Constants (in press).
4 . A.G. Mungall, H. Daams and J-S. Boulanger, IEEE Trans. Instr. and Meas.
IM-29 (1980) 291.
5. A.G. Mungall, W.A. Skholm and C.C. Costain, Metrologia (in press).
