Installation of hydrogen maser for very long baseline interferometr

Levine, M. W.

Vessot, R. F. C.

English

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2Hydrogen-Maser Time and Frequency Standard at Agassiz Observatory
Martin W. Levine and Robert F. C. Vessot
Presented at the
-
Symposium on Very Long-Baseline Interferometry
W	 Charlottesville, Virginia
April 13-15, 1970
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April 1970
Smithsonian Institution
Astrophysical Observatory
Cambridge, Massachusetts 02138
004-20
ABSTRACT
A hydrogen maser was installed at the Harvard College Observatory/
Smithsonian 84-ft radio telescope in Harvard, Massachusetts, for VLBI
experiments 14 to 1j October 1969 and 22 November to 14 December 1969.
The maser is a compact unit, of relatively low power consumption, speci-
fically designed for field use and readily transportable. A precision 5-MHz
crystal oscillator, phase-locked to the maser, provides all frequency and
_- l
	
=
time references for the VLBI receivers, recorde.s, and clocks. The maser
was tuned against a rubidium standard by .a rapid flux-switching technique;
maser frequency resettability was estimated to-be approximately t3 x 10-13
with an overall accuracy of approximately t1 x 10-12.
A
Loran-C was continuously monitored during the VLBI experiments, and
the results of the Loran-C -maser compar i s-)ns are included.
t'.S
4
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z
Hydrogen-maser time and frequency standard at Agassiz observatory
Martin W. Levine and Robert F. C. Vessot
Smithsonian Astrophysical Observatory, Cambridge, Massachusetts 02138
1. INTRODUCTION
The hydrogen maser is generally acknowledged to be the ideal time and
frecrsency standard for very long-baseline interferometry (VLBI), exhibiting a
unique combination of superior short-term phase stability and freedom from
drift. Further; the maser is a primary-
 standard with unsurpassed reset-
ability of frequency. In the past, the size, weight, and general awkwardness
of the equipment have limited the hydrogen maser to fixed installations. For
those observatories with only periodic or infrequent need for the phase
stability of-the maser standard, the investment required for the maser and
its supporting electronics could not be justified.
The SAO hydrogen maser described in this paper was designed to be
easily transportable and rugged enough to withstand the norr,-^al rigors of
being moved from one station to another. It was trucked from Cambridge to
Agassiz Station, Harvard, Massachusetts, for use during two V LBI experi-
ments from 1 1 to 15 October and 22 November to 14 December 1069. The
mas--r was installed at Agassiz, tuned aga`nst a rubidium-cell reference and
operated as the master time and frequency standard for the Observatory for
the duration of the observations. At the completion of the experiment, the
maser was returned to Cambridge for use in other experiments. Figure 1
shows the SAO hydrogen maser on its shipping pallet enroute to Agassiz
station.
1
004-20
2. HYDROGEN-MASER OSCILLATOR
The SAO hydrogen maser originated from a program sponsored by NASA
Headquarters to dev,-Lop a -clock for orbital applications for a precise test
of the principle of equivalence [ Kleppner et al. , 1970. NASA's Marshall
Space Flight Center funded a second program to build a small ground
hydrogen maser incorporating the advanced technology of the space maser.
The design features of the SAO hydrogen maser are described in detail
in a previous publication [Vessot et al., 19631. Figure 2 is a cutaway illus-
trating the major assemblies of the maser. It should- be noted that the design
philosophy of this maser emphasizes good long-term frequency stability
without the necessity for continuous automatic tuning._ The cavity resonator
is silver-coated CER-VIT, thermally compensated and enclosed in a-two-
stage oven. Provisions have been made for manual or automatic tuning
using only a stable external reference oscillator and a digital-period counter.
The tuning procedure, normally .required only when the maser is moved to
a new location, is detailed in a later section.
The short-term performance of the SAO hydrogen maser is summarized
in Figure 3, which is a conventional 6(T) plot. The RMS frequency stability
as a function of averaging time is shoe. - far quartz-crystal oscillators, and
for rubidium and cesium standards, as well as for the maser. The same
data are shown in Figure 4 in a form that might be more useful for VLBI
experimenters. The RMS phase deviation as a function of observation time,
normalized at 1 GHz, is shown for the four types of frequency standards.
3. MASER CLOCK SYSTEM
The hydrogen -maser is an active oscillator with an output of approxi-
mately -95 ebm at 1420 MHz. A 5-MHz crystal oscillator is 1:'iase-locked to
the maser signal to provide standard frequencies at levels useful to the clock
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rContract NASW-1337.
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Contract NAS 8-2604.
2
-	
--	 --
system. A simple dual-conversion receiver is used to generate the phase-
locking signals in conjunction with a synthesizer to set the time scale. The
synthesizer is not an integral component of the maser clock system; the
instrument used in this experiment was borrowed from the Agassiz station.
The entire maser clock system is shown in block diagram in Figure 5.
The maser output at 1420 MHz is fed to a balanced_ crystal diode mixer
where it is hetrodyned with the 1400-MHz output of a 5-MHz to 1400-MHz (or
X280) multiplier. The first IF signal at 20.405 MF.z is amplified by 40 db
and then hetrodyned with the 20 -MHz output of the X4 multiplier. The se c<
and IF signal, at 405 kHz, is amplified by approximately 50 db and fed to one
input of a balanced phase detector. The reference signal for the phase
detector is supplied by a Hewlett-Packard 5102A digital synthesizer. The
synthesizer is normally set at 405, 794. 3 Hz to approximate the UTC time
scale. The output of the phase detector is used to phase lock a precision--
5-MHz quartz-crystal oscillator through a very high-gain operational intP-
grator. Double integration is incorporated inta the phase-locked loop so that
the phase tracking error (or velocity error) is very nearly zero. The time
constant of the integrator is selected so that the closed-loop response =s
critically damped and the closed-loop bandwidth is approximately 10 Hz.
The output of the phase-locked 5-MHz oscillator is used to drive the X280
and X4 multipliers and the digital synthesizer so that the entire maser clock
system is phase-coherent. A I-MHz signal, derived from internal dividers
in the crystal oscillator, is used to drive the VLBI timing system.
The Agassiz VLBI installation includes a Hewlett-Packard- 5065A rubidium
standard. This instrument provides the 5-MHz quartz-crystal oscillator used
in the hydrogen-maser clock system. The rubidium-cell reference loop in
the 5065A is normally disabled and, as shown in Figure 5, the quartz oscil-
lator is slaved to the maser. Using the rubidium standard in this fashion has
a nurnber of advantages. The cost of a„ additional quartz oscillator is elim-
inated, a stable backup standard is always available on-line, and the tuning of
the maser is simplified. To change from hydrogen-maser operation to rubi-
dium requires only the turn of a 6witch to transfer the electronic-frequency
3
control of the quartz oscillator from the external maser circuitry to the
internal rubidium cell. The amplitude of the clock signals at 5 MHz, 1 MHz,
and 100 kHz is not affected by-the changes in the phE.se-lock circuitry, nor
are any interconnection changes required.
4. TUNING THE HYDROGEN MASER
The oscillation frequency of a hydrogen maser is "pulled" by the-maser
cavity resonator. The amount of pulli.q is a function of cavity detuning and
the linewidth of the atomic resonance [Vanier and Ve`ssot, 1964]
v	 v	 0.29va2fiV
V = vH + cv H Q. -
	
2 0	 c Ovt
(_
 (1)
c	 Q' µ 0 '1 V b	-
where
V = maser oscillation frequency.
V  = center frequency of the atomic resonance line.
v c = center frequency of the cavity resonance.
Q = quality factor of the cavity.
v = mean velocity of the hydrogen atoms.
a0 = first Bohr orbit radius.
th = Planck's constant divided by 27r.
11 0 = Bohr m4gneton.
V c = volume of the cavity.
V  = volume of the storage bulb.
r) = ratio of average electromagnetic field energy density in the
storage bulb to average energy density over the entire cavity.
Avf = atomic resonance linewidth.
4
If the cavity is tuned to a frequency v c0 such that
1(2)Vc0 
vH	
0. 29va0 Th Vc
1-
Q2 µ^ q Vb
then the term in brackets in equation (1) vanishes and
V ' V 	 (3)
The maser oscillates at the center of the atomic resonance line.
The cavity is tuned to v c0 by modulating the flux- of atoms entering the
storage bulb. It can be shown that the spin - exchange contribution to the line
width is directly proportional to the flux, and referring to equation (1), when
the cavity is at= the tuning point v^0, then v is independent of line width and
therefore also independent of the total hydrogen flux.
The maser can be tuned by using a frequency reference must less stable
than that of the maser itself. The technique used is to switch the hydrogen
flux periodically between a very low level, near threshold, and a very high
level and to look for corresponding changes in the oscillation frequency of
the maser. The flux l-vel must be modulated as rapidly as possible to
minimize the phase-noise contribution from the reference oscillator. The
phase-noise power spectral density of quartz-crystal oscillators, rubidium
standards, and cesium standards decreases as the offset from the carrier
frequency is increased.
The effect of linear frequency drift in the reference standard can be
eliminated by using an algorithm devised by L. C. Cutler (private communi-
cation, 1968). The period of the beat between the maser wnd the reference
oscillator is measured an odd number of times, alternately at low and at
high flux. The measurements at low flux are assigned a negative sign, the
5
measurements taken at high flux a positive sign. The first and last meas-
urements are divided by two and all the measurements are then summed
algebraically. This residue,
-(a + a ) n-1
NR =	 12 
n +F ( - 1) k
 ak	(4)
k= 2
is a measure of the cavity detuning. It can be readily shown that the value of
the residue is unaffected by linear frequency drift of the reference.
The maser at Agassiz was tuned against- the HP 5065A rubidium standard
by the procedure outlined in the previous_ paragraph. Control of the atomic
hydrogen flux into. the maser _storage bulb is achieved by varying the RF power
level in the hydrogen dissociator. The power level can be switched between
two preset values by means of a manual control on the front panel or by a
logic signal from an external controller. The output of the phase detector
(Figure 3) was disconnected from the 5065A rubidium standard and tempo-
rarily connected to a digital frequency counter set to read 10-period averages.
The 5065A was slaved to its internal rubidium-cell reference to provide
maximum stability. The frequency of the digital synthesizer was offset by
1. 2 Hz to yield a beat period of 0. 833 sec at the phase-detector output for a
10-period measurement interval of 8. 333 sec.
Figure 6 shows the results - of a tuning run at Agassiz taken just before
the VLBI observations. The flux switching was done manually with the
front-panel control on the maser. The data, eleven 8. 3-sec measurements
for each point on the curve, were recorded by a digital printer. A desk
calculator was then used to calculate NR from equation (4).
The precision of this tuning procedure is estimated to be approximately
f7 X 10-13. That is, the maser can be reset to the same oscillation fre-
quency with a standard deviation of about one millihertz. The actual oscil-
lation frequency, the mean of a large number of such measurements, is a
Tile entire process has since been automated by the addition of an index
register and a reversible counter.
6
function of the bulb wall shift, the second-order doppler shift in the bulb,
and the magnetic field. These effects can be measured or calculated and
are discussed in the next section.
5. MASER ACCURACY AND LORAN-C
The wall shift of the SAO hydrogen maser has been directly measured
against the Harvard University reference maser, the SAO reference maser,
and a maser at the National Bureau of Standards The second-order doppler
was calculated for the known temperature of the storage bulb, and the
-nagnetic field was estimated-from the measur>d frequency of the Zeeman
transition. The oscillation frequency of the maser is determined as follows
[Hellwi et al. , 1970]
Unperturbed hydrogen frequency in
terms of the cesium second:
Conversion for Al to UT C:
Second-order doppler:
Measured wall shift:
Magnetic field shift:
Total (vH in UT  time scale):
1, 420, 405, 751. 768 Isz f 0. 003
42.612
- 0. 063
-	 - 0. 02 0
+ 0.001
1, 420, 405, 794.298 Hz t 0. 003
The digital-synthesizer -r esolution was limited to 0. 1 Hz, so the closest
synthesizer setting was 405, 794. 3 Hz. We thus introduced a known "error"
of -0. 002 Hz; the VLBI timing-system clocks should have run 1. 4 parts in
-10 -12 fast with respect to LORAN-C.
LORAN-C timing pulses were continuously monitored at Agassizfrom
24 November to 15 December 1969. The Agassiz VLBI timing system gained
6 msec with respect to LORAN-C, while the USNO Daily Phase Values pre-
dict a gain of 1. 6 µsec. The difference of 4. 4 µsec for a 21-day period
7
corresponds to a local clock Z. 42 X 10-12 nigh in frequency. Since it is
known that the Agassiz clo:-k was 1. 4 X 10-12 low owing to the granularity
of the synthesizer, the total average discrepancy between the maser and
LORAN-C was Z. 42 X 10 -12 + 1. 4 X 10-12 or 3. 8 X 10-12.
The LORAN-C transmissions are monitored and steered by the U. S.
Navai Observatory. The USNO establishes and maintains a coodinated
time scale based on the average of a large number of clock systems
in this country and abroad. The unperturbed hydrogen frequency shown in
an earlier paragraph was-measured against the ensemble of cesium clocks
at the National Bureau of Standards in Boulder, Colorado. The discrepancy
of 3. 8 parts ;.n 10-12 observed in this experiment lies well within the com-
bined error limits of the LORAN-C time scale and the precision of the
hydrogen-cesium intercomparison at NBS, Boulder.
6. VLBI TIMING SYSTEM
The hydrogen maser clock provides the 1-MHz signal to drive the VLBI
timing system shown in Figure 7. The SAO digital clock accumulates and
displays the time of -day with a resolution of 1 µsec. The digital clock also
generates a 1-pps signal for the SAO digital delay generator. The delay
generator is used to preset the LORAN-C propagation delay and to determine
the time differential between a portable clock and the system clock.
Time of day is set into the VLBI timing system by means of either a
portable quartz-crystal clock or LORAN-C transmissions. The digital clock
is designed so that it can be directly reset to the 1-pps portable-clock signal.
Setting the digital clock to the LORAN-C epoch is accomplished visually by
means of an oscilloscope triggered by the LORAN rate generator.
The DEMAND inputs of the timing system freeze the clock display at the
time of a demand input pulse. The LORAN rate generator, the delayed
1 pps, a VLF 1-pps signal, and the VLBI event encoder can be used to demand
the clock.
8
The complete VLBI timing system is described in greater detail else-
where in this issu. r . , (Michelini).
7. CONCLUSION
For VLBI observations of more than a few minutes or for observations
at wavelengths less than 5 cm, the hydrogen maser is the only satisfactory
frequency standard. The SAO hydrogen maser has demonstrated that it can
be used rout.nely and reliably for VLBI. The ease with which the maser can
be transported makes feasible the fabrication of complete traveling backends
for radio telescopes so that maximum utilization of existing observatories
and baselines can be obtained.
REFERENCES
Cutler, L. S. , and R. Vessot (1968), Present status of clocks and fre-
quency standards, NEREM Record, pp. 68-69.
Hellwig H., R. Vessot, M. Levine, P. Zitzewitz, and H. Peters (1970),
Determination c,,' the unperturbed hydrogen hyperfine transition frequenc ­
with the hydrogen maser, Transactions of the IEEE—Instrumentation
(in press).
Iileppner, D., R. Vessot, and N. Ramsey (1970), An orbiting clock experi-
ment to determine the gravitatiorai red shift, in Astrophysics and Space
Science, vol. 6, pp. 13-32, D. Reidel Publ. Co., Dordrecht-Holland,
Amsterdam.
Vanier, J. , and R. Vessot (1964), Cavity tuning and pressure dependence
of frequency in the hydrogen maser, App i. Phys. Lett. , 4, pp. 123-124.
Vessot, R., M. Levine, L. Cutler, M. Baker, and M. Mueller (1968),
Progress in the development of hydrogen masers, in Proceedings, 22nd
Annual Symposium on Frequency Control, Atlantic City, New Jersey.
List of Figure Captions
Fig. 1. The SAO hysrogen maser prepared for shipment from Cambridge
to Harvard, Massachusetts. The phase-lock receiver can be seen to the
rear of the maser.
Fig. 2. A section through the SAO hydrogen maser showing the major sub-
assemblies. This unit, which constitutes the maser oscillator, is mounted -
with the electronic support systems in a steel -cabinet 42" x-22 1 ' x 22".
Fig. 3. The RMS frequency stability of the hydrogen maser as a function
of observati-on time. The characteristics of typical quartz-crystal, rubidium,
and cesium standards are shown for comparison.
Fig. 4, The data of Fig. 3 are replotted to express directly the phase sta-
bility of the four types of frequency standards as^ a function of observation time.
All the data hale been normalized to 1 GHz [from Cutler and Vesso t, 1968].
Fig. 5. The maser clock system in block diagram form. The rubidium 	 =
cell in the Hewlett-Packard 5065A is disabled, and only the internal quartz-
crystal oscillator is used.
Fig. 6. Data from a tuning run of the hydrogen mi-ser using the rubidium
standard as a reference.
Fig. 7. The VLBI timing system shown controlled by the hydrogen-maser
clock system. The LORAN-C capability permits continuous monitoring of
the epoch.
10
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Hydrogen-maser time and frequency standard at Agassiz Observatory

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