In the NASA/JPL Deep Space Network (DSN), radio science experiments (probing outer planet atmospheres, rings, gravitational waves, etc.) and very long-base interferometry (VLBI) require ultra-stable, low phase noise reference frequency signals at the user locations. Typical locations for radio science/VLBI exciters and down-converters are the cone areas of the 34 m high efficiency antennas or the 70 m antennas, located several hundred meters from the reference frequency standards. Over the past three years, fiber optic distribution links have replaced coaxial cable distribution for reference frequencies to these antenna sites. Optical fibers are the preferred medium for distribution because of their low attenuation, immunity to EMI/IWI, and temperature stability. A new network of Beam Waveguide (BWG) antennas presently under construction in the DSN requires hydrogen maser stability at tens of kilometers distance from the frequency standards central location. The topic of this paper is the design and implementation of an optical fiber distribution link which provides ultra-stable reference frequencies to users at a remote BWG antenna. The temperature profile from the earth's surface to a depth of six feet over a time period of six months was used to optimize the placement of the fiber optic cables. In-situ evaluation of the fiber optic link performance indicates Allan deviation on the order of parts in 10(exp -15) at 1000 and 10,000 seconds averaging time; thus, the link stability degradation due to environmental conditions still preserves hydrogen maser stability at the user locations. This paper reports on the implementation of optical fibers and electro-optic devices for distributing very stable, low phase noise reference signals to remote BWG antenna locations. Allan deviation and phase noise test results for a 16 km fiber optic distribution link are presented in the paper

Calhoun, Malcolm

Kuhnle, Paul

Law, Julius

English

NASA Technical Reports Server

N95- 32346
_f/ (i" j
FIBER OPTIC
REFERENCE FREQUENCY DISTRIBUTION
TO REMOTE BEAM WAVEGUIDE ANTENNAS*
I
t
I5,
MALCOLM CALHOUN, PAUL KUHNLE, and JULIUS LAW
Jet Propulsion Laboratory
California Institute of Technology
Pasadena, California 91109
Abstract
In the NASA/JPL Deep Space Network (DSN), radio science experiments (probing outer planet
atmospheres, rings, gravitational waves, etc.) and very long-base interferometry (VLBI) require
ultra-stable, low phase noise reference frequency signals at the user locations. _pical locations
for radio science/VLBl exciters and down-converters are the cone areas of the .]4 m high eJJiciency
antennas or the 70 m antennas, located several hundred meters from the reference frequency
standards. Over the past three years, fiber optic distribution links have replaced coaxial cable
distribution for reference frequencies to these antenna sites. Optical fibers are the preferred medium
for distribution because of their low attenuation, immunity to EMi/RFI, and temperature stability.
A new network of Beam Waveguide (BWG) antennas presently under construction in the DSN
requires hydrogen maser stability at tens of kilometers distance from the frequency standards central
location. The topic of this paper is the design and implementation of an optical fiber distribution
link which provides ultra-stable reference frequencies to users at a remote BWG antenna.
The temperature profile from the earth's surface to a depth of six feet over a time period of six
months was used to optimize the placement of the.fiber optic cables. In-situ evaluation of the fiber
optic link performance indicates Allan deviation on the order of parts in !0 l._ at 1000 and 10,000
seconds averaging time; thus, the link stability degradation due to environmental conditions still
preserves hydrogen maser stability at the user locations. This paper reports on the implementation
of optical fibers and electro-optic devices for distributing very stable, low phase noise reference
signals to remote BWG antenna locations. Allan deviation and phase noise test results for a 16 km
fiber optic distribution link are presented in the paper.
INTRODUCTION
The NASA/JPL Deep Space Network is expanding its spacecraft tracking capability with a
network of 34 meter Beam Waveguide antennas. A cluster of three of these antennas at
the Goldstone Tracking Station (GTS) is located a distance of 16 kilometers from the Signal
'The reseatrch described in this paper was carried out at the Jet, Propulsion Laboratory, California Institute of
Technology, under a contract sponsored by the National Aeronautics and Space Administration.
415
https://ntrs.nasa.gov/search.jsp?R=19950025925 2020-06-16T06:09:26+00:00Z
ProcessingCenter (SPC). Deep SpaceStation 24 (DSS 24), the first of the cluster to be
completedis scheduledto go on-line in late 1994. In svpport of antenna tracking functions
as well as radio science and VLBI experiments, precise time and stable reference frequency is
required at this remote antenna site. The Frequency and Timing Systems Engineering Group at
JPL is responsible for providing reference frequency and precise time to users at the antenna.
Certain applications at the antenna require frequency stability and phase noise of the quality
of a hydrogen maser. Since the hydrogen maser frequency standard is located at the SPC, the
problem becomes one of distributing the signals to remote locations without signal degradation.
The distance between the SPC and the antennas is too great to consider coaxial cable for
the reference frequency and time signals; also, microwave links do not provide the required
stability. The method of choice for the distribution implementation is fiber optic links. Due
to cost constraints, commercial off-the-shelf equipment was utilized as much as possible. The
optical fibers in the link are standard, single mode SMF-28, 96 fibers contained in a direct
burial cable. The burial depth is approximately 1.5 meters.
The hardware implementation for timing and reference frequency along with tests results
measured after the installation was completed are presented in this paper. Also, stability
considerations based on temperature effects on the optical fibers are discussed.
DISTRIBUTION HARDWARE
The 5 MHz reference frequency signal and the modified IRIG-G time code signals are
transmitted over separate fiber optic links to avoid corruption of the reference signal. The
terminal equipment for the fiber links is the Wavelink model 3290 manufactured by the Grass
Valley Group, a subsidiary of Tektronix, Inc. The transmitter consists of a 1300 nm laser diode
along with the required bias and modulation circuits. The receiver contains a photodetector
for converting the 1300 nm light to RF which is demodulated to recover the signal. In order
to meet the phase noise requirements for radio science applications, the 5 MHz reference
freqt, ency recovered from the optical receiver is phase locked by a FTS 1050 Disciplined
Frequency Standard (DFS) with a 1 Hz loop bandwidth . The signal from the DFS is then
distributed to the antenna users. A block diagram of the frequency and timing distribution is
shown in Figure 1.
The filtered 5 MHz from the disciplined frequency standard is applied to a distribution assembly
where it is multiplied to 10 MHz and to 100 MHz for users who require these frequencies.
The distribution assembly employs low noise, high isolation amplifiers . The entire assembly is
temperature controlled for improved stability.
The modified IRIG-G time code signal utilizes a 100 KHz carrier frequency. The source
signal is derived from the Time Insertion Distribution Assembly at the SPC, applied to the
laser transmitter, and recovered at the fiber optic receiver in the antenna pedestal room. A
travelling clock was used to set the time at the remote antenna; the time offset between DSS
24 and the SPC is less than 100 nanoseconds. Approximately 82 microseconds time delay was
removed at the Time Code Translator because of the 16 km of optical fiber.
416
TEMPERATURE EFFECTS ON STABILITY
The cables which distribute the reference signals to the remote antennas are buried at a
depth of approximately 1.5 meters. This burial depth is sufficient to mitigate the effects of
large diurnal temperature variations; however, seasonal changes and weather fronts can still be
sensed even at a depth of 2 meters. Figure 2 is a plot of surface temperature variations at
Goldstone Tracking Station, which is located in the California Mojave desertlll. Temperatures
were recorded at four hour intervals for the period 11 June 1992 to 14 June 1992. Observe
the extremes from a low near 12°C to a high near 55°C, with an average T of 35°C per 12
hour interval.
The 1.5 meter burial depth was determined by observing the temperature profile of the earth
in the Mojave desert for several monthsf II. Thermocouples were buried at depths of 0.6, 0.9,
1.2, 1.5, and 1.8 meters, respectively. A data logger with a computer was used to record these
data. The results of the measurements are shown in Fignre 3. Measurements were begun on
14 January 1992 and terminated on 26 June 1992. Analysis of the data indicates that a burial
depth of 1.5 meters is sufficient to attenuate the short term temperature variations. In Figure
3, the line with the larger variations is the daily average surface temperature.
The thermal coefficient of delay for the optical fiber is approximately 7 ppm/°C. The length of
buried cable is 16 km. At a depth of 1.5 meters, a peak to peak temperature variation of 35°C
is reduced to less than O.l°C, peak to peak.
may be calculated as follows:
The phase variations due to temperature effects
A¢ = AL × 360°/),0
AL = LkAT
where A¢ is the change in phase delay introduced by the temperature variation T, k is the
thermal coefficient of delay of the fiber in ppm/°C, L is the optical fiber length in meters, and
Ao is the wavelength of the reference signal in the medium. At a measurement frequency of
100 MHz, the wavelength in the fiber is 2.1 meters. Calculating the phase change for a 35°C
surface excursion and a worst case 0.1°C peak to peak at the fiber yields 1.92 ° phase change at
100 MHz for the 16 km fiber link. This calculated value of A_ is compared with test results
in the next section of this paper.
STABILITY AND PHASE NOISE TEST RESULTS
Since there is no reference signal at the remote antenna site to compare the fiber optic
distributed signal, the scheme shown in Figure 4 was used to measure the stability of the
reference signal. The 5 MHz signal from the DFS was applied to the Reference Frequency
Distribution Assembly where it is multiplied to 10 MHz and to 100 MHz. The 100 MHz
output from this assembly was applied to the transmitter of a Fiber Optic Reference Freqency
Distribution Assembly (FODA) which is known to have stability and phase noise performance
an order of magnitude lower than a hydrogen maserlZl. The signal was then returned to the
417
SPCover a test fiber in the samecablebundle that wasusedto sendthe referencesignal to
DSS24. Figure5 showsthe stabilitytest resultsusingthe configurationshownin Figure4. The
Allan deviation shownin Figure 5 was takenwith a temporaryfiber optic cableto complete
the cable run to DSS24 before the installationwas completed. Approximately 420 meters
of fiber cable was exposed at the surface of the Mojave desert during these measurements.
The temporary cable failed to meet system requirements. Figure 6 shows the change in phase
delay as a function of time. The temporary fiber cable caused a change in time delay of
approximately 14 ° per 12 hot, r period at 100 MHz. The corresponding Allan deviation at the
half-day period is 1.5 × 10 -14 which does not meet the system requirements.
Figure 7 shows the results of the stability test after the installation of the permanent fiber optic
cable. The test results shown are for a fiber optic cable buried at approximately 1.5 meters,
with a total length of 16 km. Note that the Allan deviation is well below the specification
limits with the exception of Tau = 1. This stability anomaly is believed due to the DFS which
has a loop bandwidth of less than 1 Hz and a slight overshoot at 1 Hz. The change in phase
delay over the fiber optic link is shown in Figure 8. The results indicate a peak-to-peak time
variation (at 100 MHz) of approximately 110 picoseconds, which equates to 1.981_/rc (for 16
km ) per 12 hour interval, almost an order of magnitude improvement over the temporary fiber
installation. Using the
l)eltao equation from the previous section yields a calculated value of 1.92 ° per 12 hour
interval, which closely agrees with the measured phase delay. Observe in Figure 7 that the
Allan deviation value at the half-day interval is approximately 1.5 × 10-1'_, which is an order
of magnitude better than the temporary fiber and also meets the system requirement for long
term stability.
Phase noise tests at DSS 24 were run using the test configuration shown in Figue 9. The
test system included a high quality test oscillator which was phase locked to the distributed
reference signals. Test results are summarized in Table 1.
Table 1. SPC 10 to DSS 24 Phase Noise Test Results
PHASE NOISE TEST RESULTS AT DSS 24
FREQUENCY
OFFSET
FROM
CARRIER
(Hz)
ESTIMATED
PERFORMANCE
AT X BAND:
FROM D-LEVEL
REVIEW
12-17-92
(dBc)
MEASURED
AT 5 MHz
(dBc)
MEASURED
AT 100 MHz
(dBc)
EQUIVALENT
AT X-BAND
(E(f) 5 MHz
-64 dB)
(dBc)
1 -52 -121 -96 -57
10 -66 -140 -115 -76
100 -77 -148 -123 -84
1000 -77 -150 -125 -86
10000 -77 -151 -125 -87
100000 -77 -154 -126 -90
418
TIMING DISTRIBUTION
The timing distribt,tion signal for DSS 24 is obtained from the master clock at SPC 10. The
signal is a modified IRIG-G time code which is derived from the Time Insertion Distribution
Assembly (TIDS) at SPC 10. The signal flow from the source to the remote antenna is shown
in Figure 1. In order to have the time offset at DSS 24 within the required 1 microsecond of
the SPC 10 master clock, a travelling Cesium Clock was used to determine and remove the
time delay over the 16 km fiber optic cable. Approximately 82 microseconds of time delay was
removed by a special Time Code Translator (TCT) at the remote antenna. Consequently, the
remote clock at DSS 24 is within 50 nanoseconds of the SPC 10 clock and the measured jitter
at the antenna is less than 2 nanoseconds.
CONCLUSIONS
The fiber optic reference frequency and timing distribution from SPC 10 to DSS 24 is complete.
Testing was begun with a temporary fiber optic cable with 420 meters exposed to the desert
extremes of hot and cold temperatures. Test results did not meet system requirements, and
thus were delayed until a permanent, boried fiber cable was installed. Test results with the 16
km of buried cable indicate that the system phase noise performance meets requirements with
some margin. The stability of the reference signals is within system requirements except at Tau
= 1, where the commercial fiber optic terminal eqt, ipment and the DFS slightly degrade the
Allan deviation. Commercial, off-the-shelf-equipment was used in order to stay within cost
constraints of the project.
After removing 82 microseconds of cable delay, the remote clock at DSS 24 is within + 50
nanoseconds time offset of the master clock with a jitter of less than 2 nanoseconds. The
timing distribution meets all system requirements at the remote antenna site.
REFERENCES
[1.] M. Calhoun, E Kuhnle, and J. Law, "E_t,iron, mental Effects o7_ the Stability of Opti-
c¢Ll FibeT's used for RefereTtee FT"equency DistributioTf', Proceedings of the Institute of
Environmental Sciences, Las Vegas, NV, May 1993.
[2.] M. Calhoun and R Kuhnle, " Ultrastable Reference b)'eque_ey DistT"ib'ation Utilizing a
Fiber Optic Link", Proceedings, 24th Precise Time and Time Interval Applications and
Planning Meeting, Tysons Corner, VA, December, 1992.
419
5 MHZ
FROM HYDROGEN MASER
ITANDARD, BPC 10
TIME CODE
FROM TIME INSERTION
AND DISTRIBUTION, BPC 10
I
FIBER OPTIC FIBER OPTIC
TRANSMITTER TRANSMITTER I
FIBER OPTIC
RECEIVER
FIBER OPTIC
RECEIVER
I TID$ CODE i
_- AND 5 MHz ;_
DIBTRIBUTION I
I ITO 8PC ALARM .91___ DIICIPUNEDFREQUENCYPANEL STANDARD
SMHz
! R_-,.mcE I _
FREQUENCY
lO MHz _. _DIBTRIEU_ON ___ lOO MHz
-" I AssEMBLY I --
5 MHz
SPC 10
DSS 24
BUFFER AMP __
j TOTcT,
Figure I. BLOCK DIAGRAM OF REFERENCE FREQUENCY AND TIMING DISTRIBUTION,
SPC I0 TO DSS 24.
420
6O
11 JUNE- 14 JUNE 1992
5O
"-" 40 fLD
I,I
rY
30
<_
r'r"
LI.J
EL_
Ld 20
I.--
/
0 43200 86400 129600 17280(D '2 1600(} '259200
IOOO 1000
6/11/92 6/14/92
Figure 2. SURFACE TEMPERATURE MEASURED AT GOLDS'TONE TRACKING STATION
4O
C)
LJ
D_
<
rY
LJ
LLJ
t---
55
5O
25
HI_AverGgeDo,,ySur oce
_f_Io m
2o
15 _'-
10
5
0 5,1 .59 67 7.5 83 91 99 107 115 12.3 1.31 139 147 15.5 163 171
DAY OF YEAR
Figure 3. GROUND TEMPERATURE MEASURED AT GOLDSTONE TRACKING STATION
421
JPL
SPECIAL 100 MHz - 1Hz
H-MASER OFFSET 1
GENERATOR
I l JPc
H.TEO LOW
100 MHz MIXER PASS I J VARIANCE
CRG FILTER I ]MEASUREMENT
I [ sv._.A0 dBm
i
"_ 100 MHZ
5 MHz
FODA
FIBER OPTIC RECEIVER SPC 10
TRANSMITrER
ASSEMBLY
I 16 km
| SINGLE MODE
....... l................................................... -L--°P-T!-c--A-L-F! BER.........................../|
FODA
TRANSMITTER
16 km
SINGLE MODE
OPTICAL FIBER
X20
FTS 1050
DISCIPLINED
FREQUENCY
STANDARD
DSS 24
FIBER OPTIC
RECEIVER
ASSEMBLY
8 MHz
TIDS CODE AND
5 MHz
DISTRIBUTION
ASSEMBLY
Figure 4. TEST CONFIGURATION FOR ALLAN DEVIATION MEASUREMENT,
SPC I0 TO DSS 24.
422
1.00E-12
1.00E-13
1.00E-14 _m
1.00E-15
i i i i
1.00E-16
1 3 10 30 100 300 1000 3000 10000 30000
TAU(SECONDS)
Figure 5. ALLAN DEVIATION AT DSS 24 WITH 420 METERS OF EXPOSED FIBER OPTIC CABLE
3oo
lOO
©
_) oU_
oo
© -loo
O_
v -200
D
-'/00
\
o 50 lOO 15o
TIME (SECONDS X 1000)
20O
Figure 6. TIME RESIDUALS AT DSS 24 WITH 420 METERS OF EXPOSED FIBER OPTIC CABLE.
423
1.00E-12
1.00E-13 \
1.00E-14
rJ'3
1.00E-15
31
1.00E-16 .....
1 3 10 30 100 300 1000 3000 101300 30000
TAU(SECONDS)
Figure 7. ALLAN DEVIATION AT DSS 24 WITH 16 km OF BURIED FIBER OPTIC CABLE
150
O
r_) 100
0
D
_ o_
-5O
50 1130 150 200
TIME (SECONDS X 1000)
Figure 8. TIME RESIDUALS WITH 32 km OF BURIED FIBER OPTIC CABLE
424
CRG I_t_--
5MHz
RBER OPTIC
TRANSMITTER
ASSEMBLY
HJAASER
SPC 10
_ 8INGLE MODEOPTICAL FIBER
RBER OPTIC
RECEIVER
ASSEMBLY
| MNJ
HP 391
FFT
ANALYZER
g
JPL VLG 10 JIPHASE NOISEMEASUREMENTSYSTEM
16km
SINGLE MODE
OPTICAL FIBER
IOOMHz III°MHz I
I REFERENCE 1
FREQUENCY
DISTRIBUTION
ASSY
_I oM_
m1050
DlSaPUNED
FREQUENCY
STANDARD
I IMN_
TID8 CODE AND
5 MHz
DISTRIBUTION
ASSEMBLY
S MHZ
DSS 24
Figure 9. TEST CONFIGURATION FOR PHASE NOISE MEASUREMENT AT DSS 24
425
QUESTIONS AND ANSWERS
FREDERICK WALLS (NIST): How much of the phase noise degradation was due to the
optical transmit received and how much due to fiber noise?
MALCOLM CALHOUN (JPL): Anything within about three or four Hz was degraded by
the optical fiber transmitted and received. The band width of this frequency standard brought
the noise floor down very rapidly. At 10 Hz, we're minus 140 dB below the carrier. And
the floor on this was limited by the oscillator in the disciplined frequency standard; it's about
minus 155. So it's just a little bit -- very close into the carrier.
426


Fiber optic reference frequency distribution to remote beam waveguide antennas

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