Time & Frequency requirements
vs kinds of observations
Roberto Ambrosini
Institute of Radio Astronomy
Bologna
ambrosini@ira.inaf.it
Technology in Radio Astronomy & SS
Sardinian Summer School – 2nd course
R. Ambrosini
11-16 June 2012
1
Definitions
• TIME ( t ): obvious for everybody… but
“the indefinite continued progress of existence and events that occur in apparently
irreversible succession from the past through the present to the future.”
• Frequency ( n ): the number of occurrences of a
repeating event per unit time.
While n = 1 / t , their derived observable quantities
can assume different behaviors. For example
an interruption of a Time Scale – will destroy it
Technology in Radio Astronomy & SS
Sardinian Summer School – 2nd course
R. Ambrosini
11-16 June 2012
2
Measuring T&F
• A time measure requires a CLOCK made of:
–
–
–
–
a Frequency Standard (pendulum, quartz, atomic…);
an accumulator (clock display of MJD, HMS, ….);
a Synchronizer (Start – Stop);
an operating life longer than the interval under test
• Frequency is measured by a COUNTER:
– hardware is almost the same (even if arranged in a
different way, only digital).
Technology in Radio Astronomy & SS
Sardinian Summer School – 2nd course
R. Ambrosini
11-16 June 2012
3
Characteristics of Frequency Standards
For Relevance read top down,
for Verification read bottom up:
• ACCURACY - traceability to International Definition of Unit
• STABILITY - precision
» mass inertia (Astronomic standards)
» isolation from environment (Atomic standards)
• ACCESSIBILITY – type of measurement
Any stable oscillator can be a Frequency Standard.
This can become an (atomic) clock only if it is directly traceable to
the SI unit of time (second).
Technology in Radio Astronomy & SS
Sardinian Summer School – 2nd course
R. Ambrosini
11-16 June 2012
4
Standards with increased STABILITY
Astronomical (events with even larger masses or dimensions)
– Earth rotation (time of the day)→ UT0
– Earth revolution (time of the year) → UT1
– …….
– PULSAR
Atomic (better isolation from the environment, in a small volume)
–
–
–
–
–
Rubidium
Cesium (laser-cooled Cs fountain) defines Current Time Unit=1s
Hydrogen Maser (smaller atoms, pushed by a resonant cavity)
Ion Trap (only very few atoms)
Supeconducting Cavity Oscillator (only for better short term)
Technology in Radio Astronomy & SS
Sardinian Summer School – 2nd course
R. Ambrosini
11-16 June 2012
5
H- maser layout
Technology in Radio Astronomy & SS
Sardinian Summer School – 2nd course
R. Ambrosini
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6
Ways to compare instabilities (1)
Spectral Density of Phase Fluctuations
Sφ(f) = [rad2/Hz] →L(f) [dBc/Hz]
– A faithful description of all types of instabilities
Phase = (angle) time difference between two
standards tuned at the same frequency
– Diverges as time goes by, due to inevitable frequency
drifts of indipendent atomic clocks or poor standards
– Best for short term instabilities (less than 1 second)
– Called time jitter in digital systems;
– L(f) SSB directly measured by Spectrum Analyzer
Technology in Radio Astronomy & SS
Sardinian Summer School – 2nd course
R. Ambrosini
11-16 June 2012
7
Graphic examples
£ (f) [dBc/Hz] Single Sideband Noise = ½ Sφ(f)
Technology in Radio Astronomy & SS
Sardinian Summer School – 2nd course
R. Ambrosini
11-16 June 2012
8
£ degrades at least with N²
Multiplication
Factor n
Degradation
( dBc / Hz )
Multiplication
Factor n
Degradation
( dBc / Hz )
n=2
6.02
n = 16
24.08
n=3
9.54
n = 20
26.02
n=4
12.04
n = 24
27.60
n =5
13.98
n = 32
30.10
n=6
15.56
n = 48
33.62
n=8
18.06
n = 64
36.12
n=9
19.08
n = 128
42.14
n = 12
21.58
n = 256
48.16
Technology in Radio Astronomy & SS
Sardinian Summer School – 2nd course
R. Ambrosini
11-16 June 2012
9
Why using a Phase Lock Loop?
Technology in Radio Astronomy & SS
Sardinian Summer School – 2nd course
R. Ambrosini
11-16 June 2012
10
Ways to compare instabilities (2)
ALLAN Deviation
σ(y)t - dimensionless
– SQR of the Variance of the differences of the
frequency differences
– Overcomes the divergence issue, but “hides”
some information
– Best for medium and long term instabilities (
> 1 second)
Technology in Radio Astronomy & SS
Sardinian Summer School – 2nd course
R. Ambrosini
11-16 June 2012
11
Time Stability Analyzer
http://www.alma.nrao.edu/memos/html-memos/abstracts/abs310.html
• The Allan Variance algorithm ( for each t )
F (0)
time
t
F (1)
Dn1
t
F (2)
Dn2
3 - temporal phases
t
Dni =
F (i  1) - F(i )
t
sy2 (t) = 1/2 < (Dn1 - Dn2)2 >
2 - frac. frequencies
1 - data valid
t = 1, 2, 5, 10, 20, 50, . . . . . , 50 000, 100 000 seconds
Technology in Radio Astronomy & SS
Sardinian Summer School – 2nd course
R. Ambrosini
11-16 June 2012
12
Graphic examples
ALLAN Deviation
Technology in Radio Astronomy & SS
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σ(y)t - dimensionless
R. Ambrosini
11-16 June 2012
13
Graphic examples
ALLAN Variance
σ(y)t - dimensionless
Technology in Radio Astronomy & SS
Sardinian Summer School – 2nd course
R. Ambrosini
11-16 June 2012
From T4science web site
14
Time Stability Analyzer
http://www.alma.nrao.edu/memos/html-memos/abstracts/abs310.html
Frequency Standard #1
TSA
f mix = comparison frequency
A/D card
Frequency Standard #2
f mix
Vout = Kv sin( f(t) ) + Off
f(t) = arcsin (Vout –Off) / Kv
Technology in Radio Astronomy & SS
Sardinian Summer School – 2nd course
R. Ambrosini
11-16 June 2012
15
Transfer formulas (Sφ(f) << 1 rad2)
http://www.hpmemory.org/an/pdf/an_283-3.pdf
Technology in Radio Astronomy & SS
Sardinian Summer School – 2nd course
R. Ambrosini
11-16 June 2012
16
Same noise processes: different slopes
£ (f)
http://www2.rohde-schwarz.com/en/service_and_support/Downloads/Application_Notes/?type=20&downid=5168
Technology in Radio Astronomy & SS
Sardinian Summer School – 2nd course
R. Ambrosini
11-16 June 2012
17
Coherence loss (VLBI)
http://www.vlba.nrao.edu/memos/sci/sci04memo.pdf
Technology in Radio Astronomy & SS
Sardinian Summer School – 2nd course
R. Ambrosini
11-16 June 2012
18
Effect of a SMALL temperature gradient
http://www.ira.inaf.it/Library/rapp-int-2004/237-97.pdf
Technology in Radio Astronomy & SS
Sardinian Summer School – 2nd course
R. Ambrosini
11-16 June 2012
19
Where T&F become fundamental (1)
Antenna pointing
Antenna beamwidth ~ c / ( Dant • Freq )
Timing required is UT1,
but only UTC is distributed worldwide
(GPS, WWW, Radio, etc).
SRT at 100GHz needs a few millisecond sync
IERS Bulletin D – announces DUT1 value
Technology in Radio Astronomy & SS
Sardinian Summer School – 2nd course
R. Ambrosini
11-16 June 2012
20
Where T&F become fundamental (2)
Data acquisition
Path A - RF front end
1. Preampifier (cryostat, filters,..)
2. Local Oscillator chain is made of:
Station Freq. Standard
Multiplier x N (degrades with N²)
3. Amplitude Calibration (Noise gen.)
4. Phase Calibration
Antenna Unit
Ground unit
Path A
Path A
Path B - Backend
1. Passband Filters
2. Fractional Synthesizer
3. ADC – Digitizer and Formatter
Technology in Radio Astronomy & SS
Sardinian Summer School – 2nd course
Path B
R. Ambrosini
11-16 June 2012
Path B
21
T&F specs vs types of Observations (1)
Single dish
• Total Power
– Almost no spec neither on T, or on F
• Spectral Line
– From n and Dn/n → Frequency accuracy
– No special timing
• Pulsar
– 10-14 / Year
– Local Freq Standard acts as a Flying Wheel to TAI
• Tracking Doppler of Interplanetary spacecraft
– Radio Science Sky freq. = 32 GHz
– 10-14 / 1000s
– Round trip light time 72 minutes
Technology in Radio Astronomy & SS
Sardinian Summer School – 2nd course
R. Ambrosini
11-16 June 2012
22
Tracking Doppler of
the Cassini
spacecraft
Coherent frequency translators on board of Cassini
X; Ka
X ; Ka 
Transmission
from a
Deep Space
Antenna
Round Trip Light Time = 72 minutes
Technology in Radio Astronomy & SS
Sardinian Summer School – 2nd course
R. Ambrosini
11-16 June 2012
Downlink
received at the
Noto (I)
Radiotelescope
23
A new Ka-band receiving capability
at the Italian Noto radiotelescope
•
•
•
•
Tip and tilt adjustments of the feed
Thick passive insulation
Peltier cooling of the receiver box: a fan inside avoids stratification of
the air
Power supplies in a separate section
Technology in Radio Astronomy & SS
Sardinian Summer School – 2nd course
R. Ambrosini
11-16 June 2012
24
A new Ka-band receiving capability
at the Italian Noto radiotelescope
Fuoco Primario
ORN X-KA
RF=31.85-32.25
IF1=1.45-1.85
DBS DB00-0734
LO=15.2GHz Lev el 10dBm
I.F.= 1.65GHz +/- 200MHz
G=20dB 600mA 9W
N.F.= 6dB
Filtro BP LORCH 32GHz BW 2.6GHz
QUEST 1450-1850MHz
1
L
R
I
Filtro BP KL 300/400
0.3 dB
L
I
X2
MIC 1650/516 cav ity
3dB
ATT.
DORADO 32GHz
0.3 dB
LNA MITEQ AFS3-08000850-07-10P-4
N.F.=0.7dB
GAIN=30dB
QUEST SM0812T01
8-12.4GHz
OUT Ka
0.3 dB
R
LNA 32GHz G35dB
ZFL-1000VH
GAIN=20dB
WJ-M8TC
N.F.=4.5 dB
RF/OL=0.001-3.4 GHz IP3 33dBm
I.F.= 0.001-2GHz
320mA
Lev el 10dBm
3.75W
QUST 1.2-2.4GHz
0.3 dB
4.2W
MIC 8400/560 cav ity
CTI PDRO 15200 locked to 50MHz a 0dB
280mA
+13dBm
MiniCircuit ZEDC-10-2B
1-1000MHz
OUT
IN
.
.
5MHz
ATT.
alimentazioni +5 e +15V
Mitek LP-1350-50-1-15P
+13dBm min.
250mA
IN
4.2W
MiniCircuit ZESC-2-11
10-2000MHz
OUT
OUT
CTI PDRO 6750 locked to 50MHz a 0dB
280mA
+13dBm
ATT.
10dB
MITEQ PLD-5-50-15-P
lev el=+16dBm
130mA
.
OUT
MiniCircuit ZESC-2-5
10-1500MHz
3dB
RF=8.2-8.6
QUST 1.2-2.4GHz
IF=1.45-1.85
R
MiniCircuit ZJL-3G
N.F.=3.8dB
QUEST 6.4-8.5GHz
GAIN=19dB
30-3000MHz
IP3=22 45mA
QUEST 1450-1850MHz
0.54W
QUEST 1450-1850MHz
I
WJ-M76C
RF/LO=4.5-9.5GHz
I.F.= DC-2.0 GHz
Lev el 10 dBm
ZFL-1000VH2
GAIN=28dB
N.F.=4.5 dB
IP3 38dBm
320mA
L
L
JCA812-201
N.F.=3.5dB
GAIN=18dB
8-12GHz
IP3=23 100mA
QUEST SM0812T01
8-12.4GHz
1.5W
R
MIC 1650/516 cav ity
0.3 dB
OUT X
I
WJ-M8TC
RF/OL=0.001-3.4 GHz
I.F.= 0.001-2GHz
Lev el 10dBm
4.8W
Filtro BP KL 300/400
0.3 dB
Technology in Radio Astronomy & SS
Sardinian Summer School – 2nd course
Mixing products (IF and LO frequencies),
filters and amplifier gains are selected for
best Tsys, Phase Noise and IP3.
All oscillators are locked to an H-Maser,
the station Atomic Frequency Standard.
Instantaneous BW is 400MHz in both
bands.
R. Ambrosini
11-16 June 2012
25
T&F specs vs types of Observations (2)
Interferometer
• Astronomical VLBI
– Sky Frequency determines max Phase Noise L(f) (short term)
– Max Integration time determines Tau in Allan Deviation
– Theoretically: NO TIMING (VLBI itself makes clock comparison)
– Practically: to reduce Max Fringe Search = GPS sync ~ 10ns
• GEO VLBI
– Delay and Delay rate, Bandwidth synthesis, Iono correction,
– 1 mm goal = 3 picoseconds !!!!
Technology in Radio Astronomy & SS
Sardinian Summer School – 2nd course
R. Ambrosini
11-16 June 2012
26
Conclusions
• Each type of Observation pushes for its own separate
requirements on Time AND Frequency.
• The Hydrogen Maser by itself is not enough to
guarantee a specific overall Stability: consider the
contribution of each block of the data acquisition chain.
• Express each contribution in Time Units (picoseconds)
to avoid scaling them.
• Phase Noise (short term) fixes maximum Sky frequency
• Allan Deviation puts a limit on the max integration Time
(do not forget to include other effects, such as:
tropospheric turbulence, antenna deformations,
temperature gradients in all devices).
• In VLBI the total coherence loss accounts for the real
performance of each station
Technology in Radio Astronomy & SS
Sardinian Summer School – 2nd course
R. Ambrosini
11-16 June 2012
27
Even SRT was not built in a day !
Technology in Radio Astronomy & SS
Sardinian Summer School – 2nd course
R. Ambrosini
11-16 June 2012
28