Technology in Radio Astronomy & SS Sardinian Summer School – 2 nd course R. Ambrosini 11-16 June...

Technology in Radio Astronomy & SSSardinian Summer School – 2nd course

R. Ambrosini11-16 June 2012

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Roberto AmbrosiniInstitute of Radio Astronomy

[email protected]@ira.inaf.it

Time & Frequency requirements vs kinds of observations



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DefinitionsDefinitions

• TIME ( t ): obvious for everybody… butTIME ( t ): obvious for everybody… but “ “the indefinite continued progress of existence and events that occur in apparently the indefinite continued progress of existence and events that occur in apparently

irreversible succession from the past through the present to the future.”irreversible succession from the past through the present to the future.”

• Frequency ( Frequency ( ): ): the numberthe number of occurrences of a of occurrences of a repeating event per unit time.repeating event per unit time.

While While = 1 / t= 1 / t , their derived observable quantities , their derived observable quantities can assume different behaviors. For examplecan assume different behaviors. For example

an interruption of a Time Scale – will destroy an interruption of a Time Scale – will destroy itit



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Measuring T&FMeasuring T&F

• A time measure requires a CLOCK made of: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:Frequency is measured by a COUNTER:– hardware is almost the same (even if arranged in a

different way, only digital).



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Characteristics of Frequency StandardsCharacteristics of Frequency Standards

ForFor Relevance Relevance read top down,read top down, forfor Verification Verification read bottom up:read bottom up:

• ACCURACY - ACCURACY - traceability to International Definition of Unittraceability to International Definition of Unit

• STABILITY - STABILITY - precision precision » mass inertia (Astronomic standards)mass inertia (Astronomic standards)» isolation from environment (Atomic standards)isolation from environment (Atomic standards)

• ACCESSIBILITY – ACCESSIBILITY – type of measurementtype of measurement

Any stable oscillator can be a Frequency Standard. Any stable oscillator can be a Frequency Standard. This can become an (atomic) clock only if it is directly traceable to This can become an (atomic) clock only if it is directly traceable to

the SI unit of time (second).the SI unit of time (second).



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Standards with increased STABILITYStandards with increased STABILITY

Astronomical Astronomical (events with even larger masses or dimensions)(events with even larger masses or dimensions)

– Earth rotation (time of the day)→ UT0– Earth revolution (time of the year) → UT1– …….– PULSAR

AtomicAtomic (better isolation from the environment, in a small (better isolation from the environment, in a small volume)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)



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H- maser layout



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Ways to compare instabilities (1)Ways to compare instabilities (1)

Spectral Density of Phase Fluctuations Spectral Density of Phase Fluctuations

SSφφ(f) = [rad(f) = [rad22/Hz]/Hz] →→LL(f)(f) [dBc/Hz]– A faithful description of all types of instabilities

Phase 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;

– LL(f)(f) SSB directly measured by Spectrum Analyzer



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Graphic examplesGraphic examples

£ (f)£ (f) [dBc/Hz] Single Sideband NoiseSingle Sideband Noise = = ½ S½ Sφφ(f)(f)



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£ degrades at least with N²

MultiplicationFactor n

Degradation( dBc / Hz )

MultiplicationFactor 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



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Why using a Phase Lock Loop?Why using a Phase Lock Loop?



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Ways to compare instabilities (2)Ways to compare instabilities (2)

ALLAN Deviation ALLAN Deviation

σσ(y)(y)dimensionlessdimensionless

– 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)



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Time Stability Analyzer

• The Allan Variance algorithm ( for each

(0) (1) (2) 3 - temporal phases

time

y)2

seconds

)()1( ii

i

2 - frac. frequencies

1 - data valid

http://www.alma.nrao.edu/memos/html-memos/abstracts/abs310.html



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ALLAN Deviation ALLAN Deviation σσ(y)(y)dimensionlessdimensionless



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ALLAN Variance ALLAN Variance σσ(y)(y)dimensionlessdimensionless

From T4science web site



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Time Stability Analyzer

Frequency Standard #1

A/D card

f mix = comparison frequency

Vout = Kv sin( (t) ) + Off

(t) = arcsin (Vout –Off) / Kv

f mixFrequency Standard #2

http://www.alma.nrao.edu/memos/html-memos/abstracts/abs310.html



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Transfer formulas (SSφφ(f) << 1 rad(f) << 1 rad22))

http://www.hpmemory.org/an/pdf/an_283-3.pdf



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Same noise processes: different slopes£ (f)£ (f)

http://www2.rohde-schwarz.com/en/service_and_support/Downloads/Application_Notes/?type=20&downid=5168



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Coherence loss (VLBI)

http://www.vlba.nrao.edu/memos/sci/sci04memo.pdf



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Effect of a SMALL temperature gradient

http://www.ira.inaf.it/Library/rapp-int-2004/237-97.pdf



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Where T&F become fundamental (1)

Antenna pointingAntenna pointingAntenna 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 syncIERS Bulletin D – announces

DUT1 value



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Where T&F become fundamental (2)Data acquisitionData acquisition

Path A - RF front endRF front end

1. Preampifier (cryostat, filters,..)2. Local Oscillator chain is made of:

Station Freq. StandardMultiplier x N (degrades with N²)

3. Amplitude Calibration (Noise gen.)4. Phase Calibration

Antenna UnitGround unit

Path B - Backend1. Passband Filters2. Fractional Synthesizer3. ADC – Digitizer and Formatter

Path A Path A

Path B Path B



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T&F specs vs types of Observations (1)

Single dish• Total Power

– Almost no spec neither on T, or on F

• Spectral Line– From and → 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



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Tracking Doppler of Tracking Doppler of the Cassini the Cassini spacecraftspacecraft

Transmission from a Deep Space Antenna

Coherent frequency translators on board of Cassini

X ; Ka X; Ka

Downlink received at the Noto (I) Radiotelescope

Round Trip Light Time = 72 minutesRound Trip Light Time = 72 minutes



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• 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

A new Ka-band receiving A new Ka-band receiving capability capability

at the at the Italian Noto radiotelescopeItalian Noto radiotelescope



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A new Ka-band receiving A new Ka-band receiving capability capability

at the at the Italian Noto radiotelescopeItalian Noto radiotelescopeATT.

3 d B

L N A M I TE Q A F S 3 -0 8 0 0 0 8 5 0 -0 7 -1 0 P -4N . F . =0 . 7 d BG A I N =3 0 d B

4.2W

4.2W

Z F L -1 0 0 0 V H

I P 3 3 3 d B m3 2 0 m A

N . F . =4 . 5 d BG A I N =2 0 d B

Z F L -1 0 0 0 V H 2

I P 3 3 8 d B m3 2 0 m A

N . F . =4 . 5 d BG A I N =2 8 d B

X2

4.8W

F ilt ro B P K L 3 0 0 / 4 0 00 . 3 d B

3.75W

M it e k L P -1 3 5 0 -5 0 -1 -1 5 P+1 3 d B m m in .

2 5 0 m AC TI P D R O 6 7 5 0 lo c k e d t o 5 0 M H z a 0 d B

2 8 0 m A+1 3 d B m

1.5W

M I C 1 6 5 0 / 5 1 6 c a v it y

0 . 3 d B

1

O R N X-K A

IF=1.45-1.85

0.54W

M in iC irc u it Z J L -3 G

G A I N =1 9 d BN . F . =3 . 8 d B

3 0 -3 0 0 0 M H zI P 3 =2 2 4 5 m A

D B S D B 0 0 -0 7 3 4

I . F . = 1 . 6 5 G H z + / - 2 0 0 M H zL O =1 5 . 2 G H z L e v e l 1 0 d B m

6 0 0 m AN . F . = 6 d BG =2 0 d B

R I

L

Q U E S T 1 4 5 0 -1 8 5 0 M H z

C TI P D R O 1 5 2 0 0 lo c k e d t o 5 0 M H z a 0 d B2 8 0 m A+1 3 d B m

9W

M I C 8 4 0 0 / 5 6 0 c a v it y

0 . 3 d B

Q U E S T 1 4 5 0 -1 8 5 0 M H z

RF=31.85-32.25

F ilt ro B P K L 3 0 0 / 4 0 00 . 3 d B

Q U E S T S M 0 8 1 2 T0 18 -1 2 . 4 G H z

M in iC irc u it Z E D C -1 0 -2 B1 -1 0 0 0 M H z

.

. .

O U T K a

J C A 8 1 2 -2 0 1

G A I N =1 8 d BN . F . =3 . 5 d B

8 -1 2 G H zI P 3 =2 3 1 0 0 m A

D O R A D O 3 2 G H z

M in iC irc u it Z E S C -2 -51 0 -1 5 0 0 M H z

I N

OU

TO

UT

O U T X

RF=8.2-8.6

W J -M 7 6 CR F / L O =4 . 5 -9 . 5 G H zI . F . = D C -2 . 0 G H zL e v e l 1 0 d B m

R I

L

Q U S T 1 . 2 -2 . 4 G H z

Fuoco Primario

ATT.

3 d B

Q U E S T S M 0 8 1 2 T0 18 -1 2 . 4 G H z

W J -M 8 TCR F / O L =0 . 0 0 1 -3 . 4 G H zI . F . = 0 . 0 0 1 -2 G H zL e v e l 1 0 d B m

R I

L

M in iC irc u it Z E S C -2 -1 11 0 -2 0 0 0 M H z

I N

OU

TO

UT

M I TE Q P L D -5 -5 0 -1 5 -P

1 3 0 m Ale v e l=+1 6 d B m

Q U S T 1 . 2 -2 . 4 G H z

M I C 1 6 5 0 / 5 1 6 c a v it y

0 . 3 d B

alimentazioni +5 e +15V

F ilt ro B P L O R C H 3 2 G H z B W 2 . 6 G H z

0 . 3 d B

ATT.

1 0 d B

Q U E S T 1 4 5 0 -1 8 5 0 M H z

IF1=1.45-1.85

L N A 3 2 G H z G 3 5 d B X2

5 M H z

W J -M 8 TCR F / O L =0 . 0 0 1 -3 . 4 G H zI . F . = 0 . 0 0 1 -2 G H zL e v e l 1 0 d B m

R I

L

Q U E S T 6 . 4 -8 . 5 G H z

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.



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T&F specs vs types of Observations (2)

Interferometer• Astronomical VLBI

– Sky Frequency determines max Phase Noise LL(f)(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 !!!!



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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 UnitsTime 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



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Even SRT was not built in a dayin a day !

Technology in Radio Astronomy & SS Sardinian Summer School – 2 nd course R. Ambrosini 11-16 June...

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