Time frequency requirements vs kinds of observations
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Time & Frequency requirements vs kinds of observations. Roberto Ambrosini Institute of Radio Astronomy Bologna [email protected] Definitions. TIME ( t ): obvious for everybody… but

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Time & Frequency requirements vs kinds of observations

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Time frequency requirements vs kinds of observations

Time & Frequency requirements vs kinds of observations

Roberto Ambrosini

Institute of Radio Astronomy

Bologna

[email protected]

R. Ambrosini

11-16 June 2012


Definitions

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 numberof 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

R. Ambrosini

11-16 June 2012


Measuring t f

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).

R. Ambrosini

11-16 June 2012


Characteristics of frequency standards

Characteristics of Frequency Standards

For Relevanceread 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).

  • R. Ambrosini

    11-16 June 2012


    Standards with increased stability

    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)

    R. Ambrosini

    11-16 June 2012


    H maser layout

    H- maser layout

    R. Ambrosini

    11-16 June 2012


    Ways to compare instabilities 1

    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

    R. Ambrosini

    11-16 June 2012


    Graphic examples

    Graphic examples

    £ (f)[dBc/Hz] Single Sideband Noise =½ Sφ(f)

    R. Ambrosini

    11-16 June 2012


    Degrades at least with n

    £ degrades at least with N²

    R. Ambrosini

    11-16 June 2012


    Why using a phase lock loop

    Why using a Phase Lock Loop?

    R. Ambrosini

    11-16 June 2012


    Ways to compare instabilities 2

    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)

    R. Ambrosini

    11-16 June 2012


    T ime s tability a nalyzer

    TimeStabilityAnalyzer

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

    • The Allan Variance algorithm ( for each t )

    F (0)

    F (1)

    F (2)

    3 - temporal phases

    t

    t

    t

    time

    Dn1

    Dn2

    2 - frac. frequencies

    sy2 (t) = 1/2 < (Dn1 - Dn2)2 >

    1 - data valid

    t = 1, 2, 5, 10, 20, 50, . . . . . , 50 000, 100 000 seconds

    R. Ambrosini

    11-16 June 2012


    Graphic examples1

    Graphic examples

    ALLAN Deviation σ(y)t - dimensionless

    R. Ambrosini

    11-16 June 2012


    Graphic examples2

    Graphic examples

    ALLAN Variance σ(y)t - dimensionless

    From T4science web site

    R. Ambrosini

    11-16 June 2012


    T ime s tability a nalyzer1

    TimeStabilityAnalyzer

    TSA

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

    Frequency Standard #1

    f mix = comparison frequency

    A/D card

    Frequency Standard #2

    Vout = Kv sin( f(t) ) + Off

    f(t) = arcsin (Vout –Off) / Kv

    f mix

    R. Ambrosini

    11-16 June 2012


    Transfer formulas s f 1 rad 2

    Transfer formulas (Sφ(f) << 1 rad2)

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

    R. Ambrosini

    11-16 June 2012


    Same noise processes different slopes

    Same noise processes: different slopes

    £ (f)

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

    R. Ambrosini

    11-16 June 2012


    Coherence loss vlbi

    Coherence loss (VLBI)

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

    R. Ambrosini

    11-16 June 2012


    Effect of a small temperature gradient

    Effect of a SMALL temperature gradient

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

    R. Ambrosini

    11-16 June 2012


    Where t f become fundamental 1

    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

    R. Ambrosini

    11-16 June 2012


    Where t f become fundamental 2

    Where T&F become fundamental (2)

    Data acquisition

    Path A - RF front end

    • Preampifier (cryostat, filters,..)

    • Local Oscillator chain is made of:

      • Station Freq. Standard

      • Multiplier x N (degrades with N²)

    • Amplitude Calibration (Noise gen.)

    • Phase Calibration

      • Antenna Unit

      • Ground unit

    Path A

    Path A

    • Path B- Backend

    Path B

    Path B

    • Passband Filters

    • Fractional Synthesizer

    • ADC – Digitizer and Formatter

    R. Ambrosini

    11-16 June 2012


    T f specs vs types of observations 1

    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

  • R. Ambrosini

    11-16 June 2012


    Time frequency requirements vs kinds of observations

    Tracking Doppler of the Cassini spacecraft

    Coherent frequency translators on board of Cassini

    X; Ka

    X ; Ka 

    Downlink received at the Noto (I) Radiotelescope

    Transmission from a Deep Space Antenna

    Round Trip Light Time = 72 minutes

    R. Ambrosini

    11-16 June 2012


    Time frequency requirements vs kinds of observations

    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

    R. Ambrosini

    11-16 June 2012


    Time frequency requirements vs kinds of observations

    A new Ka-band receiving capability at the Italian Noto radiotelescope

    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


    T f specs vs types of observations 2

    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 !!!!

  • R. Ambrosini

    11-16 June 2012


    Conclusions

    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

    R. Ambrosini

    11-16 June 2012


    Even srt was not built in a day

    Even SRT was not built in a day !

    R. Ambrosini

    11-16 June 2012


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