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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

R. Ambrosini

11-16 June 2012

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

- 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

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

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

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

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

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 examples

ALLAN Variance σ(y)t - dimensionless

From T4science web site

R. Ambrosini

11-16 June 2012

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 (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

£ (f)

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

R. Ambrosini

11-16 June 2012

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)

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)

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)

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

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

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

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)

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

- 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

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