Technology in Radio Astronomy & SS Sardinian Summer School – 2 nd course R. Ambrosini June Roberto Ambrosini Institute of Radio Astronomy Time & Frequency requirements vs kinds of observations

Technology in Radio Astronomy & SS Sardinian Summer School – 2 nd course R. Ambrosini June Definitions TIME ( t ): obvious for everybody… butTIME ( 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.” “the indefinite continued progress of existence and events that occur in apparently irreversible succession from the past through the present to the future.” Frequency ( ): the number of occurrences of a repeating event per unit time.Frequency ( ): the number of occurrences of a repeating event per unit time. While = 1 / t, their derived observable quantities can assume different behaviors. For example an interruption of a Time Scale – will destroy it an interruption of a Time Scale – will destroy it

Technology in Radio Astronomy & SS Sardinian Summer School – 2 nd course R. Ambrosini June Measuring 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).

Technology in Radio Astronomy & SS Sardinian Summer School – 2 nd course R. Ambrosini June Characteristics of Frequency Standards For Relevance read top down, for Verification read bottom up: ACCURACY - traceability to International Definition of UnitACCURACY - traceability to International Definition of Unit STABILITY - precisionSTABILITY - precision » mass inertia (Astronomic standards) »isolation from environment (Atomic standards) ACCESSIBILITY – type of measurementACCESSIBILITY – 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 – 2 nd course R. Ambrosini June 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 – 2 nd course R. Ambrosini June H- maser layout

Technology in Radio Astronomy & SS Sardinian Summer School – 2 nd course R. Ambrosini June Ways to compare instabilities (1) Spectral Density of Phase Fluctuations S φ (f) = [rad 2 /Hz] → L (f) S φ (f) = [rad 2 /Hz] → L (f) [dBc/Hz] Phase –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) –L (f) SSB directly measured by Spectrum Analyzer

Technology in Radio Astronomy & SS Sardinian Summer School – 2 nd course R. Ambrosini June Graphic examples £ (f) Single Sideband Noise = ½ Sφ(f) £ (f) [dBc/Hz] Single Sideband Noise = ½ Sφ(f)

Technology in Radio Astronomy & SS Sardinian Summer School – 2 nd course R. Ambrosini June £ degrades at least with N² Multiplication Factor n Degradation ( dBc / Hz ) Multiplication Factor n Degradation ( dBc / Hz ) n = 26.02n = n = 39.54n = n = n = n =513.98n = n = n = n = n = n = n = n = n =

Technology in Radio Astronomy & SS Sardinian Summer School – 2 nd course R. Ambrosini June Why using a Phase Lock Loop?

Technology in Radio Astronomy & SS Sardinian Summer School – 2 nd course R. Ambrosini June Ways to compare instabilities (2) ALLAN Deviation σ(y)  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 – 2 nd course R. Ambrosini June Time Stability Analyzer The Allan Variance algorithm ( for each   (0)  (1)  (2) 3 - temporal phases  time    y      ) 2   seconds 2 - frac. frequencies 1 - data valid

Technology in Radio Astronomy & SS Sardinian Summer School – 2 nd course R. Ambrosini June Graphic examples ALLAN Deviation σ(y)  dimensionless

Technology in Radio Astronomy & SS Sardinian Summer School – 2 nd course R. Ambrosini June Graphic examples ALLAN Variance σ(y)  dimensionless From T4science web site

Technology in Radio Astronomy & SS Sardinian Summer School – 2 nd course R. Ambrosini June Time Stability Analyzer Frequency Standard #1 A/D card f mix = comparison frequency V out = K v sin(  (t) ) + Off  (t) = arcsin (Vout –Off) / Kv TSA f mix Frequency Standard #2

Technology in Radio Astronomy & SS Sardinian Summer School – 2 nd course R. Ambrosini June S φ (f) << 1 rad 2 ) Transfer formulas (S φ (f) << 1 rad 2 )

Technology in Radio Astronomy & SS Sardinian Summer School – 2 nd course R. Ambrosini June Same noise processes: different slopes £ (f)

Technology in Radio Astronomy & SS Sardinian Summer School – 2 nd course R. Ambrosini June Coherence loss (VLBI)

Technology in Radio Astronomy & SS Sardinian Summer School – 2 nd course R. Ambrosini June Effect of a SMALL temperature gradient

Technology in Radio Astronomy & SS Sardinian Summer School – 2 nd course R. Ambrosini June Where T&F become fundamental (1) Antenna pointing Antenna beamwidth ~ c / ( D ant 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 – 2 nd course R. Ambrosini June Where T&F become fundamental (2) Data acquisition RF front end 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 B - Backend 1.Passband Filters 2.Fractional Synthesizer 3.ADC – Digitizer and Formatter Path A Path B

Technology in Radio Astronomy & SS Sardinian Summer School – 2 nd course R. Ambrosini June 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 – / Year –Local Freq Standard acts as a Flying Wheel to TAI Tracking Doppler of Interplanetary spacecraft –Radio Science Sky freq. = 32 GHz – / 1000s –Round trip light time 72 minutes

Technology in Radio Astronomy & SS Sardinian Summer School – 2 nd course R. Ambrosini June Tracking Doppler of the Cassini spacecraft 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 minutes

Technology in Radio Astronomy & SS Sardinian Summer School – 2 nd course R. Ambrosini June 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 capability at the Italian Noto radiotelescope

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

Technology in Radio Astronomy & SS Sardinian Summer School – 2 nd course R. Ambrosini June T&F specs vs types of Observations (2) Interferometer Astronomical VLBI L (f) –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 – 2 nd course R. Ambrosini June 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. Time UnitsExpress 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 – 2 nd course R. Ambrosini June in a day Even SRT was not built in a day !