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Basic Detection Techniques

Basic Detection Techniques. 1b (2009/09/04): Single pixel feeds Theory: Brightness function Beam properties Sensitivity, sky noise, system noise, Aeff/Tsys Receiver systems, mixing, filtering Case study: the LOFAR Low Band Antenna Reference frequency Bandwidth Beampattern. EM waves.

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Basic Detection Techniques

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  1. Basic Detection Techniques • 1b (2009/09/04): Single pixel feeds • Theory: • Brightness function • Beam properties • Sensitivity, sky noise, system noise, Aeff/Tsys • Receiver systems, mixing, filtering • Case study: the LOFAR Low Band Antenna • Reference frequency • Bandwidth • Beampattern

  2. EM waves • Directionality (RA, dec, spatial resolution) • Time (timing accuracy, time resolution) • Frequency (spectral resolution) • Flux (total intensity, polarization properties)

  3. Coherent detectors • Responds to electric field ampl. of incident EM waves • Active dipole antenna • Dish + feed horn + LNA • Requires full receiver chain, up to A/D conversion • Phase is preserved • Separation of polarizations • Typically narrow band • But tunable, and with high spectral resolution • For higher frequencies: needs frequency conversion schemes

  4. Sensitivity • Key question: • What’s the weakest source we can observe • Key issues: • Define brightness of the source • Define measurement process • Define limiting factors in that process

  5. Brightness function • Surface brightness: • Power received /area /solid angle /bandwidth • Unit: W m-2 Hz-1 rad-2 • Received power: • Power per unit bandwidth: • Power spectrum: w(v) • Total power: • Integral over visible sky and band • Visible sky: limited by aperture • Band: limited by receiver

  6. Point sources, extended sources • Point source: size < resolution of telescope • Extended source: size > resolution of telescope • Continuous emission: size > field of view • Flux density: • Unit: 1 Jansky (Jy) = 10-26 W m-2 Hz-1

  7. Reception pattern of an antenna • Beam solid angle (A = A/A0) • Measure of Field of View • Antenna theory: A0 Ωa = λ2

  8. Black-body radiation • General: Planck’s radiation law • Radio frequencies (hf << kT): • Rayleigh-Jeans law (or rather: approximation) • B = 2kT/λ2

  9. Antenna temperature, system temperature • Express noise power received by antenna in terms of temperature of resistor needed to make it generate the same noise power. • Spectral power: w = kT/λ2 AeffΩa = kT • Observed power: W = kT Δv • Observed flux density: S = 2kT / Aeff • Tsys = Tsky + Trec • Tsky and Tant: what’s in a name • After integration:

  10. Sensitivity • Source power from Ta: • Source power from flux: • Antenna area A, efficiency ha • Rx accepts 1/2 radiation from unpolarized source • Define scaling factor K • K is antenna’s gain or “sensitivity” • unit: degree Jy-1

  11. System Equivalent Flux Density • Covers Tant, not Tsys • Define SEFD: • What’s in Tsys? • 3K background and Galactic radio emission Tbg • Atmospheric emission Tsky • Spill-over from the ground and other directions Tspill • Losses in feed and input waveguide Tloss • Receiver electronics Trx • At times: calibration source Tcal

  12. Example: VLA and MFFE

  13. Receiver chain at WSRT

  14. Horn antennas

  15. Inside the MFFE

  16. Wire antennas, vivaldi

  17. Sampling

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