radio telescopes in the netherlands: advanced rf ......radio telescopes in the netherlands: advanced...
TRANSCRIPT
Radio telescopes in the Netherlands: Advanced RF technologies to observe miniscule celestial radiowaves amidst DAB, DVB and 4G
Wim van Cappellen
29 November 2017
Outline
• RF design for Radio astronomy• LOFAR and the Westerbork Telescope• Impact of interference• RF modeling and design• Summary
29 November 2017 Dwingeloo, Drenthe
“Making Discoveries in Radio Astronomy Happen”
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Today’s Astronomy isThe History of the Universe
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Radio Astronomy
• Determine the intensity and polarization state of the radio waves emitted by celestial objects
• Wide frequency range– Observing frequency corresponds to how the Universe looked like a
certain time ago.– Few protected bands
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Typical Radio Telescope specifications
• As sensitive as possible– Large collecting area– Low noise
• As stable and smooth as possible
• As wideband as possible– Not just protected astronomy bands
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Low Band Antenna• 10 – 80 MHz• Active antenna• Sparse pseudo-random configuration
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High Band Antenna
High Band Antenna• 768 x 2 dipoles per station• Sparse rectangular grid• Analog beamformer per
tile (4x4 elements)
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But now the trouble begins…
FM radio
LOFAR HBA
DAB
LOFAR LBA
Short-wave DAB
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“Harmless” interference
• Only a few channels are affected• Interference is limited in bandwidth and time• Observations continue with minimal impact
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Higher impact
Rollout DAB+ (2013-2017)
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Flagged data
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Westerbork Synthesis Radio Telescope
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• Transform the WSRT into an efficient 21-cm survey facility• 17x Survey speed increase• 1130 – 1750 MHz, 300 MHz processed bandwidth• System temperature: 70 K
APERTIF: Phased Array Feeds for the WSRT
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Out-of-band RFI
• Vivaldi is more sensitive to out-of-band RFI than a horn feed– No cut-off at low frequencies
GSM GSMDVB-T
APERTIF band
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Intermodulation
LNA
Antenna
x(t) y(t)=a0∙x(t)+a1∙x2 (t) +a2∙x3 (t) +…
APERTIF band
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RFI mitigation
LNA
Antenna
Filter
Worst-case situation
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Block diagram
24
1500x
~€ 350/channel
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Summary
• RF Interference leads to a loss of astronomical data
• Careful design is essential to deal with interference
• RF design approach:– Keep out-of-band interference out-of-band– Minimize self-generated spurs– Make sure your system can deal with the in-band interference– Amplify as much as possible, then filter, amplify, filter, etc