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The ALMA Front End
John C. Webber
National Radio Astronomy Observatory,
Charlottesville VA 22903, USA
ALMA Front End Organization
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Work is done by:
– The National Radio Astronomy Observatory (NRAO, USA)
– The European Southern Observatory (ESO, Germany)
– The National Astronomical Observatory of Japan (NAOJ, Japan)
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The Herzberg Institute for Astrophysics (HIA, Canada)
The Institut de Radioastronomie Millimétrique (IRAM, France and Spain)
The Netherlands Research School For Astronomy (NOVA, The Netherlands)
Onsala Space Observatory (OSO, Sweden)
Rutherford Appleton Laboratory (RAL, UK)
The Centro Astronomico de Yebes (CAY, Spain)
The University of Cambridge / Astrophysics (CA, UK)
– The Academia Sinica (Taiwan)
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Front End Key Design Goals
• Noise performance as good as the best mm and sub-mm receivers
• Single sideband systems when possible, to minimize noise from the image
sideband (all except Bands 9 & 10)
• Beam pointing controlled by mechanical tolerances (except Bands 3 & 4,
which have mirrors on top of the cryostat)
• No moving parts except for cryogenics and amplitude calibration loads
• Short-term phase stability good enough for >90% coherence at 950 GHz
• Long-term phase stability good enough to go 30 minutes without
instrumental calibration
• Ambient and heated RF loads for amplitude calibration
• WVR for atmospheric phase calibration by radiometry
• Rapid switching between bands for atmospheric phase calibration by position
switching on a 10-second time scale
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Front End Key Programmatic Challenges
• Build 70 complete FE assemblies (66 on antennas plus 4 spares) and 3 more
sets of spare subassemblies on time and on budget
• Distribute the work to multiple groups around the world and have everything
play together when integrated
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Typical results for Bands 3, 6, 7, and 9. Bands 4, 8, and 10 are in
development (dashed lines). The horizontal lines are the specification
which applies over 80% of the band. Above 600 GHz, mixers are DSB.
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Front End Wiring Diagram
POWER
M&C
OFFSET LO REF
IF OUT
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OPTICAL LO REF
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3D Model of Front End Assembly
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Front End Assembly Bottom View
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Cryostats
Cryostat and prototype chassis mounted on tilt table
• Cryostat is 1 meter in diameter, mass ~450 kg
• Sumitomo 3-stage refrigerator cools below 4K
• Internal clamp rings provide thermal contact to cold stages
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Optics
1. Optics design concept completed:
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All ALMA Bands 1-10 and WVR pick off
mirror have been designed and incorporated into
cartridge designs
Detailed designs of all windows and IR filters
Widget space containing:
• amplitude calibration device
• ¼ wave plate holder for Band 7
• Solar filter
2. Design verification:
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Complete physical optics calculations made for
bands 3-10
Prototypes tested warm except for bands 5 & 10
Cold tests: done during testing of assembly
Tests on the sky at 5000m elevation to begin
before the end of 2008
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Cartridge 7 Block Diagram
Warm Cartridge
Cold Cartridge
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Cold Cartridges
Bands 3 (84-116 GHz), 6 (211-275 GHz), 7 (275-373 GHz), and 9 (602-720 GHz) SIS mixer receivers
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Cold Cartridges
Band 4, 125-169 GHz
Band 8, 385-500 GHz
(qualification model)
(4K stage in test dewar)
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Front End Local Oscillators
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Warm Cartridge Assemblies (WCA)
– Contain YIG-Tuned Oscillator, Phase Lock Loop, Active Multiplier Chain, and
output Power Amplifiers
– Locked to output of photomixer driven by photonic signal from Back End
– Phase and amplitude noise meet ALMA specifications
– Band 3 WCA drives SIS mixer directly
– For other bands, WCAs drive cold frequency multipliers mounted in cold cartridge
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Frequency Multipliers
– Wideband varistor designs from Virginia Diodes, Inc.
– Designs final for Bands 4, 6, 7, and 9
– Band 8 and 10 prototypes being tested
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183 GHz Water Vapor Radiometer
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Development status
– Two prototype WVRs (Cambridge and
Onsala) were completed and fully tested
– Extensive testing was carried out at the
SMA
– Final design is a single channel, Dickeswitched radiometer
– First production unit is under test and
will be delivered in mid-2008
Dicke switched WVR
RF Front End
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Ambient/Hot Load Amplitude Calibration Device
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Integration of Front End
Warm Optics
Warm Cartridge
Assembly Band 3
(1st LO, Bias, M&C)
Band 3 Cartridge
Warm Cartridge
Assembly Band 6
(1st LO, Bias, M&C)
Band 6 Cartridge
Warm Cartridge
Assembly Band 7
(1st LO, Bias, M&C)
Band 7 Cartridge
Warm Cartridge
Assembly Band 9
(1st LO, Bias, M&C)
Band 9 Cartridge
Cryostat
FE Assembly
FE M&C unit
Front end IF
Front end chassis
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Assembly and Test
The first FE assembly rear view with side panels removed (minus the LO Photonic Receiver)
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Principal Test Requirements
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Noise temperature
Image rejection
Beam patterns
Amplitude stability
Phase stability
IF output power
Gain flatness
Gain compression
Polarization alignment
Vacuum compatibility
Electromagnetic compatibility
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Schedule
• FE#01
– Assembled and tested at the North American integration center
(Charlottesville, Virginia)
– Delivered as an “engineering model” without phase stability
measurements (waiting for photonic reference test module)
– Installed on a Mitsubishi antenna in November 2008
• FE#02
– Assembled and tested at the East Asian integration center (Academica
Sinica, Taiwan)
– Delivered as an “engineering model” without phase stability
measurements (waiting for photonic reference test module)
• FE#03
– Assembled and under test at the European integration center
(Rutherford Appleton Laboratory, England)
– Will be delivered in February 2009 as an “engineering model” without
phase stability measurements (waiting for photonic reference test
module)
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Schedule
• FE#04-06
– One each to be assembled and tested at the three integration centers
– Qualification and verification of integration center equipment and
procedures
– Delivery as fully qualified units June-August 2009
• FE #07-70
– To be delivered over the period October 2009-October 2012
– Output rate of each integration center: one every 51 days!
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