The ALMA Front End

John C. Webber

National Radio Astronomy Observatory,

Charlottesville VA 22903, USA

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ALMA Front End Organization

• 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)

– 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 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 is 1 meter in diameter, mass ~450 kg• Sumitomo 3-stage refrigerator cools below 4K• Internal clamp rings provide thermal contact to cold stages

Cryostat and prototype chassis mounted on tilt table

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Optics

1. Optics design concept completed:• 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:• 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

Cold Cartridge Warm 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

(qualification model)

Band 8, 385-500 GHz

(4K stage in test dewar)

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Front End Local Oscillators

• 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

• 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

• 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, Dicke-switched radiometer

– First production unit is under test and will be delivered in mid-2008

Dicke switched WVRRF Front End

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Ambient/Hot Load Amplitude Calibration Device

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Integration of Front EndWarm Optics

Band 3 Cartridge

Band 6 Cartridge

Band 9 Cartridge

Band 7 Cartridge

Cryostat

Front end chassis

Front end IF

FE M&C unit

Warm Cartridge Assembly Band 3

(1st LO, Bias, M&C)

Warm Cartridge Assembly Band 6

(1st LO, Bias, M&C)

Warm Cartridge Assembly Band 7

(1st LO, Bias, M&C)

Warm Cartridge Assembly Band 9

(1st LO, Bias, M&C)

FE Assembly

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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

• 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!