Antenna, Feed, and LNA Integration

S. Weinreb, May 7, 2009, LAX

Antenna Working Group Meeting

1. Issues

2. Feed Summary

3. LNA Summary

4. Differential LNA’s

5. Feed LNA Interface

6. Noise budget

7. Summary and integration plan

Feed/LNA Interconnect IssuesLow Frequencies (< 3 GHz) at 300K

• Losses – Increase in Tsys due to small losses in the interconnection. Difficult to keep these < 0.2 dB which adds 14K to Tsys. The addition can be as high at 30K when the considerations below are included.

• Differential LNA’s – Most wideband feeds have a differential (floating with respect to ground) output with impedance in the 100 to 270 ohm range. There is limited experience with the design, connections, and testing of these LNA’s.

• Input Reflections and Correlated Noise Out of Input – Especially important for phased-array feeds.

• Environmental – The LNA and feed must be protected against moisture, insects, rodents. lightning, temperature cycling and hail

• Mechanical – The feed connection points are small and difficult to mechanically support. A robust, easily manufactured design is needed.

Feed/LNA Interconnect IssuesHigh Frequencies (1.4 to 10 GHz) at < 70K

• Losses – At higher frequencies the interconnections must be smaller (or they will radiate) and this increases the loss. However conductivity increases with cooling and a loss of 0.2 dB gives a tolerable ~4K noise at 70K physical temperature.

• Differential LNA’s – Same comments as for low frequency feeds..• Vacuum – Any cooling below dew point temperature requires

sealing against condensation and vacuum insulation is usually required. This leads to a need for microwave windows and hermetic seals. Vacuum at 70K is more difficult than vacuum at 15K.

• Cooler – Compact, long life (>106 hours MTBF) coolers to 70K are available for ~$2,000 in quantity of

Summary of Wideband FeedsFrom Bradley and Gawande, URSI Meeting, Boulder, Jan 2009

Lindgren 3 to 12 GHz Quadridge Feed • Measured Tsys < 35K and efficiency > 40% on GAVRT 34m antenna.

• Patterns improve with surrounding vacuum dewar

• Have 50 ohm single-ended output so simple LNA’s can be used

• Robust mechanical structure inexpensive to replicate/

Noise vs Frequency of SiGe Transistor LNA at 3 Temperatures

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LNA 300K

@ 300K Connector with LNA @ 54K

LNA 54KLNA 20K

ST first stage, NXP 2nd stage, tested May, 2008

Typical gain 35 dB, typical bias 2V, 12mA

NXP BFU 725 2 stage LNA @17K April 15, 2008

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Low-Cost SiGe 0.5 to 4 GHz Cryogenic LNA• 7K noise at 17K with $.44 NXP transistor

• With STM transistor input stage noise is 2.5K at 17K, and 7K at 55K.

SiGe transistors in 2mm plastic package on printed circuit board

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Frequency (GHz)

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300K, 1.8V, 50mA

195K, 1.8V, 44mA

105K, 1.2V, 20mA 77K, 1.2V, 20mA

60K, 1.2V, 20mA

15K, 1.2V, 20mA

Noise Temperature vs Frequency at 300K, 195K, 105K, 77K, 60K, and 15KInP HEMT MMIC, WBA13, Tested at Caltech May, 2007

Interface with Feed at 300K and LNA at 60K

Input Twin-Lead LinesActive Balun (Differential) LNA for ATA

Output coax

Dual Differential LNA Module• Can house either of 3 differential LNA chips on hand: ATA InP or 2 SiGe IBM

• Module and PCB’s in fabrication, April, 2009, expect test results by Aug, 2009

• Transition from 270 ohm quadline to 135 ohm microstrips has been tested

Dual 270 ohm resistors simulating feed impedance

Component Current Technology2009

Noise, K, 1.4 GHz

Innovation Path2011

Noise, K,1.4GHz

Sky Background + atmosphere 4 No improvement here! 4

Spillover & Blockage 15 dB edge taper + 2.5% blockage, total 4% at 300K 12 Low spillover optics 7

Feed loss 10cm of .085”, 7K + 5K feed loss 12 Twin-lead feed terminals 5

LNA to feed loss 10cm of 0.141 Cu coax bend to dewar, .04 dB at 300K 3 40mm twin-lead 2

Vacuum feedthru Glass/Kovar bead, 0.1 dB 7 Quartz/gold bead, 0.04 dB 3

Coax in dewar 10cm or .141 SS/BeCu.09 dB at 190K 4 Air line 2

Coupler at 70K Werlatone C7753, 0.2 dB 3 or noise lamp coupling 2

Total Total Above 45 Total Above 25

LNA @ 300K Commercial 0.5 to 4 GHz LNA 35 Improved LNA @ 300K 20

LNA @ 60K Current LNA 14 Improved 70K LNA 7

Total Tsys, 300KLNA 80 Total Tsys, LNA @ 300K 45

Total Tsys, 60K LNA 59 Total Tsys. 60K LNA 32

SKA Tsys Budget – Current and Expected 2011

Summary of Current Feed/LNA Status

• There are 5 candidates for wideband feeds currently under development. Pattern and noise measurements at a common facility are needed

• Cooled LNA’s for 0.5 to 3 GHz (< 7K noise at 60K) and 2 to 10 GHz (< 18K noise at 60K) are available.

• Room temperature LNA’s to meet the SKA Tsys < 35K requirement are difficult for 0.5 to 3 GHz and not feasible for > 3 GHz.

• Integration of feeds with LNA’s is in an initial phase with much progress expected in the next year.

• A US prototype antenna that can index to 3 different feed/LNA packages is recommended.

Integration Plan – May 2009

Feed pattern tests and optics optimization

(by others)

Test 3 differential LNA’s first in coaxial and

then quadline fixture at 300K and 60K

Dewar Construction and Cryogenic Test

Test feed integrated with dewar at USC

Noise and pattern test of integrated dewar,

feed, and LNA

System test on antenna, efficiency

and Tsys

2010

2011