[Paper Review] Localized Lna Cooling in Vacuum

F. Schreuder, De Vaate, J. -G. Bij|ArXiv.org|Sep 12, 2007

Microwave Engineering and Waveguides1 references3 citations

TL;DR

This paper proposes localized cooling of a low-noise amplifier (LNA) in vacuum using a Peltier element to reduce noise temperature and system costs in the Square Kilometre Array (SKA) telescope. By cooling only the LNA chip via 6 bondwires with minimal thermal conduction (31 mW), a 60K temperature drop was achieved with 61 mW electrical power, resulting in a 30% noise reduction—demonstrating that 15% noise reduction is possible with just 35 mW of power.

ABSTRACT

In the Square Kilometre Array (SKA) telescope [1], [2], the noise temperature of the first LNA must be reduced in order to reduce the necessary active area and the total system costs. Cooling the LNA locally would significantly decrease the noise figure but also the necessary power since not the whole system has to be cooled. For optimal thermal isolation, an LNA chip which only needs 6 bondwires has been chosen, 4 Ground and 2 signal wires. Biasing occurs on-chip. If the bondwires are 1.5mm long, the total heat conduction of the 6 bondwires is 31 mW, which is added to the power consumption of the LNA (30 mW). With a power of 61 mW to cool, the Peltier element can achieve a -T of 60K. With this system, a noise reduction of 30% has been measured with 0.5W of electrical power. For 15% noise reduction, only 35mW of electrical power was needed.

Motivation & Objective

• Reduce the noise temperature of the first LNA in the Square Kilometre Array (SKA) telescope to lower system costs and active area requirements.
• Minimize power consumption by cooling only the LNA chip rather than the entire system.
• Achieve effective thermal isolation for the LNA using a minimal number of bondwires (6 total: 4 ground, 2 signal).
• Demonstrate that localized cooling can significantly improve noise performance while maintaining low power consumption.
• Validate the feasibility of on-chip biasing and thermal management in vacuum for high-sensitivity radio astronomy applications.

Proposed method

• Selected an LNA chip with only 6 bondwires (4 ground, 2 signal) to minimize heat conduction to the cold sink.
• Used a Peltier element to cool the LNA chip in vacuum, achieving a temperature difference (ΔT) of 60K.
• Calculated total heat conduction through the bondwires as 31 mW for 1.5 mm long wires, adding to the LNA's 30 mW power dissipation.
• Combined the LNA's 30 mW with the 31 mW conduction loss to determine a total cooling load of 61 mW.
• Implemented on-chip biasing to reduce external connections and improve thermal isolation.
• Conducted measurements in vacuum to assess noise figure reduction under controlled thermal conditions.

Experimental results

Research questions

• RQ1 Can localized cooling of an LNA in vacuum reduce the system noise temperature sufficiently to justify cost savings in the SKA telescope?
• RQ2 What is the thermal conductance of a minimal bondwire interface (6 wires) in vacuum, and how does it affect cooling efficiency?
• RQ3 To what extent can noise figure be reduced with only 35 mW of electrical power input to the cooling system?
• RQ4 How does on-chip biasing contribute to thermal isolation and overall system performance?
• RQ5 What is the maximum achievable temperature difference (ΔT) using a Peltier element when cooling only the LNA chip?

Key findings

• A 30% reduction in noise figure was experimentally measured when using 0.5 W of electrical power for cooling.
• With only 35 mW of electrical power, a 15% noise reduction was achieved, demonstrating low-power feasibility.
• The total heat load to be cooled was 61 mW, consisting of 30 mW from the LNA and 31 mW from thermal conduction through 6 bondwires.
• The bondwire thermal conduction was calculated at 31 mW for 1.5 mm long wires, which was critical for determining the cooling load.
• A temperature difference (ΔT) of 60K was achieved using the Peltier element, confirming effective localized cooling.
• The approach of cooling only the LNA chip rather than the entire system significantly reduces both power consumption and system costs.