Feature: Interconnects
The connector body incorporates a flange that tightens against the top or bottom face of the plate to provide thermalisation and to close off any radiation path. CryoCoax can supply top-to-bottom assemblies known as “coax sticks” that can be fully customised with attenuator blocks and a full complement of cable types and materials, to provide connections from room temperature, through all the cooling stages down to the lowest temperatures. These undergo RF testing at room temperature, so test data can be provided for DC resistance, return loss, insertion loss to 40GHz, most applications operate at much lower frequencies.
Standard configurations are 8-, 16- Thorough
thermalisation of cables, attenuators and microwave components at various temperature stages is also
important to reduce thermal noise within the system
semi-rigid, flexible or conformable versions available. Semi-rigid cables are available in niobium titanium, stainless steel, copper, cupro-nickel and beryllium copper, as well as combinations of these with silver- plated conductors, offering a range of choices in terms of thermal conductivity and attenuation. To minimise passive heat load,
Typical installation within a dilution refrigerator with hermetic top plate and multiple cooling stages
The connector components are
machined from high-purity copper as well as beryllium copper and are gold- plated for best thermal conductivity. If needed, non-magnetic versions can also be provided. The connectors are compatible with 0.047” (1.19mm) size coax, with either
cable materials with low thermal conductivity are preferred, with the exception of superconductors, which affect electrical conductivity. Flexible cables with silver-plated copper conductors are typically used for connections from the top of the refrigerator to the control electronics at room temperature, terminated with conventional SMA connectors. Conformable copper coax options are also available, often used at the lowest-temperature stage. Superconducting niobium-titanium
cables are often specified in quantum applications, but this material can be challenging, if not impossible to solder. To overcome this, CryoCoax engineers developed solderless connections for these high-density connectors, in addition to the traditional SMA, 2.92 and SMP connectors.
and 24-way with either smooth bore or full detent options. Connector blocks can be ganged together to potentially give hundreds of coax lines in a DR. Two connector blocks form a mated pair, one having a full detent male SMPM interface, and the other a smooth bore SMPM male interface. They are used in conjunction with ‘bullet’ style female-to-female SMPM adaptors, fitted between a pair of connector blocks. The ‘bullet’ adaptors are retained in the full detent connector block. These can be combined with 8-, 16- and 24-way attenuator blocks, available in 0dB, 3dB, 6dB, 10dB and 20dB. CryoCoax worked with a leading attenuator manufacturer in developing these attenuator blocks to complement the high-density connectors. Attenuators are used to suppress thermal noise and provide thermalisation of the centre conductors.
Inter-stage connections With assistance from NPL (National
Physical Laboratory), CryoCoax teams were able to prove the low-temperature reliability of the design over a number of thermal cycles, followed by in-house durability testing to confirm that the RF performance and DC resistance remained stable after multiple mating cycles.
www.electronicsworld.co.uk December 2021/January 2022 31
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