Using Cooling and Cryogenics to Reduce Noise Figure

Ultra-sensitive systems depend on receiver components with as low an added noise figure as possible. Any added noise figure in the receiver chain reduces the signal-to-noise (SNR) of the receiver, and is cumulative. Hence, for each component in the receiver chain contributing noise, the noise floor of the receiver raises. A common method to reduce the added noise figure of RF elements is the use of cooling or cryogenic systems. In the case of many RF components and devices, lowering the temperature will at least reduce the thermal noise component of the added noise figure. In the case of amplifiers, lowing the temperature of the device may also improve the gain and linearity performance of the low-noise amplifier (LNA) or power amplifier (PA).

There are a variety of ways to reduce the operating temperature of RF components and devices. One such way is to provide some type of passive cooling, such as a heat sink attached to the device. The cooling effectiveness of heat sink depends on the ambient environment, and is ultimately limited in effectiveness between the difference in the devices temperature and the ambient temperature. Adding fans or other active cooling elements, such as liquid cooling systems that use thermally conductive fluids to transfer thermal energy from the heat sink contact to an external radiator, can be more efficient at reducing the temperature on a device, but even active thermal management systems like these are still limited by the ambient temperature. This includes cold plates in which RF devices are placed on top and the thermal energy is transferred to the cold plate

Another method of providing even greater cooling is using refrigeration methods, such as compressor based active cooling systems, or using phase change cooling systems. These systems, through the use of gas/liquid compression and phase change phenomena are able to reduce the temperature at the heat sink well below the ambient temperature. Larger, more powerful, and/or more efficient refrigeration systems are able to lower temperature at the device to even lower levels. Other active cooling devices that fall into this category include thermo-electric cooler (TEC) or Peltier heater module. TECs use semiconductor devices that when stimulated with electrical energy reduce the thermal energy at one junction by transferring the energy to another.

Another method that can be used to reduce the noise temperature of RF devices to just a few kelvin are cryogenic cooling systems. These systems use cryogenic fluids, such as liquid nitrogen, to lower a device’s temperature to as low as 80 K. Cryogenic cooling systems can be as simple as immersing an active device in a cryogenic bath, using pumped cryogenic fluids to cool a liquid cooling system with a heatsink connected to the device, or direct connection between the device and a cryogenic heat pump. These systems are typically costly, large, and require substantial amounts of energy at some stage of the cryogenic liquid generation process. Hence, the use of cryogenic systems is limited by application constraints that would otherwise reduce the size, weight, power, and cost of a system.