Is it 290K or 293K?

A reader of my articles commented on them and some of my calculators regarding the use of 290K as the reference temperature (T0) for Noise Figures.

(Friis 1944) suggested that temperature as reference temperature and it has been widely used since. One may also see 293K (eg in certain ITU-R recommendations), but in my experience, 290K is most commonly used and is for instance the basis for calibration of Keysight noise sources in Excess Noise Ratio (ENR).

The assumption in measurement of Noise Figure or of sensitivity is that the ‘cold’ source has a known source resistance with Johnson noise equivalent to 290K (16.85° C). That noise producing resistance is commonly achieved using a large attenuator at the generator output.

References / links

  • Friis, HT. Noise figures of radio receivers. Proceedings of the IRE, Jul 1944 p420.
  • Keysight. Jul 2018. Keysight 346A/B/C noise source operating and service manual.