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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.

Minimum ambient noise level – ITU-R P.372-13 guidance discussed S/N degradation in a receive system with given noise figure (NF) based on ITU-R P.372-13. This article uses the same data to determine the maximum acceptable receiver noise figure for a given S/N degradation.

The analysis assumes linear systems (eg no signficant intermodulation distortion).

What is the minimum ambient noise level?

Above is Fig 2 from ITU-R P.372-13 which shows some key components of total ambient noise. The solid line is entitled “minimum noise level expected”, and it is a combination of curves B, C and D. Above 0.7MHz, only curves C and D are at play. Continue reading Maximum acceptable receiver noise figure – derived from ITU-R P.372-13 guidance

This article is a follow up to Optimum receive system noise figure for given ambient noise – Flex 6600 using data published at (Farson 2014) to make similar estimates for the Flex 6700.

How to determine the amount of RF Preamp gain to apply for band conditions suggests that the 6700 figures might also apply to the 6500 and 6400(M).

Farson gives a table of MDS in 500Hz bandwidth figures for the 6700 on certain bandws, including MDS for 4 RF Gain configurations, 0, 10, 20, and 30dB.

Above is Farson’s data with my chosen RF Gain option (selected for SND<3dB) and calculated values in yellow and orange for: Continue reading Optimum receive system noise figure for given ambient noise – Flex 6700

Gerald Youngblood (K5SDR) of FlexRadio wrote of optimal receiver noise figure relationship to antenna noise in a blog posting about SDR receivers.

This article discusses that posting in the context of linear receivers, ie effects of intermodulation distortion are not included.

His gives the following advice:

For optimal weak signal performance near the atmospheric (antenna) noise floor you want your receiver noise floor (sensitivity/MDS) to be 8 to 10 dB below the noise coming from the antenna.  For strong signal reception, less sensitivity is almost always better.

The terminology is not industry standard, but that is quite  usual for hams who have a need to redefine well known terms, and this is really loose with implied equivalence (eg sensitivity/MDS).

ITU-R P.372-14 speaks of natural noise as including atmospheric noise due to lightning, and also speaks of man made noise.

It is likely Youngblood is actually talking about man made noise since he uses man made noise figures from an earlier revision of P.372.

Optimal is a compromise between weak signal performance (ie S/N degradation due to internal receiver noise) and handling of strong signals that might clip in the ADC of an SDR receiver.

He gives a table of measured MDS (minimum discernable signal, which actually is synonymous with Noisefloor) for recommended configurations of a Flex 6600 radio on several bands.

Above is Youngblood’s data with my calculated values in yellow and orange for: Continue reading Optimum receive system noise figure for given ambient noise – Flex 6600

Comments were received from some readers of the article S/N degradation is related to external noise level and receive system internal noise.

Essentially, two questions were asked:

• what is the minimum HF ambient noise level; and
• explain observation of lower HF ambient noise level.

What is the minimum ambient noise level?

Above is Fig 2 from ITU-R P.372-13 which shows some key components of total ambient noise. The solid line is entitled “minimum noise level expected”, and it is a combination of curves B, C and D. Above 0.7MHz, only curves C and D are at play. Continue reading Minimum ambient noise level – ITU-R P.372-13 guidance