Optimum receive system noise figure for given ambient noise – Flex 6700

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:

  • system noise figure calculated from MDS and assuming that the quivalent noise bandwidth is 500Hz;
  • ambient noise figure for Quiet Rural precinct from P.372-14;
  • total noise power (ie internal plus external); and
  • calculated signal / noise degradation due to receiver internal noise.

Note that Quiet Rural may be seen as a lower bound for man made noise from 0.7 to 30MHz, but it should be remembered that at frequencies above foF2 as varies from time to time, Galactic noise is greater… so in a sense this is a very conservative analysis towards 30MHz.

A lossless antenna system is assumed.

A further issue is that the quivalent noise bandwidth (ENB) of a nominal 500Hz filter may be significantly different, and we have no knowledge of the actual ENB.

Notwithstanding these issues, the calculated values in the SND column give indicative expectations.

The same table reworked for P.372 Rural precinct gives better SND.

A reality check for your own scenario is to compare the Pt column with receiver power measurement in 500Hz. Higher than Quiet Rural will increase the figure, antenna system loss will decrease the figure. It will be rare for most of us to observe Quiet Rural ambient noise.

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