Optimum receive system noise figure for given ambient noise – Flex 6600

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:

  • system noise figure calculated from MDS and assuming that the effective 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.

Note also that Youngblood states the table below shows the gain setting and the expected MDS with no antenna attached…. This suggests a misunderstanding of the meaning of MDS. MDS or Noisefloor is the total internal and external noise with a nominal dummy load on the input as demonstrated by (Allison et al 2011).

A lossless antenna system is assumed.

A further issue is that the effective 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 estimated SND is perhaps higher than desirable on some or even most bands if the user was actually experiencing typical Quiet Rural ambient noise with a reasonably efficient antenna.

Readers will note that where the total noise power Pt is barely above MDS (ie most of the noise is internal), SND is relatively high.

If the configurations did achieve Youngblood’s suggested internal noise of at least 8dB below external noise, the SND would be 0.64dB, a fairly satisfactory statistic for many.

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.

References / links