Minimum s-meter reading

[pa3bwe]

Hi,

Is there any reason why the s-meter reading doesn't go below -140 dBm?

I

The signal in the picture above is at (appr) -145 dBm while the S-meter shows -140 dBm.

Or is there a setting that I can adjust to change the minimum value?

73's TooN

[jdow]

The long of it:
You have, roughly, a 3 kHz bandwidth for the S-Meter. You have a signal a touch over -150 displayed and a noise level, per pixel, of maybe about 15 dB lower level. The 3 kHz bandwidth is about 300 pixels wide. If you add all the noise in that 3 kHz bandwidth you get 300 times the total noise. That would add 25 dB to the noise placing the "signal" 10 dB into the noise. Simon's S-Meter algorithm is sort of funky, and in my gentle opinion broken, so instead of reading something like -135 it is reading much lower.

The short of it:
You are reading total noise power in a bandwidth much larger than the signal.

Set the bandwidth down to 100 Hz and see what happens,

{^_^}

[Simon G4ELI]

The reason: I designed the S-meter with a minimum value of -140 dBm.

[pa3bwe]

Thanks for the clarification!

[jdow]

That is reasonable since -133 dBm is S0. {^_-}

Still, he has a 3 kHz bandwidth with the very narrow signal just barely over the noise. So I'd expect to see an S-Meter reading well above the signal peak on the spectrum report.
{^_^}