Noise evaluation in REW

[Braca]

Hello,

When measuring noise I notice a disagreement between the noise levels displayed in the RTA upper right and Distortion panels.
Referring to the attachment, I'm measuring thermal noise of a 50 ohm resistor, amplified by an LNA with a gain of 60dB and a flat noise floor at 0.39nV/rtHz RTI.
The noise floor I expect to see is 0.98uV which is only slightly higher than the value of 0.96 that I see in REW, i.e. a very good result.
Integrating the former vallue over a BW of 44kHz and taking into account the LNA gain I get 206uV, and expect to see 202uV in REW.
Combining the two noise values from the upper right panel in RTA I get 205uV, which is again very good.

However, in the Distortion panel, RTA displays a noise value of 123uV, which I can not explain to myself.

Thank you for a great software!

Kind regards,
Braca

 

[Braca]

Sorry, forgot the attachment...

 

[John Mulcahy]

Hi Braca, there is a problem in the displayed N figure when the fundamental is close to the noise floor, causing it to read low. The conversion from a relative to an absolute value was using the fundamental power instead of the total input power, I have fixed it for the next build.

 

[Braca]

Thank you for the swift reaction!