Voltage probe handles noise voltages incorrectly

[tejeez]

In an AC simulation, the voltage probe appears to simply calculate the difference in total noise voltage between its terminals, which generally does not provide the correct result.

As a simple demonstration of the problem, let's try measuring the voltage between two uncorrelated noise sources of 1 V^2/Hz:

The resulting Pr1.vn is 0, whereas I think the correct result would be √2 V/√Hz.

Now, if I replace the voltage probe with an ideal transformer and a label to perform the same measurement, I get the correct result:

The "transformer and a label" trick can be used a workaround for now. It seems to be a bug in the implementation of the voltage probe itself.

I'm running the latest Qucs version compiled from develop branch 29432325df75d0c1512624a2c5811e99e4342058 and Qucsator 0.0.20, a08b0a1 . (Btw, I'm not sure whether this issue should in the qucs or in the qucsator repository).

[felix-salfelder]

Thanks for the bug report.

(Btw, I'm not sure whether this issue should in the qucs or in the qucsator repository).

That will require some investigation, either way. e.g. Could you have a look at the Qucsator output? Is it correct? How does the voltage probe handle noise voltages?

[ckoegler]

qucsator output:
<Qucs Dataset 0.0.20>
<indep acfrequency 2>
+1.00000000000000000000e+06
+2.00000000000000000000e+06


+0.00000000000000000000e+00
+0.00000000000000000000e+00


+0.00000000000000000000e+00
+0.00000000000000000000e+00


+0.00000000000000000000e+00
+0.00000000000000000000e+00

looks identical to the tabular output in the schematic

vol_probe_noise.sch.txt

How does the voltage probe handle noise voltages?
I had a look in \qucsator\src\components\vprobe.cpp, but I have no idea how this code works.