A quick read of the code would say yes, that is what it does. The bandwidth = the noise bandwidth. I’m pretty sure I wrote it that way because it would seem strange to measure Noise+Distortion where you take the noise from one frequency range but distortion from a wider frequency range. I view it as you have a brick-wall filter with that bandwidth applied to the signal.
If you’ve got a reference that says this is wrong I’m happy to take a look at it again.
You are correct that THD is done by adding up the first N harmonics (up to 7 harmonics are taken up to sampling rate limit). I think I’m more likely to reduce the harmonics in THD to the noise bandwidth than vice versa take harmonics past the noise bandwidth in THD+N, but I’m open to other specs.
it were ongoing stalls, so that finally the programm need to be shut down by taskmanager.
This happened for example when changing generator voltage under a running spectrum measurement.
It´s no major issue. When one knows one can work around …. and yes, I know … changing a parameter on a software whilst it is running can always bear risks of stalling, or strange behaviour.
I’m still not sure what an ongoing stall is. Maybe a picture of the output would help but it sounds to me like you raised the voltage high enough that it was overloading the inputs which causes a relay click every second or so and produces high levels of distortion. Just a guess.
When a test starts each sweep it reads the gui parameter changes and caches them then during the sweep it only uses those cached parameters (with some leeway for things like turning plot lines on and off). It’s pretty robust when it comes to changing stuff during the run.
By the way – if you check the video on this page–> Attenuation · MZachmann/QA40xPlot Wiki at around 2:25 it actually shows an example of overloading the inputs and you can watch the plot as I let it overload.
Using also QA40xPlot a question came up. Maybe the function is already implemented and I didn’t find it. What I would like to have is that the calculation THD and THD+Noise and NOISE only can be filtered to a definable bandwidth. This because I use still analog distortion measurement instruments and there are a hi-pass filter (400 Hz) and two low-pass filters (30 and 80 kHz) available in order to depress hum and possible hi-frequencies which are not created by the measurement specimen. If such a function would be included then best the filter cut-off values may be freely set within the measurement frequency limits. Thanks in advance. Of course if the original QuantAsylum analyzer could have it too, this would be great.
Currently QA40xPlot supports a noise bandwidth setting (so low pass filtering) but there’s no high pass option to remove hum. I think you could do this with the factory QA40x by defining a user-defined weighting function that drops off the low and high frequencies.
I’ll take this as a feature request. It’s not that hard to implement.
No problem. It was so easy and I was so unhappy with the constant hardcoded 20Hz throughout that I just took care of it. It will be in the next release. It’s a little clumsy for now - you have to set the min frequency and bandwidth in Settings - at which point they apply to all test pages.
Mark
And note this only applies to noise math. It does not affect THD calculations. It does affect THD+N math.