No, the answer is correct. You have to put a signal into the receiver with a calibrated signal generator, for example at -70 dBm.
Nope. Not if I'm comparing the free space noise level at freq. x between two software packages too see how they calculate it. As I've said, I'm not interested in a calibrated answer, only in a comparative answer between two software systems looking at the same input and the same exact hardware. This is perfectly valid. You don't seem to be able to grasp what I'm getting at.
Some SDR software lets you then adjust the spectrum display and s meter display levels to agree with the known input signal level. Other SDR software, such as HDSDR lets you only calibrate the s meter reading, not the spectrum display level. That must be accounted for in the ExtIO DLL that interfaces your hardware with HDSDR.
The other issue is that the noise floor, after calibration using the method above, ...
I'm not interested in calibrating the systems, only in comparing the answer they give, given substantially the same noise input and substantially the same settings.
... If you double the sample rate, the noise floor increases each doubling by approx. 6 dB. If you decrease the sampling rate by one half, the calibrated noise floor drops by 6 dB. You will also notice that as you increase or decrease the filter bandwidth in the SDR programs, the s meter reading of the noise floor in your selected bandwidth changes too.
I've already said, I can't get the two programs to give the same answer even if I adjust all the parameters available.
This is all fundamental sampling theory stuff and you should read up on it.
The answer to your original question is the no SDR software displays the correct noise floor unless you calibrate the displayed level with a signal source of known level.
I'm not interested in calibrating them as I've said now several times.
There is variations from SDR hardware to hardware of the same kind as well as different SDR hardware that makes it necessary to calibrate the software calculation for it to be accurate and display the real noise floor.
I'm not interested in the calibrated noise floor - to avoid repeating myself, see above responses ...
Here, let me try a simple analogy.
First we take it that we are using systems that are made by companies that have a sound understanding of the engineering of the products they have produced.
Now, suppose I put masking tape on the front of my variable 0 - 30V P.S. readout so I can't see the voltage. I then randomly turn the voltage knob and turn it on. I have no idea what the voltage is. I then measure the output with a Fluke DMM. It reads 12.50V.
I then get a second DMM, made by Agilent, and measure the voltage. It reads 12.59V. What is the actual voltage of the P.S.?
The answer is - I don't care. All I'm doing is comparing two systems using a given input to see if they give substantially the same answer - not the "right" answer. It makes no difference whatsoever if the input is calibrated or not. Now what happens if the DMMs read substantially different values, such as 12.50V and 13.8V? All that means is that for a given input something is causing a difference in the readings and it isn't the output voltage. If I try to get them to agree by adjusting either one or both, and I cannot do that, it tells me that there is something very wrong with the DMMs, something at a fundamental level. It has nothing to do with the input being calibrated or not. If I can adjust one or the other so they read the same, it then tells me that there is no substantial problem with the DMMs, and if I did get a calibrated voltage, that I could also adjust to that, but that isn't the issue at the level I'm referring to here. The issue is, with a measurable input voltage, do the soundly engineered DMMs agree or not and can they be made to agree - not whether they are calibrated to a standard.
So it goes with the noise floors. The two programs should be able to substantially agree given the same exact hardware and substantially the same running noise level at freq. x. Of course there are parameters to adjust, but we're dealing with mathematics now, not hardware, in the computer. The math theory is sound, so the answers should be closer than orders of magnitude difference after all means of adjustment has been attempted.
See, I knew this would result in a argument which is why I didn't respond back initially, but ah well ... here we go.