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I have manual waterfall scaling active (autoscale off) but it is not possible to keep the selected min/max values while zooming.
Is there a way to totally fix the selected min/max values?
No, it supposed to do that. The WF min value changes +/- 3 dB for every factor of two in bandwidth (one zoom step). For the same reason a passband filter noise floor gets quieter/louder every time you half the bandwidth.
This has the beneficial side effect of keeping the color of the noise floor relatively constant as you zoom in and out (only works with a real antenna connected supplying an actual noise floor).
Without this adjustment the color of the noise floor would rise through green and yellow as you zoomed out until the WF was compressed into an unreadable mess. Or fall to black as you zoomed in. Just try it yourself -- manually adjust to the same WF min value at different zoom levels.
This logic is even more evident on the spectrum display. You can see the noise floor change as the WF bandwidth changes. So WF min needs to change to keep the vertical position of the noise floor in roughly the same place on the spectrum display.
thank you for the detailed explanation.
I get the point.