Hello Miran,
It appears that the 1kHz minimum is an artificial limitation in the software. We could look into removing this limitation in a future release of the software.
The best you could do right now create an offset between 1kHz to 100kHz.
Additionally, given your current settings, if you create an offset that is higher than the measured channel power, it would never fail. Is this close in offset for visual indication only?
Also, the channel power bandwidth used when “Auto” is selected is based on your current offsets. If you created an offset that comes all the way in to 1kHz or below (given a software update), you will want to manually set the channel power bandwidth.
With your current RBW of 300Hz, being able to specify a start frequency of < 1kHz would only test a few additional points. Are you testing against a standard mask? I look forward to your response. Regards