Hi,
it seems that something is wrong with the input check carried out when the user specifies a lower and upper frequency for the Frequency step. The upper bound is always recognized as smaller than the lower bound:
Apparently, the only way around this is to leave the lower bound as default and specify the upper bound, then change the lower bound to the desired value. It doesn’t work the other way around (leaving the upper bound as default and specifying it after the lower bound is set).