Reverse frequency sweeping on Rohde&Schwarz ZNB40 via QCoDeS #7100

Hi,

This is mainly a feature request but it would be nice if the user has the option to define a frequency sweep in the opposite direction for a measurement channel. Currently for instance, if measuring S21 for a DUT, we set

from qcodes.instrument_drivers.rohde_schwarz.ZNB import ZNB
station.add_component(
    ZNB(
        name="VNA",
        address="<ip_address>",
        init_s_params=True,
    )
)     
station.VNA.S21.frequency_start(500e6)
station.VNA.S21.frequency_stop(1000e6)

to perform the measurement. However, QCoDeS gives out an error when setting the attribute frequency_stop to be lesser than frequency_start.

Looking up the manual for the ZNB40 VNA in particular, we find a section on independently defining the port configuration via the K4 option (see this FAQ from R&S). Interestingly enough, atleast on the VNA we have at hand I was unable to find this option via the in-built local GUI of the VNA.

It would be nice if the driver could include this feature.