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Starting from Field System version 9.11.18:
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Switching between PFB and DDC modes of the DBBC2 requires a change 
in the equip.ctl of the Field System!

as user prog:

cd /usr2/control/

edit equip.ctl
at the very top (in the line that specifies ""type of rack") put in:
dbbc_ddc   for the DDC (tunable) mode
dbbc_pfb   for the PFB mode

Don't forget to check the setting prior to any observations