As per Simrad (Robertson) specs the rudder feedback converts the rudder angle to a variable frequency of 3000 to 3800 Hz and it is calibrated to span the full range.
Usually Arneson provides an output 0 to 5V.
Before you start looking for convertion of Voltage to Frequency, ask the supplier of Arneson or the shipyard if the VHVT Cylinder Interface is available on board.
This interface has an output of 3000 to 3800 Hz on the AutoPilot output (Section A) Pins 2 and 3
Note that the rudder feedback indication is available only after starting engines.