All transformers are designed to work over a certain frequency range their output is not just a function of turns ratio other factors will determine the frequency range of the device, core material, air gap, winding style are just three. The device that you have is likely to form part of an...
To complicated for a simple answer, however have you considered the use of LPT1 as the control source for your D-A the code could then be written using a high level langauge?