I am new to using YIG Filters so I bought a number of them to see how well they work. After buying a few from EBay I tried using them without a driver but by simply varying the current to its magnetic. I hoping to see a shift in the filter band-pass as I changed the current, but saw no response. the filters worked from about 2 GHz and the current was increased up to around 500 ma.I checked the current on my meter and the frequency response on my spectrum analyzer. Some YIG filters only showed a slight increase in signal strength with increased current but others showed on change at all. I could not find anything wrong with the field coils in the YIGs so I thought maybe I just got some bad units but the EBay seller said they came from working instruments. In conclusion, I just wonder if they need special types of drivers?. .