The ornamental design for a frequencyconverter, as shown and described.
FIG. 1 is a front, bottom, right perspective view of a frequency converter showing my new design;
FIG. 2 is a front, top, left perspective view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines illustrating the unclaimed portions of the frequency converter form no part of the claimed design. Dot-dash broken lines defining the boundaries of the claimed design form no part of the claimed design.