A frequency converter includes a symbol string editor in which the potential input symbols are grouped into at least two symbol lists. One of the lists is treated as a current symbol list, which can be scrolled and from which the symbols can be selected as the user input. In addition, a user is allowed to control change of the current symbol list such that the symbols in all of the symbol lists are available for selection. This arrangement allows for scrollable lists to be significantly shorter than a single list of potential symbols would be, which makes entering symbol strings faster.