filter only text and space in textbox using ajax

<asp:TextBox ID="txt_fatherHasbandName" runat="server" Width="153px"></asp:TextBox>
                                                    <asp:FilteredTextBoxExtender ID="Filter_fathername" runat="server" TargetControlID="txt_fatherHasbandName" ValidChars="abcdef ">
                </asp:FilteredTextBoxExtender>