Its maximum threshold voltage is 6V for some of them to conduct only 0.25mA. Then some of them will not do anything with a gate to source voltage of only 5V. It needs a gate-source voltage of 10V for it to turn on pretty well.
Yes i know, i am considering this function generator which can output 20Vpp. I will also need a driver since impedances are 4 to 50, so this FG couldn't drive it into saturation by far. At IXYS they recommended one of these.