problem with Low output voltage level in hmc7043


I'm using hmc7043 in my design, so that I drive it with lmx2581 at 2GHz with -1.5dbm output voltage level.i enable only clkout0, then disable sync and slip in it. i set  divider ratio to 1 in asynchronous mode. the output driver mode is lvpecl. when i look at output signal level, i found it to be very low. and it is -15dbm, then i opened the load from it. at this manner the output level is -5dbm. then i set divider ratio to 4  ,8 ,16 and 32 respectively and I see that the output level will be increased when output frequency decreased. At now I'm confused with it, please help me what do i do for obtain optimum swing level