I designed the LFMCW transmitter with ADF4159 and ADF5901, using triangular wave modulation, designed to output 24~24.2GHz. The problem is that ADF5901 cannot be locked at 24GHz～24.2GHz， It was later found that the frequency range of ADF4159 modulation can be locked in 24.3～24.5, and the peak value of the VTUNE triangle wave of ADF5901 is 0.2V～0.8V. This also does not comply with the datasheet VTUNE modulation voltage should be 1 V ～2.8V. In order to locate, I disconnected the output of PLL filter from VTUNE, and developed the output frequency of Vtune at 0V by signal source test. The output frequency is 24.3 GHz ，VtuneV at 0.5V，output of frequency is 24.5GHz. At the same time ADF5901 output phase is very poor. The official development version of VTUNE was compared with the manual curve. Later, the ADF5901 single-chip test board with no other circuits was designed to work with the manual in line with the manual, and can work at 24～24.2GHz. what is the cause of this situation for ADF5901?