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ADF5902 TX_DATA as Ramp Clock Problems


I want to generate a FMCW Ramp from 24.0 to 24.25 GHz. I want to trigger the Ramp steps with the TX_DATA pin from a STM uC. The Step word is set to 8192 and the Dev-word to 5120.  My Ref_IN is ac coupled with a 100MHz oscillator. For normal countinius Ramp generation there are no problems. But with the TX-DATa as ramp clock trigger  I need more than twice the number of trigger pulses as set in register 15 (see Figure 2).

For example, if I use the TX_data as trigger for each ramp and the intern clock for the Steps, than I can see that approx. every 10 ramps a trigger puls is not  recognized and the ramp starts with the next but one pulse (see figure 1). But that not explains why I need  twice of the pulses (more than 16384) by triggering each step of ramp.

 FIgure 1 Figure 2

Yellow line are Tx_DATA pulses, green line: V_tune voltage

Here are my Register settings:

//Initialisation ADF5902

0x02000007, //Register 7

0x0000002B, //Register 11

0x0000000B, //Register 11

0x0018000D, //Register 13

0x1D32A64A, //Register 10

0x2A20B929, //Register 9

0x40003E88, //Register 8

0x800FE520, //Register 0

0x01800827, //Register 7

0x00000006, //Register 6

0x01E38005, //Register 5

0x00001804, //Register 4

0x0189F883, //Register 3

0x00020642, //Register 2

0xFFF7FFE1, //Register 1

0x800FE720, //Register 0

0x800FE560, //Register 0

0x800FED60, //Register 0



//Ramp Defining

0x00000011, //Register 17

0x00000010, //Register 16

0x0004000F, //Register 15 STepSize 8192 slope0

0x0204000F, //Register 15 STepSize 8192 slope0

0x0404000F, //Register 15 STepSize 8192 slope0

0x0604000F, //Register 15 STepSize 8192 slope0

0xC002800E, //Register 14 slope0

0xC202800E, //Register 14 slope1

0xC402800E, //Register 14 slope2

0xC602800E, //Register 14 slope3

0x0018010D,//0x0018078D, //Register 13

0x0018012D,//0x001807AD, //Register 13

0x0018014D,//0x001807CD, //Register 13

0x0018016D,//0x001807ED, //Register 13 

0x004F000C, //Register 12 

0x2800B929, //Register 9

0x01002027, //Register 7

0x00000006, //Register 6

0x20F00005, //Register 5

0x00002004, //Register 4

0x0189F883, //Register 3

0x0000000B //Register 11

Does anyone have an idea what this might be due to.

Thanks a lot for every help