ADF5902 PLL not locking

Hey,

after following the instructions of another post of mine (https://ez.analog.com/rf/f/q-a/538009/using-adf4159-for-a-cw-radar-application/396330#396330), I tried to get the ADF5902 running this week. Im trying to lock the ADF5902 to a single frequency (no ramps), but cant get it to work. When using a spectrum analyzer I see the signal over a wide bandwith:

Im using the initialization sequence (via SPI) mentioned in the datasheet (slighty modified, by neglecting the writes to register R13-R16 as mentioned here: https://ez.analog.com/rf/f/q-a/544087/adf5902-cannot-be-locked) and setting the frequency to 24.1 GHz with the following:

  • 20 MHz Oscillator
  • Reference Doubler D=1
  • Reference division factor R= 1
  • Reference divide by 2 bit T=0
  • N=301
  • F_MSB =1024
  • F_LSB=0

Here ist the sequence:

0x02000007	// R7
0x0000002B	// R11
0x0000000B	// R11
0x1D32A64A	// R10
0x2A20B929	// R9
0x40003E88	// R8
0x800FE520	// R0

// delay of 10 microseconds

0x01800827	// R7
0x00000006	// R6
0x01E38005	// R5
0x00000004	// R4
0x01897803	// R3
0x00020642	// R2
0xFFF7FFE1	// R1
0x800FE720	// R0

// delay of 1200 microseconds

0x800FE560	// R0
0x800FED60	// R0

// delay of 500 microseconds

0x800FE5A0	// R0
0x800FF5A0	// R0

// delay of 500 microseconds

0x00000011	// R17
0x004F000C	// R12
0x2800B929	// R9
0x01800427	// R7; here i deploy my pll settings
0x00000006	// R6
0x025A8005	// R5
0x00002004	// R4
0x0189F803	// R3

// delay of 100 microseconds

0x0000010B	// R11

The used loop-filter:

What I tried so far:

  • using a different ADF5902 Chip
  • using a recalibration sequence
  • changing the pfd frequency by disabling the referncy doubler
  • setting the center frequency to a different frequency
  • measured the 20 MHz Oscillator (works fine)

So far, nothing helped. I would really appreciate if someone could help me with this issue.

If you need any other information, please let me know!