Hello,
We are using the LMK04828 with an external VCO and loop filter (only PLL2 is in use).
The external VCO is used in the frequency range of 1.2GHz-2.4GHz.
OSCin changes between 75-150MHz.
In some devices, we see that the LMK04828 can’t lock on frequencies near 2400MHz (the problem starts around 2380MHz up to 2400MHz which is the highest frequency used). OSCin is 148.75-150MHz (for VCO 2380-2400MHz). PLL2_DLD indicates PLL2 is locked.
For 2400Mhz, Vtune entering the VCO is at its highest value (rail of the LF opamp), and the actual frequency is 2470MHz.
The problem seems to be constant on the devices it appears on.
I’m attaching the schematics and the register values used.
Can you advise of a possible reason for this to occur?
Thank you,
Yulia
====================================================================== LMK4828 [A] Registers ====================================================================== System-Functions Registers: -------------------------------------------------- R00 (RESET, SPI_3WIRE_DIS): 0x00 R02 (POWERDOWN): 0x00 R03 (ID_DEVICE_TYPE): 0x06 R04 (ID_PROD[15:8]): 0xD0 R05 (ID_PROD[7:0]): 0x5B R06 (ID_MASKREV): 0x20 R0C (ID_VNDR[15:8]): 0x51 R0D (ID_VNDR[7:0]): 0x04 Clocks Pair [0-1] -------------------------------------------------- R100: 0x28 R101: 0x55 R103: 0x01 R104: 0x00 R105: 0x00 R106: 0x71 R107: 0x01 Clocks Pair [2-3] -------------------------------------------------- R108: 0x60 R109: 0x55 R10B: 0x01 R10C: 0x20 R10D: 0x00 R10E: 0x70 R10F: 0x55 Clocks Pair [4-5] -------------------------------------------------- R110: 0x08 R111: 0x55 R113: 0x00 R114: 0x00 R115: 0x00 R116: 0x79 R117: 0x01 Clocks Pair [6-7] -------------------------------------------------- R118: 0x08 R119: 0x55 R11B: 0x00 R11C: 0x00 R11D: 0x00 R11E: 0x79 R11F: 0x01 Clocks Pair [7-8] -------------------------------------------------- R120: 0x08 R121: 0x55 R123: 0x00 R124: 0x00 R125: 0x00 R126: 0x79 R127: 0x01 Clocks Pair [9-10] -------------------------------------------------- R128: 0x20 R129: 0x55 R12B: 0x01 R12C: 0x20 R12D: 0x00 R12E: 0x70 R12F: 0x55 Clocks Pair [11-12] -------------------------------------------------- R130: 0x24 R131: 0x55 R133: 0x01 R134: 0x00 R135: 0x00 R136: 0x71 R137: 0x05 SYSREF, SYNC, and Device Config. -------------------------------------------------- R138: 0x40 R139: 0x03 R13A: 0x03 R13B: 0xC0 R13C: 0x00 R13D: 0x08 R13E: 0x03 R13F: 0x00 R140: 0xA0 R141: 0x00 R142: 0x00 R143: 0x11 R144: 0xF3 R145: 0x7F R146: 0x18 R147: 0x12 R148: 0x06 R149: 0x0B R150: 0x42 R15B: 0xF4 R15F: 0x0B R160: 0x00 R161: 0x01 R162: 0x48 R163: 0x00 R164: 0x00 R165: 0x10 R166: 0x04 R167: 0x00 R168: 0x08 R169: 0x59 R16A: 0x20 R16B: 0x00 R16E: 0x13 R172: 0x02 R17C: 0x15 R17D: 0x33 R182: 0x00 R183: 0x04