Other Parts Discussed in Thread: LMX2582, USB2ANY
Tool/software:
Dear Sir,
We have purchased the LMX2582 eval board so that we can evaluate the performance before using it in the system. After evaluation, we have developed the PCB as per Gerber provided by you for eval with the simple addition of an uC on the same PCB for programming.
We are using two LMX 2582 developed as per Gerber provided for EVAL in a cascade where one acts as a reference clock and another as LO for the Mixer. The first LMX2582 module is programmed with external uC. Individually standalone when the results are checked the phase noise appears acceptable for the first LMX2582 but after cascading the second LMX2582 phase noise is degrading at 10KHz suddenly since last week, it was working fine even when cascaded. The results are still fine if we simply program the first LMX2582 using USB2ANY and TI GUI in cascade. The phase noise at 10 KHz offset is dropping from -112 dBc/Hz to -93 dBc/Hz at 1205 MHz.
We do not understand what is the reason that is causing the issue, please guide us. Let me know if you need any supporting documents.
Below is the PCB image with uC removed and PLL programmed using USB2ANY. The connector we made was based on pin data.
The schematic for the above board is attached below:
In the above case, the cascaded phase noise is acceptable as shown below,
When the uC is either connected to PCB or we even tried connecting the uC on a separate PCB and programmed the PLL through the connector to check if the uC is a problem or the placement of the uC is a problem, but even when the uC is connected outside the cascaded phase noise is degraded. The results are degraded even after replacing or changing the uC with a different version. The degraded results are shown below:
The results are fine only when the first uC is completely disconnected from the system or programmed from USB2ANY.
We have used the default loop filter of the EVAL board in our design.
For the first LMX2582 (40MHz to 120.5 MHz), the following is the register file used. This is the LMX where the uC issue is observed. We have tested the register files using two controllers namely:
Controller 1: STM32F042F6P6
Controller 2: STM32F429ZIT6U(STM32F4-discovery)
R70 0x460000 R69 0x450000 R68 0x440089 R64 0x400077 R62 0x3E0000 R61 0x3D0001 R59 0x3B0000 R48 0x3003FC R47 0x2F00CF R46 0x2E0AA1 R45 0x2D00C8 R44 0x2C0000 R43 0x2B0000 R42 0x2A0000 R41 0x2903E8 R40 0x280000 R39 0x278104 R38 0x260060 R37 0x254000 R36 0x240C41 R35 0x23119B R34 0x22C3EA R33 0x212A0A R32 0x20210A R31 0x1F0601 R30 0x1E0034 R29 0x1D0084 R28 0x1C2924 R25 0x190000 R24 0x180509 R23 0x178842 R22 0x162300 R20 0x14012C R19 0x130965 R14 0x0E0DEC R13 0x0D4000 R12 0x0C7001 R11 0x0B0018 R10 0x0A10D8 R9 0x090302 R8 0x081084 R7 0x0728B2 R4 0x041943 R2 0x020500 R1 0x010808 R0 0x00221C
The register file used for the second LMX2582 (120.5 MHz to 1205 MHz) is attached below. The Controller used here is STM32F04ZIT6.
R70 0x460000 R69 0x450000 R68 0x440089 R64 0x400077 R62 0x3E0000 R61 0x3D0001 R59 0x3B0000 R48 0x3003FC R47 0x2F00CF R46 0x2E0A21 R45 0x2D0000 R44 0x2C0000 R43 0x2B0000 R42 0x2A0000 R41 0x2903E8 R40 0x280000 R39 0x278104 R38 0x260028 R37 0x254000 R36 0x240C21 R35 0x23029B R34 0x22C3EA R33 0x212A0A R32 0x20210A R31 0x1F0601 R30 0x1E0034 R29 0x1D0084 R28 0x1C2924 R25 0x190000 R24 0x180509 R23 0x178842 R22 0x162300 R20 0x14012C R19 0x130965 R14 0x0E0DEC R13 0x0D4000 R12 0x0C7001 R11 0x0B0018 R10 0x0A10D8 R9 0x090302 R8 0x081084 R7 0x0728B2 R4 0x041943 R2 0x020500 R1 0x010808 R0 0x00229C
