LMX2595: After enabling internal frequency doubling of the chip, there is a spurious Fvco/2 phenomenon

Jiahui Gao

Part Number: LMX2595

Tool/software:

Hi team，

LMX2595: 3/4 RF spur - Clock & timing forum - Clock & timing - TI E2E support forums

As mentioned in the referenced post above (and verified by myself), changing the OUTMUX register of port B to "sysref" can indeed solve the 3/4 spurious (i.e. the third harmonic of Fvco/2) of port A after enabling the multiplier.

1. May I ask why this configuration can effectively solve 3/4 spurious?

2. After configuring the OUTMUX register of port B to "sysref", the TICS PRO software will report a red. Will this configuration have any other impact?

Thanks & Best regards，

Jiahui

5 months ago

+1 Noel Fung 5 months ago

TI__Guru* 96480 points

Hi Jiahui,

When A is doubler while B is sysref, channel divider is disabled, so there is no 3/4 spurs.

The warning color in TICS Pro is used to alert the users that the configuration was not set properly, in this case, if you set B to sysref, that means we want to use sysref, so we should enable sysref and the VCO_PHASE_SYNC bit. TICS Pro turns these bits to red to remind users that they are not set to =1. It is fine to ignore the warning in this case as the purpose of setting B to sysref is to disable channel divider but not use sysref.