LMK04828: multi-board RFSOC - Clock & timing forum - TI E2E

Hi Wiliam,

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Due to the absence of a detection port for the sync signal on the clock board, I am unable to measure the sync output of the LMK. However, I can measure the phase difference between the two SDout1 outputs, which should be consistent with the sync output. I used VNC to measure the phase of the two 5M outputs. In trc1, a phase jitter of 5 degrees can be observed.

The following image shows the outputs of the LMX on two different clock boards. It appears they are not aligned and have phase jitter.

Am I missing some configurations or using a wrong sync structure? As for LMX, our case falls under cat. 1. Should an external sync signal be needed for synchronization? However, when I enable sync mode(vco_phase_sync), the clock fails to lock.

Regards,

Jingda

Hi Prado,

Do you mean I need to disable the two sync signals provided by the LMK to the ?

My case falls under the right side cat1. Does this mean I only need to select vco_phase_sync and do not need to provide an external Sync pulse signal?

Regards,

Jingda

Hi Prado,

Thank you for your explanation. 

I currently have two clock evaluation boards, each containing one LMK and two LMX. When testing two LMX on the same evaluation board, the phases can be aligned. However, when measuring the LMX from different evaluation boards, the phases cannot be aligned. I suspect this might be because the two LMK are not properly synchronized.

Regards,

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Jingda

Hi Jingda, 
After further research it appears for CAT1 SYNC, VCO_PHASE_EN must always be set, you will not require external PSYNC . All devices just need to share same reference clk source to maintain phase relationship. 
https://e2e.ti.com/support/clock-timing-group/clock-and-timing/f/clock-timing-forum//lmx-some-questions-about-category-for-sync-mode

You will need to ensure the outputs from LMK device that feed into LMX are phase aligned. If I'm not mistaken, there are two ways two do this:

1. Use an external SYNC to align outputs across the LMK devices. 

2 Utilizing ZDM (Zero delay mode) 

I will allow the apps engineer more familiar with the jitter cleaners to comment more on this though. 

Regards, 

Vicente 

Hi Jinda, 
That is definitely odd. Assuming the outputs from the LMK device are stable you should be able to lock the VCO of the LMX device even if they're out of phase. 
In your config, PFD frequency is extremely low. Typically we want to make PFD as large as possible for optimal phase noise. 

I attached a revised config. Please note the issue with device not being to lock is separate from not having phase aligned outputs across devices. We first need to ensure devices locks beforehand. Once we manage to lock devices consistently we need to focus on having the reference clks from the LMK device being phase synced as they're fed into the LMX devices. 

I will let Will provide his input on the appropriate LMK configuration.

.LMX_500M_Ref_500M_Out_M.tcs

Regards, 

Vicente

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