LMX2491: Clock distribution order

Eirik Bjerkaker

Prodigy 30 points

Part Number: LMX2491
Other Parts Discussed in Thread: LMK04828, LMK04832, LMX1204, LMX2582, LMX2572

Hi forum, we need to generate very quiet clocks at 4 GHz and 500 MHz for ADC and DAC and also some clocks for FPGA.

We plan to use the LMK04828 or LMK04832 for MHz clocks and sysrefs.

What is the best option for lowest phase noise?:

1) LMK -> LMX

2) LMX -> LMK (distribution)

It seems option 2 will introduce more jitter because a divider and extra splitter is necessary, is that correct?

Best regards,

Eirik Bjerkaker

over 2 years ago

0 Noel Fung over 2 years ago

TI__Guru* 90590 points

Hi Eirik,

For the lower frequency clocks and sysref clock, LMK04832 or LMK04828 are good candidates.

For the 4GHz clock, how many output do you need? LMX devices can support two differential outputs. If you need more output, it has to fanout with a RF buffer such as LMX1204.

What is your jitter/phase noise requirement for the 4GHz clock? You can use PLL Sim to estimate which LMX device can meet your requirement.

0 Eirik Bjerkaker over 2 years ago in reply to Noel Fung

Prodigy 30 points

Hi, I only need one 4GHz output for an ADC. If using the LMK as a distributor, I also need a splitter and divider considering the available LMX devices. The LMX1204 is not available. Below 100fs jitter would be a nice target figure.

BR, Eirik

+1 Noel Fung over 2 years ago in reply to Eirik Bjerkaker

TI__Guru* 90590 points

Hi Eirik,

LMX2572 can support 4GHz and 500MHz simultaneously. If you need better phase noise, consider LMX2582.

0 Eirik Bjerkaker over 2 years ago in reply to Noel Fung

Prodigy 30 points

Thanks, Noel!

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LMX2491: Clock distribution order