LMX2592: Noise floor improvement possible?

Hello Roger,

"The best PLL noise floor is achieved with a 200 MHz PFD."

I would refer to "Figure 17. Noise Floor Variation With Output Frequency" to determine how low the noise floor is. I do not believe you'll be able to get a significantly better noise floor. 

Thanks,

Vibhu

Hi

The noise floor I'm interested in is way outside of the loop bandwidth so I doubt PFD frequency has much influence. Yes I've seen figure 17. Question is what sets this noise floor. If it's the intern LDO noise then an external LDO could be able to improve this floor.

After re-measuring it seems the noise floor is closer to -150dBc/Hz on my board and it's basically flat for >20MHz offset from carrier.

What sort of areas of improvements and specific pins/rails on the chips should I focus on in order to try to at least reach the quoted -155dBc/Hz ?

Best regards,

Roger Kamben

Hello Roger,

The internal LDO cannot be disabled / replaced with an external LDO.

The noise floor at the offset you mentioned is dominated by the VCO and output buffer. Output matching will help with power but not with phase noise.

Have you tried changing your LBW? Is that an option?

Thanks,

Vibhu