LMX2581: phase noise between measure and simulation

Hi Shinichi,

If you are trying to meet the following specs:

Phase noise -140dBc/Hz@10kHz, -145dBc/Hz@100kHz, -145dBc/Hz@1MHz

It will not be possible to meet this with LMX2581.

So the options you have:

1)Have more flexibility with these specs

2) Use LMX2582 and change the OSCin (ref in frequency) from 10 MHz to 100 MHz, using a high performance reference oscillator.

I encourage the customer to play with PLLatinum Sim, our software used for design and simulation.

http://www.ti.com/tool/pllatinumsim-sw

Below is a simulation of LMX2581 assuming no phase noise contribution from the reference oscillator, and there might be slight variations with practical measurement. This simulation fails to meet the spec at 10 kHz and 100kHz offsets.

I recommend using LMX2582 which has better PLL and VCO performance. Below there is a simulation showing that it meets the specs:

As you can see it meets the specs but the reference oscillator had to be changed from 10MHz --> 100 MHz.

-142.4 dBc/Hz @ 10k Hz offset

-148.7 dBc/Hz @ 100 kHz offset

-158.8 dBc/Hz @ 1MHz offset

Remember this is a simulation and it's not accounting for reference oscillator phase noise contribution which typically affect it for lower offsets (1-10kHZ)

I would recommend a high quality oscillator to be able to achieve this performance for LMX2582:

1) VC-708 fixed 100 MHz frequency from Vectron

2) LMK61E2 Programmable oscillator from TI. (Can output from 10 MHz to 1GHz)

Hope this helps!

Thanks,

Julian