Part Number: LMX2571
I'm using the LMX2571 as the LO in a 400MHz PMR design (what it is actually intended for)
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I believe that the remainder of my design is "good" (filter stop bands, supply rail noise, reference waveform, decoupling, layout ... all OK) but the phase noise performance is sub-par
On adjacent (+/-25KHz) the phase noise is barely OK (just sufficient to support a 70dB adjacent channel rejection) but moving into alternate channels and further out (+/-50KHz and wider) the phase noise degrades markedly
This can be envisaged as the noise "plinth" of the PLL having a peak at about +/-100KHz
I can improve matters somewhat by tweaking the loop filter parts, but nothing I do seems good enough
So what am I missing ??
For reference, the LO frequency is 437MHz, I am using a 2nd order hash and a 32MHz TCXO