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Part Number: lmx2594
I am trying to reduce the fractional spurs that I see when adjusting the output phase of the LMX2594 using the MASH_SEED. I was hoping that Ti might have some tips.
I have been reading TIDA-01346 which states:
"Programming non-zero seed values can actually produce fractional spurs with a numerator of zero. Therefore, optimal spurs are obtained when MASH_SEED is a multiple of IncludedChannelDivide and the fractional denominator is set to k × IncludedChannelDivide, where k is a positive integer that has no factors of 2 or 3. Sub-fractional spurs can be avoided using this strategy"
To this end I have been using a fractional denominator of 127, such that k=127 which contains no factors of 2 or 3. I'm incrementing MASH_SEED by 1 which is a multiple of IncludedChannelDivide. Please see the setup below:
I'm getting quite bad fractional spurs when I increment the MASH_SEED in steps of 1. For instance, with a MASH_SEED of 1 I get:
I believe these are fractional spurs at multiples of 200MHz / 127 = 1.5748MHz.
Would Ti have any tips on how I can adjust the output phase of the LMX2594 whilst minimising SPURS? I have found that some MASH_SEED values are better than others, but I've not found a way to predict the behavour. I think PLLatinum Sim does not work with a numerator of zero.
Thnks for the help
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In reply to Dean Banerjee:
Thanks for the help and the link to the book. I'm not very familiar with the LMX2594 sigma-delta architecture so I'll have a study of your book and hopefully be able to figure out how I can optimise these spurs.
For now I think a brute for search might be the way to go to figure out what denominators and MASH_SEEDs give the best performance in my application.
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