LMX2594EVM: Phase Shifting

Mathew,

1. Where PLL_DEN > PLL_NUM + MASH_SEED i.e. MASH_SEED <= PLL_DEN - PLL_NUM. What happens if this condition is violated?

->In many situations, nothing.  But in some case, you may find that the shift due to the MASH_SEED is not as predicted.  Specifically, you might find that the phase linearly increases with MASH_SEED to a point, but then it starts decreasing.  So this is one restriction to avoid this condition

2. If I am interpreting these variables correctly from the TICS Pro GUI I do not see anything like this relationship.

->For the 2nd order modulator, PLL_N≥45, for the 3rd order modulator, PLL ≥49, and for the fourth order
modulator, PLLN≥54.  As for the variable names, you can mouse over them.  For instance, in TICSPro, if you mouse over  the "32" for "N divider" you will see "PLL_N" in the description

3. On the same page of the manual it seems to suggest (its not written clearly!) that VCO_PHASE_SYNC needs to be 1 for this phase shifting to work - is this true? 

->  No, this is not true at all.  You can do phase shifting with VCO_PHASE_SYNC=0.  One common application is to try to align two devices, so the idea is that you sync them together and then adjust out small differences with MASH_SEED.  But you are not required to sync them.  

4. I also do not know how I can determine (or alter) the FRAC_ORDER from the TICS Pro GUI.

->  That's a typo.  What is really meant is MASH_ORDER

5. Could somebody advise how the exact phase shift imparted can be determined from the parameters under one's control?

->  Let's do your example:

Phase shift in degrees = 360 (9000 / 1000)  ( 4 / 128) = 101.25 degrees

One additional comment:  MASH_SEED is cumulative.  So if you toggle MASH_RST_N, then it will be as calculated above (although maybe not as this violates two datasheet restrictions).  However, if you put the cursor in the MASH_SEED box and press enter again, it adds 9000 for a cumulative total of 18000.  It may be easier to see what is going on if you  put a small value in MASH_SEED box and repeatably press enter and see the phase advance.

Regards,
Dean

Hi Matt,

OK, here is the test result.

1. Program the device as follows with MASH_SEED_EN = 0 and MASH_SEED = 0. You should get it locked at 76MHz output.

2. Program MASH_SEED_EN = 1

3. Program MASH_SEED = 64000

4. You should get 180 degrees phase shifted (= 360 x SEED /1000 / 128)

OSCin is used as the trigger channel. 

Mathew,

Realize that since you are using the channel divider of 128, this creates 128 possible phases.   I see you are not in phase sync mode, so the output state is different between power cycles or every time you calibrate the VCO.  In this case, I would expect the phase to shift, it's just that the starting phase would be different.  Also reading above, MASH_RST_N is negative polarity, so disabling this disables the mash engine.  Maybe we should have just called it "MASH_EN" to be less confusing.  So when you say it doesn't work anymore, I'm not sure if you mean that you are not getting the phase shift you expect, or the phase shift does not work at all.

If you use phase sync mode,OSCin doubler interfers with this except for special cases, and this is not one of them.  Also, when there is an included divide in the N divider path, this adds more restrictions and typically using MASH order higher than 2 can sometimes give erratic results.

Regards,

Dean