LMK61E07: I2C communication interfered with clock output
Part Number: LMK61E2
We found the measured phase noise of LMK61E2 is quite much worse than the datasheet, though at a different frequency.
Could someone explain that if this is expected, i.e. LMK61Exx is optimised for 156.25MHz in integer-N mode and 161.1328125MHz in Fractional-N mode, rather than 125MHz?
Please find our measurement attached, the LMK61E2 is set to 125MHz and 125.000001MHz to force configuration tool going into the fractional-N mode.
The configuration RAW registers values generated with the TI tool are the following.
For fractional-N mode:
R0 0x0010R1 0x010BR2 0x0233R8 0x08B0R9 0x0901R16 0x1000R17 0x1180R21 0x1501R22 0x1600R23 0x172AR25 0x1900R26 0x1A34R27 0x1B20R28 0x1C00R29 0x1D01R30 0x1E3FR31 0x1FFFR32 0x20FFR33 0x2103R34 0x2224R35 0x2327R36 0x2424R37 0x2502R38 0x2600R39 0x2707R47 0x2F00R48 0x3000R49 0x3110R50 0x3200R51 0x3300R52 0x3400R53 0x3500R56 0x3800R72 0x4802
For Integer-N mode:
R0 0x0010R1 0x010BR2 0x0233R8 0x08B0R9 0x0901R16 0x1000R17 0x1180R21 0x1501R22 0x1600R23 0x1728R25 0x1900R26 0x1A32R27 0x1B00R28 0x1C00R29 0x1D00R30 0x1E00R31 0x1F00R32 0x2001R33 0x210CR34 0x2228R35 0x2303R36 0x2408R37 0x2500R38 0x2600R39 0x2700R47 0x2F00R48 0x3000R49 0x3110R50 0x3200R51 0x3300R52 0x3400R53 0x3500R56 0x3800R72 0x4802
We also got similar result on LMK61E07, which we actually prefer to use, but the tool and evaluation board doesn't seem avaliable.
Please advise, thanks.
The differences between datasheet measurement and your measurement:
1, Power supply noise
2, Instrument: TI used Keysight E5052A or E5052B
3, Register settings
4, Output termination: When measure one line signal in a differential pair, another line should be terminated with 50 Ohm.
I did measurement on LMK61E2, the result is good. See details in below attachment. Configuration file .tcs can be loaded or saved in GUI "LMK61xx Oscillator Programming Tool" or GUI "TICS Pro".
Output 125MHz LVPECL:
Output 125.000001 MHz LVPECL: Some spurs caused by fractional PLL.
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In reply to Shawn Han:
Dear Shawn Han,
Thanks for the reply and tests you have done.
We have been looking into this detail and got the LMK61E2 sort of working as the datasheet says.
It is noticed by turning on and off the reference doubler, the measured in-band phase noises are different by about 6 dB.
Though it is more than 3dB which suggested by the datasheet, it is fine for us (can be measurement error, instrument calibration etc,).
BTW, the instrument we are using is R&S FSWP8.
However, the results for LMK61E07 are much worse.
They do not meet the claim about the 3dB and 6dB given by the reference div-by-4 and doubler respectively.
They do seem the same to the datasheet plots neither, and the reason is not clear to us.
Please find the attached plot.
We do need the div/4 feature as the resolution is important to us, therefore LMK61E07 is preferred.
Please could you advise, thanks.
In reply to Weida Zhang:
Have you got an optimised filter setting which we can try on LMK61E07?
We found that the filter works on LMK61E2 usually sends LMK61E07 out of locked.
The LMK61xx Oscillator Programming Tool doesn't seem recognising LMK61E07 and mistakes it as LMK61E2, which avoids us from setting the Div4.
On the other hand, we don't really care what happens on the sideband >10kHz, which we believe the loop filter is responsible for.
Will it help in reducing the phase noise between the sideband of 1kHz and 10kHz by making the loop bandwidth wider?
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