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LMK61E0M: Output waveform had distortion when output frequency < 100MHz

Roy Chou1
Genius 16210 points
Part Number: LMK61E0M

Hi Team,

I used below setting to set LMK61E2M output frequency. (92Mhz and 184Mhz)

The 184MHz behavior looks good, but 92MHz is not ok. We tried 94MHz 90MHz and 91MHz, and the behavior were similar. Can you help me how to fix it?

Fullscreen 184_HexRegisterValues.txt Download 
R0	0x0010
R1	0x010B
R2	0x0200
R8	0x08B0
R9	0x0901
R16	0x1000
R17	0x1180
R20	0x1400
R21	0x1511
R22	0x1600
R23	0x1706
R24	0x1810
R25	0x1900
R26	0x1A33
R27	0x1B00
R28	0x1C00
R29	0x1D0D
R30	0x1E00
R31	0x1F00
R32	0x2019
R33	0x2103
R34	0x2224
R35	0x2327
R36	0x2418
R37	0x2502
R38	0x2602
R39	0x2707
R47	0x2F00
R48	0x3000
R49	0x3110
R50	0x3200
R51	0x3300
R52	0x3400
R53	0x3500
R56	0x3800
R72	0x4802

Fullscreen 92HexRegisterValues.txt Download 
R0	0x0010
R1	0x010B
R2	0x0200
R8	0x08B0
R9	0x0901
R16	0x1000
R17	0x1180
R20	0x1400
R21	0x1511
R22	0x1600
R23	0x170D
R24	0x1810
R25	0x1900
R26	0x1A33
R27	0x1B00
R28	0x1C00
R29	0x1D0D
R30	0x1E00
R31	0x1F00
R32	0x2019
R33	0x2103
R34	0x2224
R35	0x2327
R36	0x2418
R37	0x2502
R38	0x2602
R39	0x2707
R47	0x2F00
R48	0x3000
R49	0x3110
R50	0x3200
R51	0x3300
R52	0x3400
R53	0x3500
R56	0x3800
R72	0x4802

Schematic.

Roy

  • 0
    TI__Mastermind 28560 points

    Hello Roy,

    It looks like you are having reflections.  If you were to put 50 ohm termination on the scope, I think you will improve the waveform.  Ideally this is source terminated so no problem with high impedance load, please try adding a 10 or 20 ohm to R256.

    73,
    Timothy

  • 0 in reply to Timothy T
    TI__Genius 16210 points

    Hi Timothy,

    I try to place 10ohm/20ohm on R256, but the waveform is still abnormal. Below is waveform with R256 = 10ohm.

    Can you let me know why reflection occur when fout is low? Do you have any suggestion for the waveform? We need clear sine wave like initial 184MHz waveform.

    Regards,

    Roy

  • 0 in reply to Roy Chou1
    TI__Mastermind 28560 points

    Hello Roy,

    The output is not a sine wave but a square wave.  The frequency content of a square wave is much higher often 5 to 7 times than the fundamental frequency.  Recall a square wave is all the odd harmonics.  With a square wave and low frequency, it behaves something like a TDR in which you can see the reflections.  Typically there is a large reflection, as in the case of a CMOS termination, then the source should terminate to prevent reflections.

    Perhaps a higher value like 33 ohms would give a better response.  As for why you see this at lower frequencies vs. high.  (1) the longer time after the rising edge gives more visibility to disturbances (as TDR mentioned above) and (2) Do you have an active probe you can use?  Perhaps your probing is impacting the measurement.  Are you probing at the source, load, or somewhere between?  (3) the impedance response may be different vs. frequency.

    However if you want a sine wave, I suggest a filter (RC or LC) after the output. Don't put a capacitor directly on the output to ground.  Use an R (or L) first.

    73,
    Timothy