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LMK05318: Phase noise in close to carrier when locked to a quiet reference

John Willison
Prodigy 140 points
Part Number: LMK05318
Other Parts Discussed in Thread: LMX2572LP,

Sirs,

The part's integrated phase noise from 12 kHz and 20 MHz is awesome. Well done! In our application we also need great phase noise a 10 Hz offset from carrier, and so we provide a 10MHz OCXO with -145dBc/Hz at 10 Hz offset. My question is, "What is the phase noise figure of merit when locking to an external source? Will it be close to the performance of the LMX2572LP?"  (FOM for the LMX2572LP is –232 dBc/Hz and the normalized 1/f Noise is –123.5 dBc/Hz.)

Thanks for your help,

John

  • 0
    TI__Genius 11900 points

    Hello John,

    Please refer to Figure 1 of the datasheet.

  • 0 in reply to Vibhu Vanjari
    Prodigy 140 points

    Vibhu,

    Yes, thanks, I saw all the phase noise plots, which do much credit to the span between 12 kHz and 20 MHz (which is well outside the 2.5 kHz BW of APLL1's loop filter) so, they are essentially just showing the noise of BAW VCO (which is great).

    Within APLL1's 2.5 kHz BW, the measured phase noise at 10 Hz offset in your plots is typically -70 dBc/Hz. The problem with the phase noise plots in the data sheet is that I suspect they are just showing the multiplied-up phase noise of the crystal reference. For example, a typical 10 MHz XO would have a 10 Hz phase noise of -100 dBc/Hz, which increases by about 6 dB/octave, or about by about 32 dB, when multiplied up to 312.5 MHz, matching the plot in the spec sheet (about -70 dBc/Hz at 10 Hz offset).

    If so, your spec sheet is under representing the performance of the part at 10 Hz offset. Using the flicker noise spec of the PLL in the LMX2572LP, I compute a phase noise at 10 Hz offset of about -103 dBc/Hz (33 dB better). 

    Which leaves me with the question: What is the phase noise of the LMK05318 in close to carrier when locked to a quiet reference?

    Thanks,

    John

  • 0 in reply to John Willison
    TI__Genius 11900 points

    Hello John,

    Sorry for the delay in response. Unfortunately I couldn't find any characterization data for the PLL figure of merit for this part. This is typically not a concern. However here is a video you can refer to in order to measure it yourself. https://training.ti.com/engineer-it-how-control-synthesizer-phase-noise