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Original question:

LMX2595: LMX2595

LMX2595: LMX2595

Narendra S10
Intellectual 470 points
Part Number: LMX2595
Other Parts Discussed in Thread: LMX2492, LMX2820

Hi guys,

With a reference frequency of 70MHz and using the doubler. PFD=140 MHz. With 3rd order passive filter.

What is the minimum loop bandwidth (less than 3KHz) that can be set to get best phase noise and sharp roll off at lower offset.

It would be great if u can share a sample simulation. (with realizable resistor & Capacitor Values)

Thanks and regards

Narendra

  • 0
    TI__Guru** 112310 points

    Hi Narendra,

    Please use PLL Sim to design the loop filter.

  • 0 in reply to Noel Fung
    Intellectual 470 points

    Hi Noel,

    I have used the tool to simulate the Phase Noise.

    My Frequency to generate is 13.05 GHz.

    Ref frequency from OCXO is 70 MHz (I am using doubler.)

    My problem is , in my overall system (a down converter) there are some spurs (50KHz, 100KHz, 300KHz) which are getting  generated from some noisy DC-DC Converters which I am not able to filter beyond some point.

    So I have reduced the loop bandwidth upto 50 KHz to reduce/filter these spurs.

    Upto 50KHz  loop bandwidth my VCO phase noise is within spec.

    But if I reduce further VCO phase noise contributes to overall phase noise and phase noise degrades.

    Is there any way I can reduce the loop bandwidth further upto 1KHz and still meet the phase noise (VCO being the main factor to reduce phase noise)

    Please share a PLLatinum file with 3rd order passive filter if this is possible

    Or external VCO with synsthesizer is the only solution for narrow loop bandwidth problems

    Thanks and Regards

    Narendra

  • 0 in reply to Narendra S10
    TI__Genius 11900 points

    Hello Narendra,

    This is a trade off you cannot avoid. When you use a narrow loop BW the VCO noise starts to dominate and the overall phase noise of the PLL will increase.

    I've attached a simulation with realizable LF components, however the phase noise is high.

    https://e2e.ti.com/cfs-file/__key/communityserver-discussions-components-files/48/e2elmx2592narrowlbw.sim

    The LMX2595 is usually used for high frequency low jitter, high loop bandwidth applications.

    Perhaps you can consider the LMX2492 or the LMX2820 (in external VCO mode).

    Thanks,

    Vibhu

  • 0 in reply to Vibhu Vanjari
    Intellectual 470 points

    Thank you Vibhu.

    Will surely try out your suggestions.