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LMX2592: How to connect multiple LMX2592 ICs to same reference oscillator.

Venu Reddy
Intellectual 260 points
Part Number: LMX2592
Other Parts Discussed in Thread: LMK00105

Hi

For a single LMX2592, input clock is either connected to onboard crystal oscillator (very low jitter) or External Ref-In or programmable clock generator.

How can I connect multiple LMX2592 to External Ref-in (10MHz or 100MHz) or programmable clock gen or onboard crystal generator ?

Regards

Venu

  • 0
    TI__Expert 5370 points
    Hello Reddy,

    I would recommend to connect the Ref-in of LMX2592 using the same interconnect as our EVM and I would certainly split the reference signal with proper impedance matching if one reference source is driving multiple loads.

    Regards, Simon.
  • 0 in reply to Simon Damphousse
    Intellectual 260 points
    Hi Simon

    If i am using CWX813-100Mz as a reference source, can I drive multiple LMX2592 by splitting the signal (with 50 Ohms in parallel as shown in EVB) ? Does the crystal oscillator has enough signal strength to drive lets say two LMX2592 simultaneously ?

    or Do I need to use clock buffer (1:2 or higher) to drive multiple LMX2592 ?

    I am planning on using LMK00105SQ to switch between Ref-in and Crystal oscillator as well as have multiple clockouts at the cost of some additive jitter from it ? Is this a good choice ?

    Thanks
    Venu
  • 0 in reply to Venu Reddy
    TI__Expert 5370 points
    Dear Venu,

    The CWX813 is likely capable of driving both. You need to make sure you have keep adequate signal swing and Z matching all set and done. Not sure of your use case but you also want to be careful of coupling between both synth through this shared reference path.

    I am not sure I understand your comment on the switch. if you talk about using the 2 input on the LMK to select the reference, that is fine. Check you PN budget for your inband lmx2592 total PN and how CWX813+buffer is conributing. The CWX is likely going to be a bottle-neck for inband noise...

    I hope this helps.
    Regards, Simon.
  • 0 in reply to Simon Damphousse
    Intellectual 260 points
    Hi Simon

    I have few more questions

    1. If I am using single CWX-813 100MHz to drive both, how to avoid coupling issues (apart from isolating the two PLLs sufficiently on layout). Is there any guidelines for separating it out. Can I use a 1:2 clock buffer such as LMK00105 to reduce the coupling ?

    2. Yes, two inputs on the LMK to select the reference. CWX813 in either case (with buffer or no buffer) is the bottle neck. CWX813-100MHz provides a rms jitter of 5.03ps (10Hz to 100kHz) alone, 0.5ps rms jitter (12kHz to 20MHz). With the use of clock buffer the additive jitter is approx 30fs to 70fs. Do you recommend a good crystal oscillator with a good phase noise (rms jitter <100fs in the range of 12kHz to 20MHz) ?

    Regards
    Venu