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LMX2430 Not starting

Daniel Ribeiro
Prodigy 180 points
Other Parts Discussed in Thread: LMX2430

I'm experimenting with LMX2430 and a 6 MHZ crystal between pins OSC Out/FL Out IF and OSC IN instead of an external oscillator as designed in the evaluation board, and it seems the Ref OSC doesn't start unless I link the crystal case to GND via a 10 k resistor. What may be happening ? Did someone faced this trouble before ?

  • 0
    TI__Expert 5370 points

    Dear Daniel,

    The functionality you are referring to is supported and suggested in the D/S section 9.4.1. Crystal oscillator will sometimes need a "kick" start if not properly connected. You should make sure your Xtal is connected with all the components that are required for a Xtal oscillator; resistor and capacitors. You typically have shunt capacitors on each side of the Xtal and also resistor in series. We never had any body get back to us needing a 10k resistor on the case of the Xtal.

    Regards, Simon.

  • 0 in reply to Simon Damphousse
    Prodigy 180 points

    Dear Simon :

    Thank you for your answer.

    Once the Datasheet is very poor regarding information about the internal oscillator of LMX243x,

    it would be nice if you can suggest a LMX243x circuit  with a xtal connected between pins 7 and 8

    instead of the external oscillator we see in the demo board. I wasn't be able to find any information

    about the type of the oscillator ( Pierce ? ) and its characteristics in the datasheet, app notes or anywhere

    in the TI site. If I could find more information, this would have had a good utility in the design.

  • 0 in reply to Daniel Ribeiro
    TI__Expert 5370 points

    Dear Daniel,

    I agree with you, we could have had many more details in this section. A couple of things here that should be very helpful:

    1) This device was not designed with a Xtal as the main reference of choice. Expect some compromises in performance as a results. Your in band noise will likely be larger.

    2) The required load capacitance is typically specified by the Xtal vendor. You then need to subtract the device capacitance from the loading you will add. I would recommend to remove about 2 pF to account for the device.

    I hope this helps,

    Regards, Simon.