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AWR1243: 40MHz crystal for ARW1243

Tomotaroh Kohro
Genius 17265 points
Part Number: AWR1243

Dear Support team.

I would like to know about 40MHz crystal connected to AWR1243.

I refer page19 5.9.2.1 Clock Specifications of SWRS188-MAY2017.

(1) What Ohm is the negative resistance at startup on the AWR1243 side? (40 MHz, 50 MHz)

(2) How many times the recommended oscillation margin is?

(3) What pF is the parallel internal capacity on the AWR 1243 side?

(4) Can the AWR 1243 adjust the Drive level?

     Or is it necessary to adjust using external damping resistance?

Best Regards

KORO

 

  • 0
    TI__Guru 58035 points
    Hello Koro san,
    The negative resistance from the AWR1243 side can be as high as 50 ohms , hence we can support an XTAL of max ESR of 50 ohms (as specified in the datasheet). The drive level is fixed and cannot be adjusted by the user.
    In terms of the shunt capacitance , the next capacitance load on the XTAL would be the on-board load capacitance , the trace capacitance and the parasitic capacitance in the AWR1243 device. With these variables we have seen that external load capacitance required on board is 4.7pF in order to achieve the XTAL Cl of ~8pF.

    Regards,
    Vivek
  • 0 in reply to Vivek Dham
    TI__Genius 17265 points
    Dear Vivek-san.

    Thanks a lot and sorry for late reply.
    I understand the ESR and drive level specification.
    But I have another question.
    X'tal terminal is "diffrential mode", so 4.7pF x2 equal to 2.35pF.
    Is it really equivalent to CL=8pF?

    Best Regards
    KORO
  • 0 in reply to Tomotaroh Kohro
    TI__Guru 58035 points
    Hello Koro San,
    You are right that the external caps contribute to 2.35pf of Cl. The XTAL typically has its own parasitic caps to GND on the XTALP and M lines, then the PCB traces and AWR device have parasitic caps and all of this would add up to 8pf.
    The best way to select the XTAL caps is to look at the center frequency of the XTAL and adjust the caps such that the ppm error is brought as close as possible to 0 ppm, this is how we select the value of the caps and it could vary a bit from board to board due to layout differences.

    Regards,
    Dham
  • 0 in reply to Vivek Dham
    TI__Genius 17265 points
    Dear Dham-san.

    Thanks a lot for your support

    Br
    KORO