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LMH1980: if LMH1983 will be affected by fan airflow or any other signals

zhang ping
Genius 3835 points
Part Number: LMH1980
Other Parts Discussed in Thread: LMH1983

Tool/software:

We encountered a rather strange problem using LMH1983 here, please help analyze it.
We use LMH1983 to output clock to FPGA's 12G-SDI signal on the board. When installed in the chassis, we found that if the fan blows towards LMH1983 position, the jitter index of 12G-SDI will become relatively poor
Moving the fan direction will slightly improve the indicators, but still not as good as when there is no wind blowing.
May I ask if LMH1983 will be affected by fan airflow or any other signals?

  • 0
    TI__Guru** 107780 points

    Hi Zhang,

    This is most likely due to the VCXO, which is sensitive to vibration and probably sensitive to EMI from a motor. 

  • 0 in reply to Noel Fung
    Genius 3835 points

    Our testing shows that both the crystal oscillator and 1983 will be affected
    (1) Cover the crystal oscillator with hot melt adhesive, and the jitter is about 0.3UI;
    (2) On the basis of (1), cover the 1983 chip with glue and reduce the jitter to 0.2UI;
    (3) On the basis of (2), expand the scope of the adhesive and cover all the resistance and capacitance around 1983, reducing the jitter to 0.06UI;
    (4) Replace the device without adhesive cover, cover the 1983 and crystal oscillators with paper, and the jitter is approximately 0.1UI;
    (5) When there is no obstruction, remove the fan and fan it by hand, and you can also see the jitter getting worse.
    At present, I feel that it is related to the temperature change when the fan is turned off, but the indicators immediately improve after moving the fan away. I also think that the temperature of the chip should not change so quickly. Please help analyze the above phenomenon

  • 0 in reply to zhang ping
    TI__Guru** 107780 points

    Hi Zhang,

    There is nothing, including software or hardware, we can do to the chip to improve the performance against fan or vibration. If shielding can improve the performance, then go ahead to shield the circuitry. 

    For sure temperature will affect performance. In pcb layout, try to keep heat source away from the device and VCXO.