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ISOUSB211: Very high radiated emissions of 7.5MHz harmonics

Mitch Brosnan
Prodigy 20 points
Part Number: ISOUSB211

Tool/software:

I've been working on a device that uses the ISOUSB211 IC for USB data isolation.  The throughput is relatively high, around 300Mbps, and we are failing our radiated emissions at 225MHz.  Our testing shows that when there are a high number of signal transitions on the USB line, we see very defined spikes at multiples of 7.5MHz (the spectrum analyzer settings didn't allow me to put the marker on 67.5MHz exactly there, so ignore that value).

Does the chip have an internal clock of 7.5MHz or 15MHz?  Any tips on reducing the radiated emissions at these frequencies?

Thanks!

The following is a spectrum analyzer shot of this using a loop antenna.  We also did conducted emissions measurements on the USB pins of the isolator and we see these frequencies on both sides.

 

  • 0
    TI__Guru 54835 points

    Hi Mitch,

    Thank you for reaching out and sorry to hear about the issue.

    I am not sure about any internal clocks at 7.5MHz or 15MHz but the device does operate at 12Mbps while negotiating speed for High-Speed. Please allow me to check regarding the internal clocks at 7.5MHz or 15MHz and come back to you.

    Can you please share the conducted emissions test results?
    Did you also happen to test radiated emissions in anechoic chamber? If yes, can you please share results and test setup details?


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 20 points

    Koteshwar,

    Thanks for your response.  We do not have formal conducted emissions test results yet.  We have used a scope probe with our spectrum analyzer and seen essentially the same frequencies as the picture above.

    The picture above is with our uncontrolled environment/benchtop spectrum analyzer, on a unit without enclosure.  With the enclosure and a controlled test house, we see the primary failing issue at 225Mhz, with other major spikes at 75MHz and a few other 7.5MHz harmonics in the 150-210 range.  I don't have a formal report I can send, just some informal results.

  • 0 in reply to Mitch Brosnan
    TI__Guru 54835 points

    Hi Mitch,

    I understand your spectrum analyzer results are similar to what you have observed at a test house but do not have a test report or emissions capture from the test house to share.
    Can you please confirm what radiated emissions test standard were you testing for? Is this for CISPR 32 Class B?

    I would also like to understand a little about the test setup,

    1. What was connected as host and as peripheral to the ISOUSB?
    2. If these were inside the chamber during testing or were located outside the chamber?
    3. What was the cable length run between ISOUSB and host/peripheral?
    4. Any additional information that could be useful for the debug.

    In the meantime, I am trying to get information related to internal clocks. Thanks.


    Regards,
    Koteshwar Rao