• State
  • Replies 3 replies
  • Subscribers 34 subscribers
  • Views 55 views
  • Users 0 members are here
Support feedback
Options
Options
Related

THVD1552: Radiated emission with THVD1552

Son Nguyen
Prodigy 10 points
Part Number: THVD1552


We are having radiated emission issues with the THVD1552. there is another THVD1552 in the other end of the Cat6 cable. The signal that we are feeding into the chip is a 6.144Mhz signal single ended signal. We are failing the emission measurement with more than 20dB above the limit.

We have tried a lot of things to improve the emission. Common mode filter on the output, added more decoupling to the supply voltage. Separated battery supply to the supply voltage. Ferrite clamps on the cables. Filtering the supply voltage to the transceiver. Filtering/slowing the output signal.

All the things we have done only have a limited effect on the emission level. The whole team of 8 electrical engineers are looking into the issue. Everyone with more than 20 years of experience.

Do you have any experience with other customers using this chip or similar that had that kind of issue with radiated emission? What was the root cause? what du you see is important. Where is the transceiver especially sensitive?

Shouldn't the chip be able to pass the emission test? I see the RS485 interface as an old and proven technology. Could it be that the part is a fake part with your logo on? 

We have been working on this issue for 4 weeks, and none of us have seen anything like this before. 

image.png

  • 0
    TI__Guru* 75391 points
    Son Nguyen said:
    Common mode filter on the output

    Was this placed close to the differential output pins? From the schematic I see, placing it after the caps would have minimal effect.

    Son Nguyen said:
    Filtering/slowing the output signal.

    What are you doing to filter/slow the output?

    Son Nguyen said:
    there is another THVD1552 in the other end of the Cat6 cable.

    How long is the cable?

    Son Nguyen said:

    image.png

    Your communication looks like it's full duplex. You likely do not need the 220 ohm resistors present on the driver side.'

    Son Nguyen said:
    the other end of the Cat6 cable

    Cat6 cable has a standard characteristic impedance of 100 ohms. You typically want to terminate your receiver node with the same characteristic impedance of your cable. So in this instance a 100 ohm resistor on the receiver would likely perform better in minimizing reflections.

    Generally you will also want to choose a device that has a slower rise/fall time. Faster can be bad for EMI, usually customers who want better EMI will choose a transceiver whose max data rate just barely meets their required system.

    For further EMI protection, you typically want to use split termination with a cap between the termination.

    Other sources of imbalances could come from:

    • Layout: specifically trace length mismatch of diff. traces and trace impedance control (100 ohm characteristic impedance for RS422 differential traces are recommended)
    • Cabling: impedance controlled twisted pair cables with shielding would provide the best results (more twists the better)
    • Mismatches in passives: (example you have 10uF caps, but what is the tolerance, the bigger the difference the larger the potential reflection will be)
    • Network topologies: (Daisy chained connections provide the smallest stubs and will minimize reflections)
    • Stub length: If your RS485 transceiver is fast (THVD1552 has a 2ns typical rise/fall time) then the stub needs to be very short to minimize the reflection
    Son Nguyen said:
    Shouldn't the chip be able to pass the emission test? I see the RS485 interface as an old and proven technology

    You can still fail EMI with a differential hardware layer communication standard. The more unideal the system, the more imbalances will occur. 

    I also see you're using an AC coupled set up, this usually ends up with more EMI because it requires more toggling of the bits to break through the capacitive barrier typically done with Manchester encoding where the clock frequency being encoded ends up generating a larger frequency response. 

    RS485 is also more power hungry (more power means you will likely have more EMI), If your distance is relatively short, you could look at using LVDS which is a lower power differential communication standard that would likely perform better at reducing EMI.

    -Bobby

  • 0
    Prodigy 10 points

    Hi Bobby

    Thx for the fast response

    The common mode filter was placed just after the output of the driver. 

    for the filter we tried different configurations, both with RC filter, capacitor directly on the output, and a snubber like configuration. 

    We tried 3m and 1m cables and even a short loop back of a few centimeters.

    Yes we know that the impedance of a cat6 cable is 100ohm, the driver is able to drive 54ohm so we used 110ohm in both ends, we did also try with 100 ohm resistors. 

    The differential signal looks really nice and clean with no reflections and ringing, so I believe that the termination is good enough.

    We had a lot of focus on the impedance and the stubs during the layout phase, but i guess that is something that you always can improve a little. 

    We shorted/removed the caps to take the capacitors out of the equation.

    there is only 2 devices in our system, that makes the network topology easy

  • 0 in reply to Son Nguyen
    TI__Guru* 75391 points

    Hi Son,

    Son Nguyen said:
    The differential signal looks really nice and clean with no reflections and ringing, so I believe that the termination is good enough.

    Do you have a scopeshot of the differential pins on both the driver? (without the cable)

    And do you have a scopeshot of the differential pins on both driver and receiver? (with the cable)

    Are you able to use split termination?

    Have you measured the common mode drift/delta? 

    -Bobby