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Original question:

AWR1843AOP: Mitigation Approaches for Surface Wave Artifact

IWR6843AOP: Placement Guidance

Mitcham Tuell
Prodigy 150 points
Part Number: IWR6843AOP

I am working on a design using the IWR6843AOP and I have some questions about the ideal placement of the package on a PCB. As far as I can tell none of the documentation for this part contain any guidance except for, of all places, the silicon errata. There we are told that putting the part in the middle of a large board area will cause interference from "surface wave artifacts" and that to mitigate this the part should be placed right up to the edge of the board on two sides, or else large trapezoid cutouts should be used to simulate the edge of the board. Very little explanation is given. In my application the part would ideally be in the middle of a board so I'm interested to know some details of the tradeoffs involved.

This question asked for several useful pieces of information, but the responses indicate that the conversation went to private email so I don't know what the answers were. So here are my similar questions:

  1. Do the surface wave artifacts only affect the elevation dimension accuracy, or the azimuthal as well?
  2. Say the part were placed in the middle of a large board with no trapezoid cutouts. How much error could I expect to see? 5%? 50%? Or maybe the issue isn't measurement error but instead spurious "ghost" detections?
  3. The trapezoid cutout dimensions are oddly specific (51.19mm x 18.3mm x 17.2mm). Is that some specially tuned size, a minimum, or just some numbers someone wrote down?
  4. Do the cutouts need to be the entire FR4 stackup or just the copper?
  5. My application needs to go in an aluminum enclosure. Regardless of cutouts, how close can the part get to a vertical aluminum wall before we encounter problems? What sort of problems might we see? (EDIT: the front surface of the enclosure will be a plastic radome, not a fully enclosed aluminum box)
  6. Is there any problem with mounting the part on a small daughter board and stacking that board on a carrier board with no cutouts? Is the surface wave issue only seen in the XY plane of the antenna, or is Z-axis clearance needed?

I would appreciate any guidance you can give me.

  

Thanks,

Mitcham

  • 0
    TI__Genius 16441 points

    Hi Mitcham, 

    We are looking into your questions and will get back to you soon. 

    Best Regards,

    Josh

  • +1
    TI__Intellectual 2350 points

    Hi Mitcham!

    Thanks for your patience. I'm going to tackle your questions one by one below:

    1. There is disruption to both the azimuth and elevation radiation patterns. This is because the disruption is to the E-Field. 

    2. We would expect a 3-4 dB decrease in SNR. This becomes worrisome when trying to detect small targets, or targets with a weak return. This may not prove a large issue for     many applications where detection beyond 20 m is unneeded or undesired. If deviating from the errata, it is important to perform a careful link budget analysis to determine if the 3-4 dB drop in SNR is acceptable. 

    3. The size of the cutout was determined using HFSS simulation and iterative simulation.

    4. Yes, the cutouts should be through the full stackup. 

    5. Thanks for the edit for the front surface of the enclosure. To risk stating the obvious: this is important as very close metals in the FOV of the radar (like the front of an enclosure) will cause DC elevation and reduce ADC dynamic range, increased noise floor and result in range resolution and max range reduction, resulting in reduced detection of weak returns. Now, to your question, we don't expect to see any problems with having the aluminum backing. However, if you do see issues with FOV or radiation pattern disruption, we suggest covering the affected metal area with a radar absorbing material.

    6. Our EVM connects to a daughter card and has no issues. We would expect similar in your case. However, if you do experience problems in ROV or radiation pattern, you can cover parts of the daughter card with radar absorbing material. For both 5 above, and 6, you would want to at minimum cover the areas directly below the cutout. 

    I hope this helps!

    Blake

  • 0 in reply to Blake Kisshauer
    Prodigy 150 points

    Blake,

    Thank you for your detailed response. I think this gives us enough information to make a decision on architecture.

    Mitcham

  • 0 in reply to Mitcham Tuell
    TI__Intellectual 2350 points

    Glad to be of help! Feel free to start new threads if you have more questions!