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Thread: Teensie 4.1 RF interference

  1. #1
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    Teensie 4.1 RF interference

    I am building an IMU for a robot using the Teensie 4.x as the main compute engine.
    During development I am using the 4.1 to be able to write all the raw sensor data to the SD card for algorithm development.
    "Production" unit would use the 4.0 to save space.

    I just took the IMU for a drive to work in my car to get some live data to play with.
    I noticed that my car FM radio gets interference when the Teensie is running.
    The interference is a series of clicks.
    I am assuming this is the periodic writing of data to the the SD card or the periodic reading of the gyro/accel chips via SPI.

    Has anyone else noticed this behavior?

  2. #2
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    Its very unusual to interfere with FM when its tuned to a station. Spurs can often be detected between stations though.
    My suspicion is you are tuned to a harmonic of some clock signal by chance.

    I presume the Teensy isn't in a screened box.

  3. #3
    Senior Member vjmuzik's Avatar
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    I remember this from a while ago, though it was intentional unlike yours: https://forum.pjrc.com/threads/30152...3373#post83373

  4. #4
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    I will show my age and mention that we kept an AM radio next to the PDP-8 as a debugging tool. One could hear an infinite loop and other distinctive audio patterns from the RF interference.

    Some enterprising programmers would play songs by looping over various address ranges at different rates. I was just happy just to not crash.

    I was a bit surprised to hear this on an FM radio from modern processors, but now thinking about it, the T4 is stupidly fast and unshielded.
    I am still surprised at the strength of the interference considering the distance to the antenna.
    I guess a small shielded box is in my future.

  5. #5
    Member KD0RC's Avatar
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    I use my Teensy to control a Software Defined Radio (Flex 6400 Ham Radio). I am active on the ham bands from 1.8 MHz up to 54 MHz. I found the 24 MHz clock in the Teensy with the receiver, but it is only audible when the Teensy is sending screen refreshes, and then only when the antenna is very close to the Teensy. Since 24 MHz is not in the ham bands (and just barely audible anyway), it poses no inconvenience to me. I also transmit 500 Watts on these bands, and the RF does not bother the Teensy at all. I think in general, the Teensy is pretty RFI/EMI proof, but of course, a receiver with an antenna close to the Teensy could certainly pick up some stray RF.
    PDP-8 ah, yes... We had a PDP-11 configured as a credit card authorization front end for merchants. It interacted with telephone touch-tone pairs to initiate various actions. I was the only one with micro experience (Heathkit H-8...), so I was elected to interface the thing to the mainframe (IBM 370-158).

  6. #6
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    I've seen RF interference synchronized to SD writes on systems with sensitive high-impedance sensors such as peizoelectric shear sensors. There are two possible sources for this interference: the internal processor in the SD card and the voltage generator for the erase voltages. The erase voltage generator only gets triggered when a new 32KB (or 128KB) block has to be erased. We found that shielding the CPU and SD card and running all wires into and out of the shielded box through clamp-on ferrite filters greatly reduced the RF noise.

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