FCC Test Procedure

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  • #22558
    Narendra Singh
    Participant

    Hi,
    I am using xDot module in our application. Although xDot module is FCC certified but We need to get FCC certification of our product. I have read datasheet and under “Chapter 6 Testing and Compliance”, i could find command to send data at set interval. I had joined the network and executed “AT+SENDI=100,hello” command. This command should transmit data every 100ms as per description in datasheet but i am able to receive data every 2sec which is not correct. FCC lab is asking us to transmit data less than or equal to 1 sec so that they can generate sweep of the signals received. Also we need to test for all channels which are set using AT+FSB command(1-8). I have set AT+FSB=7.what we found is the signal is not limited to particular center frequency and it is transmitted over wide range of frequency.
    I want to know the actual test procedures which should be followed for FCC compliance testing. I want to know what commands need to execute for this test.
    Regards
    Narendra

    #22559
    Jason Reiss
    Keymaster

    Disable ACK
    AT+ACK=0

    Disable waiting after Tx for Rx Windows
    AT+TXW=0

    This will allow sending faster than normal.

    To enable waiting for Rx windows
    AT+TXW=1

    Set on a single frequency, this setting will not be sticky when saved with AT&W and AT+NJM is not peer to peer, if the device is reset then AT+TXF will need to be issued again.
    AT+TXF=902300000

    Reset to frequency hopping
    AT+TXF=0

    AT+CHM can also be used to reduce channel usage by modifying the enabled channels mask.

    #22619
    Narendra Singh
    Participant

    Hi Jason,
    Thanks for your suggestions. It helped in carrying out the tests. But it failed to complete the FCC tests. Here i have attached extract from FCC standard document available at https://www.ecfr.gov/cgi-bin/text-idx?SID=21f32428b015d0b23b71f331ec5d4f64&mc=true&node=se47.1.15_1247&rgn=div8
    “(i) For frequency hopping systems operating in the 902-928 MHz band: if the 20 dB bandwidth of the hopping channel is less than 250 kHz, the system shall use at least 50 hopping frequencies and the average time of occupancy on any frequency shall not be greater than 0.4 seconds within a 20 second period; if the 20 dB bandwidth of the hopping channel is 250 kHz or greater, the system shall use at least 25 hopping frequencies and the average time of occupancy on any frequency shall not be greater than 0.4 seconds within a 10 second period. The maximum allowed 20 dB bandwidth of the hopping channel is 500 kHz.”
    As per this standard, it needs to hop for minimum 25 frequencies but this module hops just for 8 frequencies.
    Another issue i faced is in signal strength. The signal level emitted by the module was much beyond the standard limit. As per information from FCC Lab, maximum signal strength should be 94dBuV but we were getting 107dBuV for transmit power set to 11(AT+TXP=11″) For higher power setting > 11, the strength level was increasing for each increasing power setting and strength was much higher which was around 116dBuV for AT+TXP=19.

    Can you please suggest any solution to this issue? Since the module is FCC certified so we expected it to pass through FCC tests easily but it failed to pass in Radiated Emission tests,Field Strength tests.We are not able to proceed for further tests as the module is failing in these tests.

    #22626
    Jason Reiss
    Keymaster

    FCC regulation allows a hybrid mode for FHSS devices using DTS for 8 channel operation. It is discussed how FCC Part 15.247 includes this mode of operation in this manual.
    https://www.semtech.com/uploads/documents/an1200.26.pdf

    Below is an excerpt from the LoRaWAN Regional Parameters.

    — LoRaWAN Regional Parameters 1.0.2 Section 2.2.2
    Hybrid mode, which requires that the device transmit over multiple channels (this
    may be less than the 50 channels required for FHSS mode, but is recommended to
    be at least 4) while complying with the Power Spectral Density requirements of DTS
    mode and the 400 msec dwell time of FHSS mode. In practice this limits the
    measured conducted power of the end-device to 21 dBm.

    For the issue of Radiated Emission tests and Field Strength tests please open a ticket at support.multitech.com to discuss with our certifications team.

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