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FCC Certification /Digital modulation minimum bandwitdth

Hi all,

Our company is in the process of certifying a device. This device is using a nrf52832 with radio.

We had no problem to comply with the ETSI standard, but things didn't go as expected for FCC certification.

The FCC §15.247 says that for systems using digital modulation, the minimum 6 dB bandwidth shall be at least 500 kHz.

This is the radio setting I used: Tx Power = -4dBm, Frequency = 2402Mhz, Mode = 1Mbit

This is the result obtain by the test lab:

image description

They found the highest point M3 and draw a line at -6dB. The bandwidth at -6dB from M3 should be at least 500kHz. Unfortunately from the test lab results, the bandwidth is only 483.2 kHz.

So my question is, is there a firmware way to changing the bandwidth for a fixe channel?

Thanks in advance for the help, Stéphane Lovejoy

  • Without extensive testing of your design, it is impossible to tell exactly what is going wrong on your test. However, there are a few things that stand out:

    1. There is a little too much attenuation ahead of a spectrum analyzer. As a result the noise floor seems to be unusually high at about -40dBm. Or, maybe not. Hard to tell unless one spans out. The excess noise floor tends to compress the overall dynamic range one would see here.

    2. Probably the most significant thing is the tilt in the waveform. It should be very symmetric. The most direct cause of the tilt is the lack of quadrature in the modulation. But since this is fixed inside the nRF the more likely cause is poor matching at the output of the nRF. Matching issues can wreak all kinds of havoc on the shape of the waveform from the reflected signals.

    So, look at how good your matching is first.

  • Thanks AmbystomLabs for your answer.

    I will definitely work on improving the matching between the output of the nRF and the antenna. Thanks again for your time and I will keep you updated if it worked.

  • @lovejoy: Just letting you know that you can submit your schematics and layout through our Mypage support portal for a review by one of our RF Application Engineers. All files will be treated as confidential and will only be available to our Tech.Support staff.

  • I just wanted to pass this along. Here is a sweep of our own design using the nRF52832. This is the output off of the pcb antenna close coupled to a test antenna. As you can see our 6dB bandwidth is about 654kHz. Plus we have good symmetry of the waveform. The center frequency is off by about 60kHz but that is well within the ppm spec for the crystal and bluetooth. Spectrum Analyzer Sweep

  • Thanks AmbystomLabs for the previous comment.

    As suggested by Bjørn Spockeli. I posted my question through the Mypage and they were able to help me.

    They suggested to send random data, with the use of the radio test example to increases the bandwidth of the signal. Which seemed correct since I was only sending a fix data.

    They also pointed that the radio was maybe not set up in BLE mode. Which was also correct since my radio was set to RADIO_MODE_MODE_Nrf_1Mbit

    By modifying the radio mode to RADIO_MODE_MODE_Nrf_2Mbit and using the radio test example, I was able to obtain a 673 kHz bandwidh at 6dB

    For the record, these are the result I obtain with 3 different radio mode:

    1. RADIO_MODE_MODE_Nrf_1Mbit image description

    2. RADIO_MODE_MODE_Nrf_2Mbit image description

    3. RADIO_MODE_MODE_Ble_1Mbit image description

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