Modify radio_test for FCC

[briantw]

Hi all,

We are currently under FCC certification testing for our custom BLE module based on nRF51822 but failed with the average power exceeding the limit. Nordic Semiconductor's engineer told me that the resulting graph showed a very high duty cycle, which does not match the BLE behaviour (less than 10% duty cycle).

The followings are my questions:

1. Is it possible to modify radio_test to transmit data with 10% duty cycle? Or it is already inside the code?

2. Should I modify the mode_ variable from RADIO_MODE_MODE_Nrf_2Mbit to RADIO_MODE_MODE_Ble_1Mbit?

3. Should the test be performed using 'Start TX carrier' or 'Start modulated Tx carrier'? Will this alter the test result?

Thanks in advance.

Brian

[Jørn]

Hello briantw

The RADIO_MODE_MODE_Ble_1Mbit only ensures that the frequency deviation of the radio is within the BLE spec.

You may find the answer in this post helpful.

Best regards

Jørn Frøysa

[AmbystomaLabs]

It does not matter if you modify the duty cycle. The FCC rules are written assuming 100% duty. If you reduce to 10% duty the FCC lab will simply measure the duty cycle, as they are required to do, and add 10dB to the measured power. ie, 10 log (0.1)

This is true for all conducted and radiated testing for the FCC.

Attached is the FCC Testing Guidance 558074 which clearly details measuring peak conducted power for DTS devices. To paraphrase it says that power is only measured during the time period during which the EUT is transmitting at max power (ie, when it is transmitting vs. when it is not) and when this is not possible it discusses the allowed methods for determining the duty cycle such that max power can be derived assuming 100% duty when measured using an average power meter such as a thermal RF head.

It should be noted that similar guidance exists for all manner of ISM devices. Power is always max power assuming 100% duty for any type of device. The only time duty cycle is allowed as part of the evaluation is for SAR (aka, RF exposure).

TestingGuidance_FCC558074.pdf

[briantw]

OK. Noted. Thanks.

[briantw]

Do you have the link to this particular FCC rule? Because I was under the impression that the average power may be calculated from the peak power by using the worst case duty cycle. From the discussion thread that Jørn mentioned above, it also states that. Maybe I misunderstand something here. Please help. Thanks.

[AmbystomaLabs]

See above. I edited my answer to include the FCC rules.

I believe you misinterpreted the answer on the link you sent. Duty cycle is used to calculate max average power when max power cannot be measured only during the time that the EUT is transmitting. This is not to your benefit though.

So, if one measures the power output of your EUT with an average detector (ie, thermal head) and your duty cycle is 10% then the lab just gains up the average measurement by the absolute value of 10 log(0.10) or 10dB.

Same is true for radiated testing where there are peak and average specs. On radiated, peak is measured with a calibrated spectrum analyzer employing max hold during the sweeps. In this manner the peak on a microsecond basis is captured for all frequencies. If the duty cycle is below 98% then same for conducted, average measurements are gained up by the duty cycle.