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

[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

[AmbystomaLabs]

Yes, your understanding is correct.

With GFSK modulation the peak power theoretically will be identical to the average power. The transmitted signal only transitions between frequencies but does not change in amplitude. There will be small measurement differences in real life simply because the transmitter at best is not really on 100% of the time.

Distinguishing between the two measurement types is important for regulatory bodies, such as the FCC. Many modulation techniques do not have an average power nearly equal to the peak power and having two different specifications provides for a better estimate of the interference generating capability of a transmitter.

One example where this becomes important is WiFi which uses QAM (quadrature amplitude modulation). With this type of modulation the PAR (peak to average ratio, also called the crest factor) can easily be 10dB or more.