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

[briantw]

Thank you for the detailed explanation. I think that the main thing is that no matter what the TX duty cycle (x) is, 10*log(x) is going to be added to the measured average power. So, modifying the duty cycle does not seem to be the way to go. Thanks!

However, it seems odd to me that since we are transmitting at 100% duty, isn't the average power going to be somehow close to the peak power? The report on our BLE module by the FCC test report showed that.

[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.