Modify radio_test for FCC

RE: Modify radio_test for FCC

AmbystomaLabs — Sun, 03 Sep 2017 15:33:36 GMT

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.

RE: Modify radio_test for FCC

briantw — Sun, 03 Sep 2017 15:07:14 GMT

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.

RE: Modify radio_test for FCC

AmbystomaLabs — Sat, 02 Sep 2017 13:58:49 GMT

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.

RE: Modify radio_test for FCC

briantw — Sat, 02 Sep 2017 00:45:07 GMT

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.

RE: Modify radio_test for FCC

briantw — Sat, 02 Sep 2017 00:22:22 GMT

OK. Noted. Thanks.

RE: Modify radio_test for FCC

AmbystomaLabs — Wed, 30 Aug 2017 12:58:05 GMT

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

RE: Modify radio_test for FCC

Jørn — Wed, 30 Aug 2017 12:03:13 GMT

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