What are the FCC requirements for harmonic levels?
The FCC requirements on harmonic levels (spurious emissions) depends upon the section or 'Part' of the FCC regulatory standard under which the device is operating.
Silicon Labs has observed that the most common unlicensed applications for the EZRadioPRO family of chips are governed by one of the following two FCC Parts:
FCC Part 15.247
FCC Part 15.231
Part 15.247 concerns itself with higher-power frequency hopping spread spectrum (FHSS) systems and wideband digital modulation systems, within the 902-928 MHz frequency range.
Part 15.231 concerns itself with low-power devices intended for periodic operation (e.g., garage door openers, remote keyless entry systems, security and control links, etc.). The most commonly-used frequencies for devices operating under this Part are generally within the 315 MHz to 434 MHz frequency range. The general requirements for harmonic levels under Part 15.231 are that they be no higher than -20 dBc, UNLESS the harmonic will within a 'Restricted Band' (see discussion below).
The requirements of Part 15.247 call for harmonic levels no higher than -20 dBc, UNLESS the harmonic falls within a 'Restricted Band' as defined in FCC Part 15.205. If a harmonic signal falls within a Restricted Band, then a more stringent harmonic level is specified. The maximum allowed levels of harmonics are defined in FCC Part 15.209, and are specified in terms of radiated field strength (measured at a distance of 3 meters). The maximum allowed level depends upon whether the harmonic falls above 960 MHz or below 960 MHz. If the harmonic falls below 960 MHz, the allowed field strength is 200 microvolts/meter. If the harmonic falls above 960 MHz, the allowed field strength is 500 microvolts/meter.
Users often desire to measure harmonic levels as a conducted measurement, rather than as a radiated field strength measurement. Proper measurement of radiated field strength requires a calibrated antenna chamber, and often may not be immediately available to a user. In order to convert 'conducted TX output power' into 'radiated field strength at a specified distance' (or vice versa), it is necessary to know the characteristics (i.e., spatial gain) of the antenna for the device-under-test. This will obviously vary from one user to another, and thus Silicon Labs cannot provide one unique answer that will be appropriate for all customers. However, if an ideal isotropic point-source radiator is assumed for the antenna, the following mathematical conversions may be made:
200 microvolts/meter (at 3 meters), equivalent to -49.2 dBm conducted input power to an isotropic point-source radiator
500 microvolts/meter (at 3 meters), equivalent to -41.2 dBm conducted input power to an isotropic point-source radiator
Again, we must emphasize that compliance with FCC spurious emission requirements is performed using radiated measurements, and not conducted measurements.
The provisions of FCC Part 15.35 may (in certain instances) allow use of time-averaging of a periodic or packet-like burst signal in order to reduce the apparent level of harmonics. This is often useful in achieving compliance with FCC requirements.
For more information regarding FCC requirements, please download the latest FCC documents from the government FCC website.
It will be necessary to browse or search through 'CFR 47 Telecommunications' for 'Part 15' documents.
Silicon Labs suggests that the user become familiar with the following documents:
CFR 47 Part 15.231
CFR 47 Part 15.247
CFR 47 Part 15.205
CFR 47 Part 15.209
CFR 47 Part 15.35