For unlicensed transmitters used in remote key (RKE), home automation, and security systems, both the U.S. FCC and the European ETSI impose strict limits on their transmit power levels. These regulations are not only for intentional emissions but also for unintentional or spurious emissions. The combined power limit values are tested using specific procedures to ensure compliance with established standards. The settings of test instruments significantly affect the results, determining whether a device passes or fails the tests.
This application note explores the modulation spectrum of amplitude shift keying (ASK) signals, the phase noise of the transmitter, and how VCO transient frequency pull impacts test outcomes. Understanding these factors is essential for accurate compliance testing.
The ASK modulation spectrum can be analyzed by modulating an RF carrier with a periodic square wave signal. This signal is then adjusted to account for data randomness. For example, a square wave with a period of 2T represents a non-return-to-zero (NRZ) data sequence at a rate of 1/T. Its power spectrum includes a central carrier line and odd harmonics. When real data is used, the spectral lines become more spread out, resembling a half-cycle sine wave. The mathematical expression for the normalized power spectral density is given by Equation 1.
Figure 3 shows the spectrum of the MAX1472 ASK transmitter modulated with a 4kHz square wave. Each side lobe appears at odd multiples of 4kHz. It's important to note that when the transmitter is unmodulated, its power is P0. With ASK modulation at 50% duty cycle, the total power halves to P0/2, with half of it in the main lobe and the rest distributed across side lobes.
FCC requirements for ASK transmitters include emission bandwidth limits. Section 15.231(c) specifies that the transmission bandwidth must not exceed 0.25% of the center frequency. Measured bandwidths often exceed theoretical predictions due to factors like resolution bandwidth, phase noise, and VCO transient pull. The FCC has specific measurement bandwidth guidelines, such as 100kHz for quasi-peak detectors and 100kHz for peak detection. However, narrower bandwidths like 10kHz or 30kHz may be used for more accurate results.
ETSIs requirements for ASK transmitters differ slightly. In Europe, signals up to +10 dBm are allowed in the 433.05–434.79 MHz band. Out-of-band emissions must stay below -36 dBm, while emissions in the 470–862 MHz range must remain below -54 dBm. Modulation sidebands and spurious radiation are both regulated, with different measurement techniques required for each.
Spurious radiation measurements focus on unintended emissions outside the main signal. Phase noise and other factors can increase measured power levels, making it crucial to use appropriate test settings. The MAX1472 and MAX7044 transmitters have phase noise densities around -91 dBc/Hz, which may exceed ETSI limits unless output power is reduced. The MAX1479, however, meets ETSI requirements even at +10 dBm.
In summary, understanding the modulation characteristics, phase noise, and test conditions is vital for ensuring compliance with both FCC and ETSI regulations. Proper test setup and waveform shaping can help meet stringent emission limits, especially at higher data rates.
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