3.8 MODULATION AND RF EXPOSURE
Modulation changes the time variation and spectral distribution of an RF signal, and can therefore affect how exposure is measured and interpreted. Two transmitters operating at the same carrier frequency and with the same average output power can have very different instantaneous envelopes, peak powers, and short-term field strengths.
Ideal FM, PM, and continuous-phase FSK can have essentially constant envelopes, allowing the use of efficient nonlinear power amplifiers. AM, ASK, QAM, and APSK deliberately vary the carrier amplitude. Practical filtered PSK signals may also develop envelope variation during symbol transitions. Variable-envelope signals require attention to both average power and peak envelope power.
The peak-to-average power ratio is the peak envelope power divided by the average power. Its field-strength counterpart is the crest factor, defined as peak field strength divided by RMS field strength. Under fixed far-field conditions, power is proportional to the square of field strength, so the peak-to-average power ratio is the square of the crest factor.
Duty cycle is also important for burst, pulsed, time division duplex, and time division multiple access transmissions. If a source transmits at a constant on-state power Pon for a fraction D of an averaging interval, its average power is Pavg = DPon. If the field is otherwise zero and the propagation conditions are unchanged, the RMS field over the complete interval is the on-state RMS field multiplied by √D. This averaging does not reduce the instantaneous field during the on period.
An RF exposure instrument must have sufficient bandwidth and an appropriate detector and averaging response to capture the relevant modulation. A slow average-power measurement may miss short peaks, while a peak detector alone does not represent long-term average exposure. The applicable exposure limit and averaging time determine which measurement is required; Chapters 6, 8, and 9 develop these requirements.