Constant Envelope Modulation provides significant advantages over the standard PSK forms of
modulation used with Direct Sequence Spread Spectrum Systems. The Sidebar excerpt below lists
a few of these advantages.
The Constant Envelope Modulation Power Advantage
Constant envelope modulations such as FSK, GFSK, MSK, GMSK and Feher's IJF -
nonlinearly filtered ISI-free modulations offer not only enhanced spectral efficiency
in DS systems, they also provide an inherent transmitted power advantage. All constant
envelope modulations allow a DS transmiter's power amplifiers to operate at or near saturation
levels. On the other hand, standard BPSK, QPSK, QAM and even π/4-DCQPSK modulations
contain AM components in the modulated envelope, which require from 3 to 6 dB of back off
(from saturation) in the output power amplifier to reduce or eliminate spectrum splatter
of sideband components that might cause Adjacent Channel Interference (ACI).
Most non-constant envelope modulations actually require fully linear power amplification and
thus for similar power outputs (to saturated RF PA stages) require RF power amplifiers that are
up to 4 times more capable (to handle the PEP required by the waveform). These larger power
amplifiers are less efficient, consume more primary power, generate more heat and are more
expensive than their Class C amplifier counterparts.
Next time you design a new DS system,
consider a Constant Envelope Modulation!
