Digital communications
Basic PSK constellations.
The four basic PSK constellations are summarized in
the following image. Note that the QPSK constellation follows a different mapping
pattern than the 8 PSK and 16 PSK constellations. Since this constellation is
identical to the QAM constellation of the same size, it uses the same mapping.
Figure 4: Four basic PSK constellations
CPM (continuous phase
modulation)
CPM is a form of FSK modulation. Like FSK modulation, the RF carrier maintains
a constant amplitude at all times. Only the phase is modulated. However, the
general de
fi
nition of FSK modulation allows for the phase to hop when the
frequency is shifted. Such an allowance enables the creation of simple FSK
modulators consisting of two independent oscillators and a multiplexer, driven by
the data, switching between the two frequencies. When the multiplexer switches
between the oscillators, both the frequency and the phase of the output change.
Continuous phase modulation, in contrast, guarantees that the phase will not
suffer a discontinuous jump when switching to a new frequency. As the name
implies, the phase will be continuous. The implementation of FSK in the SG390
series generators happens to be continuous phase, so in this respect, the two
modulations are almost the same. Internally, however, the implementations have
one distinct difference: the FSK implementation tracks frequency while the CPM
implementation tracks phase. The FSK implementation allows arbitrary frequency
deviations, but will, in general, slip phase relative to a
fi
xed carrier. The CPM
implementation, on the other hand, requires a rational modulation index, but will
never slip phase. Aside from this, the two modulations are identical.
The following equation describes the correspondence between an FSK peak
frequency deviation, Fdev, and a CPM modulation index, h:
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TSG4100A Series RF Signal Generators User Manual
