To generate ASK, for example, the I-input to the vector modulator would be
fed with the data stream and the Q-input tied to zero. To generate FSK requires the
generation of two symbols, one at a frequency (wc + w1) and one at a frequency (wc w1), for
example. In order to generate a frequency shift of + w1 at the output of
the vector modulator, the I and Q inputs need to be fed with cos w1
and sin w1 respectively. To generate a shift
of w1 requires inputs of cos w1 and sin w1.
This approach is now frequently used to generate some of the more elaborate filtered CPFSK formats described later in this chapter particularly in
cellular handsets. |
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