#### In-depth

## The vector modulator

| A third method of FSK generation uses a vector or quadrature modulator as shown here. The basic vector modulator was introduced in Chapter 1, and in fact can be used to generate any modulation format with the appropriate choice of in-phase and quadrature drive signals. It operates on the principle that any modulation vector can be realized by summing appropriate amounts of an in-phase (cos wct) and quadrature (sin wct) version of the carrier signal (see in-depth section).
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. |