36 Issues in RFIC Design: Noise, Linearity, and Signals2.4.2 Frequency ModulationIn PSK modulation, the phase of the carrier is changed as a means to transmit infor-mation. On the other hand, a frequency shift keyed (FSK) modulated signal encodes data by changing the frequency of the carrier signal according to the particular bits that have been transmitted. An FSK signal is discribed by FSKRF( )cos(())iStAtωωφ= ×++ (2.99)where A is the amplitude of the carrier signal (a constant), wRF is the nominal frequency of the carrier, f is an arbitrary phase, and wi is the change in carrier fre-quency that determines what bits have been transmitted. Thus, in this modulation scheme, the RF waveform can be thought of as a set of different frequencies being turned on and off at the bit rate as shown in Figure 2.18.So, what is to stop us from spacing the different frequencies corresponding to different bits infinitely close together and thus getting an infinite number of Figure 2.17 Probability of bit error versus Es/No for differential PSK. Figure 2.18 Example of a BFSK waveform.