Frequency-shift keying (FSK) and phase-shift keying (PSK)
modulation schemes are used in digital communications, radar,
RFID, and numerous other applications. The simplest form of FSK
uses two discrete frequencies to transmit binary information,
with Logic 1 representing the mark frequency and Logic 0
representing the space frequency. The simplest form of PSK is
binary (BPSK), which uses two phases separated by 180°. Figure 1
illustrates the two types of modulation. Figure 1. Binary FSK
(a) and PSK (b) modulation The modulated output of a direct
digital synthesizer (DDS) can switch frequency and/or phase in a
phase-continuous or phase-coherent manner, as shown in Figure 1,
making DDS technology well suited for both FSK and PSK
modulation . This article describes how two synchronized DDS
channels can implement a zero-crossing FSK or PSK modulator.
Here, the AD9958 two-channel, 500-MSPS, complete DDS is used to
switch frequencies or phases at the zero-crossing point, but any
two-channel synchronized solution sh