With the explosive growth of the wireless communication industry, research related to communication circuits and architectures has received a great deal of attention. The major issues being addressed are low-cost, low-voltage, and low-power designs, which combine necessary performance with the ability to be manufactured economically in high volumes. Recently, there has been an additional emphasis on integration of heterogeneous parts that constitute a communication transceiver. Modern transceivers are expected to operate over a wide range of frequencies. Although crystal oscillators offer high spectral purity, they cannot be tuned over a wide range of frequencies. Hence, some form of frequency synthesis is employed by these transceivers.
The term frequency synthesizer generally refers to an active electronic device (Fig. 1.1) that accepts some frequency reference (FREF) input signal of a very stable frequency fref and then generates frequency output as commanded by the frequency command word (FCW), whereby the stability, accuracy, and spectral purity of the output correlate with the performance of the input reference. The desired value of the output frequency is an FCW multiple (generally, a real number) of the reference frequency according to the equation.