TABLE OF CONTENTS The output frequency of an integer- N PLL changes by integer multiples of the input reference frequency ? ref. This simple fact makes it almost impossible to use the architecture in some radio standards.
A critical case is the ubiquitous GSM system. The channel spacing is about 200 kHz while the GSM spectrum is placed near 1 GHz. In an integer- N PLL architecture ? ref cannot exceed the channel spacing (200 kHz), and the division factor N must be of the order of 1 GHz/200 kHz~5000. The adoption of such a large division factor N is detrimental, essentially for three reasons, which are restated here.
Input noise amplification: any input noise superimposed on the reference signal is amplified by N 2. Taking N=5000, the factor N 2 corresponds to a 74 dB increment of the noise floor within the PLL bandwidth. This is why high N values are usually not compatible with the in-band noise requirements.
High power consumption: N divides G l oop, thus it indirectly affects other parameters of the loop and even the power consumption. As an example, taking (2.7) as an approximation of the PLL ?3dB bandwidth, the pump current is
which scales as N. Assuming N=5000, ? u=2 ? 40 krad/s, k VCO=2 ? 50 Mrad/(Vs), R=10 k ?, the current is I P=2.5 mA, which may exceed...
