Page 257 - Complete Wireless Design

Frequency Synthesizer Design

256 Chapter Five

Figure 5.2 Premixing with a PLL and a crystal oscillator.

Figure 5.3 A common PLL manual tuning method.

polarity, into the PLL filter, which then converts this charge pump output into
a DC control voltage for the VCO. The charge pump PLL permits the use of a
passive filter, which is cheaper and adds little extra noise, unlike an active op-
amp based loop filter.
A few words about prescalers. Prescalers (Fig. 5.5) take the high frequency
of the VCO and divide it down to a more manageable lower frequency in the N
divider section: The VCO frequency is fed into this prescaler, which divides the
frequency down to P 1, in which P stands for the size of the prescaler. At
each of these P 1 cycles, A B counters decrement by 1. This creates a count
of A(P 1) and (B A)P, which makes N A(P 1) (B A)P, or P(B A).

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.