Page 254 - Complete Wireless Design

Source: Complete Wireless Design

Chapter
5

Frequency Synthesizer Design

A method of combining the wide tunability of LC oscillators with the high fre-
quency stability of crystal oscillators is a necessity in modern wireless com-
munications design. We find both of these abilities in frequency synthesis,
which is a method of generating a multitude of exceptionally accurate fre-
quencies from a single, low-frequency crystal oscillator. It is the dominant
technique for variable-frequency production in most receivers, transmitters,
transceivers, and test equipment today.
By far the most widespread method of frequency synthesis is implemented
by the phase-locked loop (PLL); but a newer technique, referred to as direct
digital synthesis [DDS; sometimes called a numerically controlled oscillator
(NCO)], is becoming increasingly prevalent in certain applications. We will
concentrate on the PLL, which is easier to design, more versatile, and much
higher in frequency.

5.1 Phase-Locked Loops
5.1.1 Introduction
The majority of frequency synthesis is derived from the phase-locked loop.
Figure 5.1 demonstrates all of the vital circuits that make up a common single-
loop PLL synthesizer: A low-frequency crystal oscillator feeds a reference fre-
quency into the R divider, which decreases the reference frequency to equal the
desired f out of the N adjustable frequency divider, with the R divider allow-
COM
ing for different channel spacings. The reference frequency out of the R divider
is then inserted into the phase comparator, which compares the phase of the R
divider to that of the N adjustable frequency divider. The adjustable N fre-
quency divider receives its own input frequency from the VCO’s output, drop-
ping it down to a lower frequency that must be equal to the R divider’s output.
As the phase comparator is comparing the two frequencies at its input from the
N and R dividers to see if they are of the same phase, it will produce a rectified

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