Page 192 - Phase-Locked Loops Design, Simulation, and Applications

DESIGN PROCEDURE FOR MIXED-SIGNAL PLLS Ronald E. Best 117
results in a high value for C that is difficult to realize, we can choose a smaller value of C.
However, because the output frequency range would become too large, we must select a
smaller series capacitor C (Fig. 2.26) in order to reduce the capacitance variation by the
S
varactor. A design procedure for the VCO could look like the following:
1. Determine minimum output frequency ω , output frequency range Δω , and varactor
2min 2
capacitance variation ΔC .
V
2. Using Eq. (5.11), set a suitable value for C.
3. Determine inductor L such that the oscillator frequency is ω 2min for C = C Vmax .
V
4. When C has been chosen smaller than the value resulting from Eq. (5.11), set C such that
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the capacitance variation is large enough to produce the desired output frequency range.
In many applications, the tank circuit of the oscillator is a quartz crystal. The resonant
frequency of a quartz oscillator can be pulled over a restricted range by a varactor diode in
parallel. To design such an oscillator, the crystals specifications must be known; the data sheet
will tell you how much the resonant frequency is pulled away when switching a capacitor in
parallel to the crystal.
At frequencies in the GHz region, discrete inductors become difficult to implement.
Moreover, they offer very low Q factors, approximately in the range of 2 to 3. To get better
resonators, the discrete LC tank circuit is frequently replaced by a transmission line stub, for
example, a piece of microstrip. Radial stub lines can also be used. 53 Very detailed information
for the design of high-frequency oscillators can be found, for example, in the publications of
Rohde. 11, 48, 49

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