Page 236 - Complete Wireless Design

Oscillator Design

Oscillator Design 235

Figure 4.20 The Maxim oscillator integrated circuit with support components.

V CNTRL and the varactor. The resistor is utilized to isolate the very small value
of the varactor’s capacitance and prevent it from being affected by the rest of
the circuit’s stray reactances, as well as the high value of any decoupling
capacitor. This permits the varactor to maintain its desired capacitance value
even while attached to a highly reactive control circuit.
An effect called varactor modulation can significantly degrade phase noise
performance of the VCO. Varactor modulation is worsened when a more sen-
sitive varactor is utilized, with more phase noise now contributed to the out-
put of the VCO. This will not be as much of a problem in VCOs that tune only
over a narrow range of frequencies, but can become significant with wideband
VCOs employing highly sensitive varactors, such as the hyperabrupt types. In
this wideband VCO application the varactor has, as it must, a wide capaci-
tance shift when presented with a relatively low tuning voltage (high sensitiv-
ity), so any small noise or varactor control voltage variation will generate a
large frequency modulation. This varactor modulation noise can be lessened
by a less sensitive varactor, or by having a less wide-tuning VCO requirement.

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