Page 225 - Complete Wireless Design

Oscillator Design

224 Chapter Four

simulations predict that a reactance of 50 ohms is possible for the output
capacitor or inductor into the load, while still maintaining the proper gain
margin, then more power can safely be output into the load. Optimize this cou-
pling reactance value in the open-loop Bode simulations by decreasing the
reactance of the coupling element until the gain margin is still at a safe level
of at least 5 dB or greater. This also means that a true conjugate match direct-
ly from the oscillator output to the load input is usually impossible, since it
would excessively load down the oscillator, and prevent it from starting or
from running properly.
The approximate output power of the oscillator can be found in Spice by
employing the oscilloscope tool across the oscillator’s load and calculating:

V 2 P
RMS
P (in watts) or P 10 log (in dBm)
R 1 mW
4.2 VCOs and LC Oscillators

4.2.1 Introduction
Today, LC oscillators are normally variable frequency oscillators (VFOs) of the
voltage-controlled oscillator (VCO) type since they can readily be tuned by
adjusting the capacitance of a varactor diode to set the frequency of oscillation.
Still, very low cost products that utilize fixed-frequency LC oscillators can be
found, sometimes up to 2.4 GHz; but they have poor frequency stability over
temperature and poor phase-noise specifications because of their very low Q.
Indeed, for any LC oscillator to be even remotely frequency stable, and have
decent phase noise, it will require a high capacitance-to-inductance ratio in its
LC circuit (for a higher Q), a steady and clean power supply, stable tempera-
ture conditions, and strong isolation from its load. Still, LC oscillators will
drift in frequency by up to 1 percent or more because of aging components or
(if unshielded) when a conductive surface is in close proximity. For any quality
wireless device this is unacceptable—unless some form of frequency regula-
tion is employed.
As mentioned above, LC oscillators are found mainly in voltage-controlled
oscillator circuits, since VCOs are heavily utilized in frequency synthesis for
phase-locked loops, and in any application where a DC control voltage is
required to alter the output frequency of an oscillator.

4.2.2 Types of LC oscillators
There are numerous kinds of LC oscillators. However, both the Hartley and
the Colpitts oscillators are very common, and an understanding of their func-
tion will allow a good grasp of other LC designs.
The Hartley oscillator, as shown in Fig. 4.11, exploits a tapped coil in its tank
circuit, made of L and C , to change the phase of the feedback to the transis-
1 1
tor’s base into a regenerative signal, and to set the frequency of oscillation. C
2

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