Page 240 - Complete Wireless Design

Oscillator Design

Oscillator Design 239

Efficiency of an oscillator stage is of great interest in battery-operated
devices. When combined with the DC power dissipation created by the static
bias current and the voltage across the collector, or P V I , then we get the
DC C C
efficiency of the Class A gain stage of the oscillator, or (P /P )
100 power
AC DC
efficiency in percent.
Other considerations. If an amplifier is used at the output of an oscillator to
increase its output power, the oscillator’s phase noise performance will only
deteriorate if the amplifier’s thermal noise is higher in amplitude than the
oscillator’s own phase noise.
To prevent undesired amplitude and frequency modulation of the oscillator’s
output, the V should be heavily filtered, and the oscillator’s LC resonator
CC
should possess a high loaded Q. If filtering is insufficient, then the V may
CC
require its own voltage regulator, which should be of the low-noise type.
The minimization of all noise in an oscillator is quite important in today’s wire-
less market. To reduce flicker (1/f) noise in an oscillator, a small emitter resistor
of a very low value (10 to 20 ohms, and not bypassed by a capacitor) will function
to eliminate most 1/f noise in low-frequency oscillators. This type of noise does
not appear to be a problem over any frequency above 5 MHz. However, to lower
the oscillator’s other noise outputs irrespective of 1/f noise, a transistor that has
a low NF, and a biasing network that does not use any emitter resistor, should be
employed. This is similar to standard LNA design procedures.
As well, a packaged VCO purchased from a vendor must be terminated into
a 50-ohm load, or deleterious reflections will degenerate oscillator performance,
particularly phase noise. The most common method to guarantee that this
matching occurs is to use a 50-ohm pad at the VCO’s output of 6 to 10 dB (some
VCOs have these attenuators built in, and do not require extra padding).

Important LC oscillator terms

Frequency stability—Measurement, in MHz/°C, of the frequency variation of
a VCO while the ambient temperature is varied.
Harmonic rejection (or suppression)—Measured in relation to the power, in
dB, below the carrier (dBc) of all harmonically related frequency products.
Input capacitance—Measurement, in picofarads, of the VCO’s DC tuning
input capacitance. Important in phase-locked loop (PLL) filter design.
Modulation sensitivity—A VCO parameter, measured in MHz/V, that
specifies the frequency change per input DC tuning voltage (not a completely
linear frequency measurement versus DC tuning range).
Phase noise—Oscillators are not perfect single CW frequency sources, but
possess phase noise. Phase noise is similar to a modulated spectrum created
by a virtual noise source that is phase modulating the desired CW signal.
Posttuning drift—An undesirable slow alteration in a VCO’s frequency that
occurs after the VCO changes its output frequency after commanded to do so
by the DC tuning voltage.

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