Page 241 - Complete Wireless Design

Oscillator Design

240 Chapter Four

Pulling—A parameter that specifies the shift of frequency and/or power
when the VCO is in an output impedance mismatch condition.
Pushing—The variation in frequency or amplitude that occurs to a VCO due
to changes in V , is measured in MHz/V or dB/V, respectively.
CC
Spurious signals—Undesired and nonharmonically related CW power
output spikes present in a VCO’s output.

4.3 Crystal Oscillators

4.3.1 Introduction
Since modern wireless communications equipment could not function proper-
ly with the extreme drift present in even the finest LC sinewave oscillator, it
was necessary to develop crystal-controlled oscillators. In today’s world of lim-
ited bandwidth and tight channel spacing, not only would an RF transmitter
drift and interfere with adjacent channels, but its signal would be unreadable
as it moved into and out of the passband of the receiver, creating changes in
the volume, pitch, and distortion levels of an analog radio and degrading BER
levels in a digital system. And the lack of any serious receiver frequency sta-
bility in the LOs would only contribute to this problem. Automatic frequency
control (AFC) is one answer, as is the PLL, which, however, would not be able
to function properly without a high-quality crystal oscillator as its reference.
For a fixed-frequency RF source under a center frequency of 200 MHz, the
crystal oscillator is the dominant choice.
A crystal oscillator requires only four things to precisely oscillate at a stable
frequency and amplitude:
1. The loop gain must be 1 (but greater than 1 to start).
2. The oscillator circuit’s impedance must be equal to its crystal’s internal
resistance.
3. The oscillator circuit must not drag down the Q of the crystal excessively.
4. The total oscillator circuit feedback phase must be zero degrees from out-
put to input.

A crystal is the perfect choice for operation in an oscillator because it will
vibrate at its own natural resonant frequency if an alternating signal at that
same frequency is placed across the crystal, basically functioning as an ultra-
high-Q series resonant circuit.
The most common crystal material used in oscillators is quartz because of
its low cost, strength, and thermal stability. However, crystal-based oscillators
can not easily change frequency, except by a few hundred hertz with a capaci-
tive trimmer in parallel, or in series, with the crystal element.
The maximum frequency a crystal can reach on its fundamental is 200 MHz,
and this only by using specialized inverted mesa methods. But most common

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