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50 High Frequency Electronics
High Frequency Design
OSCILLATOR SPECS
A Review of Key
Oscillator Specifications
and What They Mean
By Gary Breed
Editorial Director
U
nderstanding the
specifications of
oscillators is
essential when selecting
a commercial unit or
evaluating the performance of your own
design. This tutorial-level
article offers a review of oscillator specifications, what they mean, and some of the applications that are most affected by particular
specifications. These notes will focus primarily on crystal oscillator specifications, although
many of the same measurements and performance factors apply to VCOs, phase-locked
loops and direct digital synthesizers.
Stability
The first specification is stability. This is as
simple as it sounds—stability is how well an
oscillator stays on the desired frequency. The
measurement of stability is the deviation in
frequency under measurement conditions.
The data is typically presented in partsper-million (ppm), although percentage is
occasionally used. High performance oscillators with stability of less than one ppm will be
specified in scientific notation (e.g. 1x10
-6
is 1
ppm). Measurement conditions include:
Temperature—Oscillators are measured
according to their intended use, which means
the temperature range may be 0 to 50 ºC for
indoor applications, –40 to +70 ºC for outdoor
applications, or some other range. The most
common crystals are “AT cut” (a specific orientation in the quartz blank), which is easy to
manufacture and has a modest, but predictable temperature variation curve. “SC cut”
crystals are harder to make, but have much
better intrinsic temperature stability, which
lessens the need to provide compensation in
the oscillator circuitry.
Time—Crystals change slightly with age,
so performance specifications include shortterm (e.g. one day) and long term (e.g. one
year) stability. Oscillators used in reference
applications may have 10-year stability specifications as well.
Shock and vibration—Frequency variation
due to shock and vibration is a key specification in oscillators that will be used in harsh
environments, including military and space
applications. The magnitude and type of stress
will vary according to the needs of each application—a DSL modem might get dropped on a
concrete floor, while some military electronics
must survive being fired from big guns.
Applications with the greatest stability
requirements include primary references for
instrumentation, data communications networks and research. Other applications with a
significant need for high stability can be classified as “secondary references” used in applications such as GPS receivers and general
purpose instruments.
Phase Noise
You might wonder why phase noise
(time/frequency domain noise) is the specification for oscillators rather than noise that
includes amplitude. The answer is straightforward. First, the frequency of the oscillator is
what’s important, and second, amplitude noise
can be largely eliminated with limiter circuitry. However, if the oscillator is so poorly
designed that it has excessive amplitude
noise, some of that energy will be converted to
phase noise by the limiter or by other nonlin-
Whether designing or buy-
ing an oscillator, the key
specifications must be
understood in order to
make the right choice, or
evaluate a design
From November 2003 High Frequency Electronics
Copyright © 2003 Summit Technical Media, LLC
52 High Frequency Electronics
High Frequency Design
OSCILLATOR SPECS
ear components in the circuit.
Phase noise is specified in dB referenced to the carrier amplitude (dBc), versus frequency offset from the carrier (f
c
). In a reasonably well-designed oscillator, the noise
energy decreases with increasing frequency offset, but the
slope of the rolloff in noise has a series of different sections, governed by several different behaviors.
Figure 1 is a simplified sketch that illustrates the various influences on crystal oscillator phase noise. The different mechanism are described in many papers on oscillator measurement, including [1]. Rather than describe
each in detail, it is sufficient to note that each is an effect
created by the physical properties of the crystal and the
active devices in the oscillator circuit. The figure shows
how different phenomena contribute to overall phase
noise characteristics of an oscillator, each being dominant
at different offset frequencies and amplitude levels.
A VCO will have a similar plot, but with significantly
higher levels of phase noise, since LC, transmission lines,
dielectric resonators etc. have much lower Q than a
quartz crystal.
In a phase-locked loop, a similar plot (Figure 2) is used
to show phase noise performance. The shape of the plot is
quite different, since noise amplitude is greatly reduced
outside the loop filter bandwidth. The loop filter determines the phase noise close to f
c
, but a well-designed VCO
is needed for best performance at large offsets. Practical
PLLs may have noise or discrete spurious responses from
circuitry between the loop filter and VCO, power supply
bounce and ground currents, digital switching, and other
circuit-related behaviors. Figure 2 is not dimensioned,
since different synthesizer implementations will have different characteristics.
Direct digital synthesizers (DDS) or numerically controlled oscillators (NCO) have unique phase noise characteristics, as well as spurs that are a result of the digital
circuitry and digital-to-analog conversion. Rather than
try to describe them in this short article, the reader is
advised to read further on this subject.
Phase noise is the single most important specification
in some communication systems. A receivers that must
operate in the presence of nearby strong signals is one
such application. A transmitter modulated with a highly
complex waveform is another. In each case, excessive
phase noise can mask desired signal content.
Additional VCO Specifications
A voltage-controlled oscillator will have its own phase
noise plot, but also requires tuning range information. The
power supply must provide the maximum tuning voltage,
while the frequency vs. tuning voltage curve will influence
the design of a synthesizer’s loop filter. Stability vs. temperature data will tell an engineer how much of the tuning range is available to be used in his or her application.
Jitter in Digital Oscillators
Jitter is related to phase noise, but is described in the
time domain rather than the frequency domain because it
relates to digital signals. Jitter, usually specified in
picoseconds, is the maximum time variation from the
ideal succession of rise and fall transitions in a square
wave. This is an important specification for high-speed
digital computing and communications systems which
must maintain precise timing. Gigabit/second optical systems are a key application requiring low jitter specs.
Reference
1. D. B. Sullivan, D. W. Allan, D. A. Howe and F. L.
Walls, editors, Characterization of Clocks and Oscillators,
NIST Technical Note 1337, National Institute of Science
and Technology, Time and Frequency Division, 1990.
Frequency Offset from
f
c
(Hz)
Random Walk frequency noise
Flicker frequency noise
White frequency noise
Flicker phase noise
–60
–90
–120
–150
10
0
10
1
10
2
10
3
10
4
Amplitude (dBc)
White phase noise
Figure 1 · The effects of different physical phenomena
on crystal oscillator phase noise.
Frequency Offset from
f
c
Close-in VCO noise
Loop filter rolloff characteristic
VCO noise plus circuit noise
VCO residual noise
Discrete spurs
Relative Amplitude
Figure 2 · The phase noise characteristics of a typical
phase-locked loop.
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