Page 357 - The Mechatronics Handbook

TABLE 17.5 Summary of Oscillator Types
Oscillator Commercial
Type Quartz (TCXO) Quartz (OCXO) Rubidium Cesium Beam Hydrogen Maser
4
Q 10 to 10 6 3.2 × 10 6 10 7 10 8 10 9
(5 MHz)
Resonance Various Various 6.834682608 GHz 9.192631770 GHz 1.420405752 GHz
frequency
Leading cause None None Rubidium lamp Cesium beam tube Hydrogen
of failure (life expectancy (life expectancy depletion (life
>15 years) of 3 to 25 years) expectancy
>7 years)
-8
-11
-11
Stability, σ y (τ), 1 × 10 to 1 × 10 -12 5 × 10 to 5 × 10 to 1 × 10 -12
τ = 1 s 1 × 10 -9 5 × 10 -12 5 × 10 -12
-9
Noise floor, 1 × 10 1 × 10 -12 1 × 10 -12 1 × 10 -14 1 × 10 -15
3
5
5
5
3
2
2
7
σ y (τ) (τ = 1 to 10 s) (τ = 1 to 10 s) (τ = 10 to 10 s) (τ = 10 to 10 s) (τ = 10 to 10 s)
Aging/year 5 × 10 -7 5 × 10 -9 1 × 10 -10 None ~ 1 × 10 -13
-12
-8
-12
-10
Frequency 1 × 10 -6 1 × 10 to 5 × 10 to 5 × 10 to 1 × 10 to
offset after 1 × 10 -10 5 × 10 -12 1 × 10 -14 1 × 10 -13
warm-up
Warm-Up <10 s to <5 min to <5 min to 30 min to 24 h to
period 1 × 10 -6 1 × 10 -8 5 × 10 -10 5 × 10 -12 1 × 10 -12
A quartz crystal inside the oscillator is the resonator. It can be made of either natural or synthetic
quartz, but all modern devices use synthetic quartz. The crystal strains (expands or contracts) when a
voltage is applied. When the voltage is reversed, the strain is reversed. This is known as the piezoelectric
effect. Oscillation is sustained by taking a voltage signal from the resonator, amplifying it, and feeding it
back to the resonator. The rate of expansion and contraction is the resonance frequency and is determined
by the cut and size of the crystal. The output frequency of a quartz oscillator is either the fundamental
resonance or a multiple of the resonance, called an overtone frequency. Most high stability units use either
the third or fifth overtone to achieve a high Q. Overtones higher than fifth are rarely used because they
make it harder to tune the device to the desired frequency. A typical Q for a quartz oscillator ranges from
6
4
7
10 to 10 . The maximum Q for a high stability quartz oscillator can be estimated as Q = 1.6 × 10 /f,
where f is the resonance frequency in megahertz.
Environmental changes due to temperature, humidity, pressure, and vibration can change the reso-
nance frequency of a quartz crystal, but there are several designs that reduce these environmental effects.
The oven-controlled crystal oscillator (OCXO) encloses the crystal in a temperature-controlled chamber
called an oven. When an OCXO is turned on, it goes through a ‘‘warm-up’’ period while the temperatures
of the crystal resonator and its oven stabilize. During this time, the performance of the oscillator
continuously changes until it reaches its normal operating temperature. The temperature within the
oven then remains constant, even when the outside temperature varies. An alternate solution to the
temperature problem is the temperature-compensated crystal oscillator (TCXO). In a TCXO, the signal
from a temperature sensor is used to generate a correction voltage that is applied to a voltage-variable
reactance, or varactor. The varactor then produces a frequency change equal and opposite to the
frequency change produced by temperature. This technique does not work as well as oven control, but
is less expensive. Therefore, TCXOs are used when high stability over a wide temperature range is not
required.
Quartz oscillators have excellent short-term stability. An OCXO might be stable (σ y (τ), at τ = 1 s) to
-12
1 × 10 . The limitations in short-term stability are due mainly to noise from electronic components in
the oscillator circuits. Long-term stability is limited by aging, or a change in frequency with time due to
internal changes in the oscillator. Aging is usually a nearly linear change in the resonance frequency that
can be either positive or negative, and occasionally, a reversal in direction of aging occurs. Aging has many
possible causes including a build-up of foreign material on the crystal, changes in the oscillator circuitry,

©2002 CRC Press LLC