Basic Technology of Quartz Crystal Oscillators
1. Categories of Quartz Crystal Oscillators
Quartz crystal oscillator is a timing device consisting of quartz crystal
resonator and an oscillation circuit to generate an output waveform at specified
frequency. It can be classified into four main categories based on its typical applications.
XO - Crystal Oscillator
The device does not have temperature control or temperature compensation.
Frequency - temperature characteristics depend on the crystal units. It is also
called SPXO – Simple Packaged Crystal Oscillator or Clock.
TCXO - Temperature Compensated Crystal Oscillator
The device has temperature compensation circuit to suppress output frequency deviation caused by surrounding temperature. That is, e.g.
the output signal from a temperature sensor (e.g., a thermistor) is used to
generate a correction voltage that is applied to a variable reactance (e.g., a
varactor) in the crystal network. The
reactance variations compensate for the crystal's F vs. T characteristic. Analog TCXO can provide about a 20X improvement over the crystal's F vs.
T variation.
OCXO - Oven Controlled Crystal Oscillator
The device usually
contains an oven block where the temperature sensor, heating element, oven
circuitry, and insulation function to maintain a stable temperature. By keeping
the temperature of the crystal and other temperature sensitive components, great
improvements in oscillator performance are realized such that the crystal's F
vs. T has zero slope. OCXOs can use either AT-, SC-, or IT- cut crystals depending
on temperature range and aging performance.
Typical OCXO can provide a >1000X improvement over the crystal's f vs.
T variation. The following table summarizes the best stability each type of oscillator can hold based on
today’s typical manufacturing process.
| |
Clock Oscillator |
TCXO |
OCXO |
| 0°C
to 70°C |
±10 ppm |
±0.5 ppm |
±0.003 ppm |
| -20°C
to 70°C |
±25 ppm |
±0.5 ppm |
±0.003 ppm |
| -40°C
to 85°C |
±30 ppm |
±1 ppm |
±0.02 ppm |
| -55°C
to 125°C |
±50 ppm |
N/A |
N/A |
VCXO - Voltage Controlled Crystal Oscillator
Crystal oscillator that has tunable output frequency or modulated output
frequency by external control voltage. The
connection of a variable capacitance diode (varactor) with crystal unit in
series can lead the diode capacitance to be changed according to the voltage
applied for frequency tuning, thus pulling frequency away from its normal value
based on the load capacitance characteristics of the crystal unit.
2. Terminology Discussion on Quartz Crystal Oscillators
Frequency Stability
Deviation from the nominal output frequency (Fmeasured – Fnorminal
)/ Fnorminal, including the frequency deviation from the following
factors:
Manufacturing process – Initial Accuracy and Aging: Initial
Accuracy is where the frequency is set close to the nominal frequency at room
temperature (25 C) ranging from ± 100 ppm to ± 0.25 ppm; Aging is the
frequency shift of the crystal over a certain time period.
The following table shows typical first year aging for 10 MHz Clock,
TCXOs, and OCXOs.
| Clock Oscillator |
TCXO |
OCXO |
| ±3 ppm |
±1 ppm |
±0.1 ppm (AT-cut); ±0.075 ppm
(SC-cut) |
|
